Quentin Bonnefoy
email: quentin.bonnefoy@polytechnique.edu

Emilian Dudas
email: emilian.dudas@polytechnique.edu

CPHT-RR080

Inspired by recent studies of high-scale decay constant or flavorful QCD axions, we review and clarify their existence in effective string models with anomalous U (1) gauge groups. We find that such models, when coupled to charged scalars getting vacuum expectation values, always have one light axion, whose mass can only come from nonperturbative effects. If the main nonperturbative effect is from QCD, then it becomes a Peccei-Quinn axion candidate for solving the strong CP problem. We then study simple models with universal Green-Schwarz mechanism and only one charged scalar field: in the minimal gaugino condensation case the axion mass is tied to the supersymmetry breaking scale and cannot be light enough, but slightly refined models maintain a massless axion all the way down to the QCD scale. Both kinds of models can be extended to yield intermediate scale axion decay constants. Finally, we gauge flavorful axion models under an anomalous U (1) and discuss the axion couplings which arise.

Introduction and Conclusions

The Peccei-Quinn (PQ) symmetry [1] and its light axion [2] (for reviews, see [3]) is probably the most elegant solution to the strong CP problem. Its implementation in string theory is natural since at tree-level in supergravity there are often continuous PQ like symmetries, usually broken to discrete subgroups by quantum corrections and nonperturbative effects. On the other hand, realistic string models often contain an "anomalous" abelian gauge symmetry, called U (1) X in what follows 1 , with anomaly cancellation à la Green-Schwarz (GS). Such a symmetry has multiple phenomenological applications: generating hierarchical fermion masses and mixing angles via the Froggatt-Nielsen mechanism [5], relating the weak angle to anomaly coefficients [6] and breaking supersymmetry [7].

In this note we comment on one additional generic property of models with an anomalous U(1): at the perturbative level, and if there is at least one charged scalar field which gets a vacuum expectation value (vev), such models always contain a potential axion candidate, which can only get a mass by turning on nonperturbative effects (and simultaneously turning on the coupling to gravity in supersymmetric models, where an R-symmetry survives even after inclusion of nonperturbative effects). We study the symmetries responsible for protecting the axion and the conditions under which the axion is light enough for solving the strong CP problem in a heterotic framework with a single charged scalar and hidden sector gaugino condensation, and we conclude that realistic supersymmetry breaking is incompatible with a light enough axion. However, we also give a refined example where nonperturbative dynamics still preserves a massless axion all the way to the QCD scale, even after coupling to gravity. Finally we show that in such a context and irrespective of the details of the model under consideration, gauge invariance fixes completely the couplings of the axion to matter when the charged scalar is used as a flavon field. The couplings to Standard Model (SM) charged fermions are proportional to their anomalous charges and the couplings to the gauge fields to the mixed U(1) X G 2 a anomalies, where G a = SU(3) c , SU(2) L , U(1) Y are the SM gauge group factors. Gauge coupling unification conditions alone then determine the ratio of the coupling to the photon to the coupling to the gluons to be E/N = 8/3 at the unification scale. These couplings are similar to the ones in the axiflavon/flaxion models [8], but the symmetry is now gauged.

The generic value of the axion decay constant in these simple setups is of order the unification scale. Such values require a tuned or nonstandard cosmology in order to ensure a consistent relic density for the axion. We therefore discuss in the final section models of moduli stabilization which display an intermediate scale axion decay constant. However, aiming for such an intermediate scale decay constant may not be required since several (recent) studies have shown that the resulting cosmology is viable and does not necessarily involve a severe amount of tuning [9]. Moreover, new proposals for axion dark matter searches are sensitive to GUT scale values for the axion decay constant [10].

2 Anomalous U (1) models 2.1 Perturbative axion in anomalous U (1) models

In this section one will prove the following result:

Theorem. In field (string) theory models of a U(1) X gauge theory with a Stueckelberg (Green-Schwarz) mechanism and at least one charged scalar field acquiring a non-zero vacuum expectation value2 , at the perturbative level there is always a massless pseudoscalar.

Proof. Let us consider an abelian gauge theory in a Stueckelberg phase, coupled to charged scalars Φ i of charges X i , of lagrangian

L = |D µ Φ i | 2 - 1 4 F 2 X,µν + 1 2 (∂ µ a S + MA X,µ ) 2 + • • • , (1)
where

Φ i = V i + h i √ 2 e iθ i V i . (2)
Gauge transformations act as

δA X,µ = - 1 g ∂ µ α , δθ i = X i V i α , δa S = M g α . (3)
From ( 1) one finds that the Goldstone boson which mixes in the usual way ∂ µ θ X A µ X with the gauge field is given by (up to a normalization factor)

θ X = gX i V i θ i + Ma S . (4)
We have therefore N + 1 potential axions/pseudoscalars, one of which is absorbed by the gauge field via the Higgs mechanism [11]. The perturbative scalar potential is of

The scalar potential (5) then gives masses to at most N -1 pseudoscalars. Consequently, there is always (at least) one leftover massless pseudoscalar, which will be a PQ axion candidate if it has the appropriate couplings. At the perturbative level, it is therefore always possible to define a PQ symmetry in models with an anomalous U(1) X gauge factor.

As one will see in the next sections, nonperturbative effects can generate gaugeinvariant potential terms of the form

V np = β e -q β s 0 -ic β a S λ β Φ p (β) 1 1 • • • Φ p (β) N N + h.c. , (8)
where s 0 is the vev of a scalar and q β , c β are numbers. Whenever such terms are generated, the leftover massless axion will get a mass from effects other than the usual QCD ones. Such terms can be generated by field-theory nonperturbative effects, instantonic effects in string theory or quantum gravity effects more generally.

Anomalous U (1): the heterotic case

In perturbative heterotic string theory constructions, there is only one possible anomalous U(1) X and one field, the universal axion-dilaton S, transforming non-linearly under gauge transformations. Those act on the different superfields involved as4

δV X = Λ + Λ , δφ i = -2 q i φ i Λ ≡ -2X i Λ , δS = δ GS Λ ≡ -2 X S Λ , (9)
where X i , X S define the holomorphic Killing vectors. The modified Kahler potential for the universal axion-dilaton is

K = -ln (S + S -δ GS V X ) (10)
and it encodes the Fayet-Iliopoulos (FI) term which appears in the D-term

D X = X I ∂ I G = X I ∂ I K = q i φ i ∂ i K + δ GS 2(S + S) , (11)
where in (11) G = K + ln |W | 2 and we used the gauge invariance of the superpotential X I ∂ I W = 0. We consider δ GS > 0 in what follows. In all known perturbative constructions there always exists in the massless spectrum a field with appropriate sign of the charge (negative in our conventions) whose vev is able to cancel perturbatively the (field-dependent) FI term and maintain supersymmetry. We consider the minimal case of one such field, called φ in what follows, and normalize its charge to -1, following [7].

Anomaly cancellation conditions relate mixed anomalies

C i = U(1) X G 2 i
, where G i are the various semi-simple factors of the gauge group G = N i=1 G i , such that

δ GS = C 1 k 1 = C 2 k 2 = • • • = C N k N = 1 192π 2 T r(q X ) , (12)
where the k i 's are the Kac-Moody levels defining the tree-level gauge kinetic functions

f i = k i S . (13)
The last term in (12) is the FI term, proportional to the mixed U(1) X -gravitational anomaly, where T r(q X ) is the sum of U(1) X charges over all the charged fermions in the spectrum. Therefore, once the FI term is generated, all mixed anomalies have to be different from zero and the theory must contain charged matter.

A light axion: gaugino condensation and anomalous U (1) in heterotic theories

Gaugino condensation in heterotic theories in the presence of the (generic) anomalous U( 1) gauge symmetry discussed in Section 2.2 has to fulfill the consistency requirements dictated by the coexistence of two local symmetries: supersymmetry and the gauge symmetry. However, although the pure Super-Yang-Mills gaugino condensation superpotential e -3S/2b 0 , where b 0 is the beta function of the hidden sector, is not gauge invariant, gauge invariance does not forbid gaugino condensation to take place, as was discussed in the heterotic string case some time ago in [7]. It was shown there that the GS cancellation of gauge anomalies restricts the nonperturbative dynamics such that the nonperturbative superpotential is precisely gauge invariant.

Taking for simplicity a SUSY-QCD model with N c colors and N f < N c flavors and denoting by Q ( Q) the hidden sector quarks (antiquarks) of U(1) X charges q (q), the GS conditions fix completely the sum of the charges to be

C h = 1 4π 2 N f (q + q) = δ GS k h , (14)
where k h is the Kac-Moody level of the hidden sector gauge group. This turns out to be precisely the gauge invariance condition of the nonperturbative superpotential

W np = (N c -N f ) e -8π 2 k h S det(Q Q) 1 Nc-N f . ( 15
)
Notice that anomaly cancellations (12) and the structure of the D-term (11) unambiguously show that the charge of the hidden sector mesons Q Q has the same sign as the induced FI term. Notice also that the charges allow for a perturbative coupling of the form

W p = λ j i φ M P q+q Q i Qj . ( 16
)
Since φ gets a large vev of the order of the FI term, below the scale of U(1) X gauge symmetry breaking the perturbative term ( 16) becomes a mass term for the hidden sector quarks and the dynamics of condensation is essentially that of supersymmetric QCD.

The light axion

In supersymmetric QCD there is no light axion. The only global anomaly-free symmetry in the UV is an R-symmetry, which is broken explicitly by the mass term. In the model introduced in [7] and briefly reviewed above, the mass term is replaced by the coupling (16). Then it is easy to check that the following global R-symmetry

θ ′ = e iα θ , (Q, Q) ′ (θ ′ ) = e i(N f -Nc)α N f (Q, Q)(θ) , φ ′ (θ ′ ) = e 2iNc α N f (q+ q) φ(θ) , S ′ (θ ′ ) = S(θ) , (17)
is exact and anomaly-free with respect to SU(N c ) 5 . It is also spontaneously broken, therefore one expects a massless Goldstone boson. More generally, one can combine the R-symmetry above with the gauge symmetry. Indeed:

φ ′ (θ ′ ) = e iq φ α φ(θ) , S ′ (θ ′ ) = S(θ) - i 2 q S α , Q ′ (θ ′ ) = e iq Q α Q(θ) , Q′ (θ ′ ) = e iq Qα Q(θ) ( 18
)
is a (non-anomalous) R-symmetry of the (non-perturbative) superpotential if:

q φ = 2N c (q + q)N f - P q + q , q Q + q Q = 2(N f -N c ) N f + P , q S = N f P 4π 2 k h , ( 19
)
where P is a number and P = 0 corresponds to the R-symmetry (17).

The model has three pseudoscalars, on which we now concentrate. In order to identify the massless axion, it is enough to parametrize the original fields by ignoring any other field than those pseudoscalars. By defining them in order to have canonical kinetic terms, we are led to the parametrization (20) where s 0 , V , and M 0 are vev's. One combination of those pseudoscalars

S = s 0 1 + i √ 2 a S M P , φ = V √ 2 e ia φ V , M = Q Q = M 0 I N f ×N f e i 2 N f M 0 a M ,
a X ∝ δ GS √ 2s 0 a S + 2V a φ -(q + q) 2N f M 0 a M ( 21
)
is absorbed by the U(1) X gauge field. Another one is shifted by the symmetry (18)(19), which we choose such that it leaves a X invariant. In the limit where M 0 ≪ V, M P , the value of P which achieves this is

P = 2N c N f 1 + δ 2 GS M 2 P 8V 2 s 2 0 + O M 0 V, M P , (22)
and the associated symmetry current gives us the expression of the physical axion a P Q :

J µ ∝ 1 1 V + 8s 2 0 V δ 2 GS M 2 P ∂ µ a φ - 2 √ 2s 0 V δ GS M P a S + O M 0 V, M P ≡ f a ∂ µ a P Q , ( 23
)
where we identified the axion decay constant

1 f a = 1 V 2 + 8s 2 0 δ 2 GS M 2 P . ( 24
)
Natural values are of order the unification scale f a ∼ M GU T , although smaller values are possible in orientifold models.

Simplified description

If the scale of hidden sector condensation is well below the scale of U(1) X gauge symmetry breaking, which we assumed in deriving expressions (22)(23), there is an approximate decoupling between the hidden sector dynamics and the U(1) X dynamics. In particular, in this limit the SU(N c ) dynamics is essentially the one of supersymmetric QCD, which has no light particles, therefore no light composite axion. It should be therefore possible to describe accurately the light axion physics by integrating out the hidden sector. By doing this, one finds an effective superpotential

W ef f = W 0 + N c (det λ) 1 Nc M 3-N f /Nc P φ M P N f (q+ q) Nc e -8π 2 k h S Nc , (25)
where the constant W 0 was added for the purpose of coupling to gravity later on. The effect of the hidden sector condensation is therefore of generating a non-perturbative superpotential, sometimes said to be of "fractional instanton" type, as compared to "stringy instanton" effects, which would be proportional to e -8π 2 S in our conventions. According to our general discussion in Section 2, the phase of the nonperturbative term in (25) defines a physical axion, which is orthogonal to the Goldstone boson a X precisely when the GS anomaly cancellation conditions ( 14) are imposed. One can write explicitly a X and the (for now) massless axion a P Q by introducing a rotation matrix a X = cos θ a S + sin θ a φ , a

P Q = -sin θ a S + cos θ a φ , (26)
with tan θ = 2 √ 2s 0 V /(δ GS M P ). Notice that a P Q coincides with the leading order expression of the axionic current obtained in (23).

At the global supersymmetry level, the axion mass is protected by the R-symmetry (22). However, after coupling to supergravity, the constant W 0 breaks explicitly the R-symmetry and as such the axion will get a scalar potential and therefore a mass [12].

Without entering details of moduli stabilization, one expects a scalar potential of the form

V (a P Q ) ∼ W 0 N c (det λ) 1 Nc M 3-N f /Nc P V M P N f (q+ q) Nc e -8π 2 k h s 0 Nc cos (q + q)N f N c a P Q f a , (27)
where the axion decay constant is given in (24). By using the order of magnitude value for the gravitino mass m 3/2 ∼ W 0 and the definition of the IR dynamical scale

Λ 3 L = (det λ) 1 Nc M 3-N f /Nc P φ M P N f (q+ q) Nc e -8π 2 k h S Nc , (28)
one finds that this axion can solve the strong CP problem if

m 3/2 Λ 3 L ≪ 10 -10 f 2 π m 2 π . ( 29
)
This is a very strong constraint, which favors in this minimal model low values of the gravitino mass and of the dynamical scale Λ L . Using the fact that in the minimal model of [7] supersymmetry was broken, and m 3/2 ∼ Λ 3 L /(V M P ), one finds, without an additional source of supersymmetry breaking, the constraint m 3/2 ≪ 10 -14 eV, which is not realistic in known mediations of supersymmetry breaking. In this model therefore, an additional source of supersymmetry breaking is necessary, whereas for a gravitino mass corresponding to standard mechanisms for supersymmetry breaking, the axion is too heavy to solve the strong CP problem.

More refined analysis

Let us perform a slightly more general analysis, by keeping the hidden sector mesons in the discussion. The hidden mesons are described by chiral (super)fields of charge q + q and have a Kahler potential, computed along the flat directions of SU(N c ), equal to

K = T r(M † M) 1 2 . ( 30
)
The hidden mesons appear in the full superpotential

W = W np + W p = (N c -N f ) e -8π 2 k h S det(M) 1 Nc-N f + λ j i φ M P q+q M i j (31)
and add a pseudoscalar axionic degree of freedom a M , that is encoded in the parametrization (20). Notice that solving for M in (31) gives back (25). Out of the original three pseudoscalars, one is the Goldstone boson absorbed by the gauge field (21) and the other two are physical, called a 1 and a 2 in what follows. They can be parametrized by the gauge invariant operators in (31) and, up to normalization, can be written as

a 1 ∼ 8 √ 2π 2 k h s 0 a S + 2N f M 0 a M , a 2 ∼ q + q V a φ + 2 N f M 0 a M . (32)
Notice that they are both orthogonal to the Goldstone boson a X , as enforced by gauge invariance. They are not orthogonal to each other, fact to be taken into account in what follows. The hidden sector nonperturbative dynamics is giving a mass to the linear combination

a h ∼ a 1 N c -N f + a 2 , (33)
whereas the orthogonal combination a l defined by (a h , a l ) = 0 is the massless (at the global supersymmetric level) axion. For general vev's its expression is relatively involved. However, in the limit we considered in the previous sections M 0 ≪ V, M P , one can easily find that

a l ∼ a 1 -N f a 2 → a P Q (34)
is precisely the light axion (23,26), obtained by integrating-out from the start the hidden sector mesons in the simplified description.

A massless axion: the 3-2 model

The main problem with the previous minimal model is that the hidden sector nonperturbative dynamics was giving a mass to the axion through supergravity interactions.

Nonperturbative dynamics is however often instrumental for stabilizing moduli, in our case the very modulus involved in the GS mechanism. The natural next step is to identify models in which the hidden sector nonperturbative dynamics leaves an exactly massless axion, even after coupling to (super)gravity. One way to achieve this goes as follows: at the perturbative level, as we proved in Section 2.1, there is always a massless axion in models with anomalous U(1) X . Suppose now that the hidden sector producing the nonperturbative dynamics has an R-symmetry itself, in the limit in which the anomalous abelian gauge dynamics is turned off. Then if the condensation breaks spontaneously the R-symmetry, there is another R-axion coming from the hidden sector. In total there are therefore two axions in the limit where gravity is decoupled. By turning on gravity and adding a constant which breaks explicitly the R-symmetry, one (linear combination) of the two axions becomes massive. But the other one remains massless down to the QCD scale and behaves as an ideal candidate for a PQ QCD axion. Essentially the nonperturbative dynamics is not adding a potential for the axion, but is just stabilizing the GS modulus.

One explicit model of this type uses for the hidden sector the 3-2 model of supersymmetry breaking [13]. The gauge group of the model is 2) is the hidden sector gauge group. The nonabelian factor SU( 3) is confining with a dynamical scale Λ 3 . The matter content in the UV contains the chiral multiplets

G = G h × U(1) X × • • • , where G h = SU(3) × SU(
Q α i (3, 2) , L α (1, 2) , Ūi ( 3, 1) , Di ( 3, 1) → Qi α = ( Di , Ūi ) , ( 35
)
in a self-explanatory notation (notice that the α index of Q is not gauged under SU( 2) and only represents a convenient repackaging). The model has two anomaly-free global symmetries, one acting like hypercharge and an R-symmetry:

U(1) Y : Y (Q) = 1 6 , Y ( Ū ) = - 2 3 , Y ( D) = 1 3 , Y (L) = - 1 2 , U(1) R : R(Q) = -1 , R( Ū ) = R( D) = 0 , R(L) = 3 . ( 36
)
Below the scale of SU( 3) condensation, the dynamics is governed by the gauge invariant operators

X 1 = Q DL , X 2 = Q Ū L , X 3 = det Qα Q β . ( 37
)
The low-energy superpotential, compatible with the symmetries and the condensation dynamics, is given by

W eff = λX 1 + 2Λ 7 3 X 3 . ( 38
)
The analysis of the potential, including the D-term contributions, shows that X 1 and X 3 are non-vanishing whereas X 2 vanishes. There are then two pseudoscalars in the hidden sector, the potential axions in the phases of X 1 and X 3 . One linear combination of them will get a mass from the nonperturbative dynamics, and the second one gets a mass from couplings to (super)gravity, as in the model described in the preceding section.

If we now couple this model to an anomalous U(1) X , we would get an additional pseudoscalar from the high-energy anomalous U(1) X sector. There is therefore one leftover axion which is massless all the way down to the QCD scale, being a good candidate for a PQ axion. To restrict the superpotential, one could use the anomalous gauge symmetry instead of imposing the hypercharge global symmetry as above. We can for instance give the following charges to the multiplets (where n is some number):

U(1) X :          X(Q) = 1 6 + n X( Ū ) = -1 3 X( D) = 1 3 X(L) = -1 2 -n 3 =⇒          X(X 1 ) = 2n 3 X(X 2 ) = 2(n-1) 3 X(X 3 ) = 1 3 + 2n X(Λ 3 ) = 1 21 + 2n 7 , (39)
where, as in the model discussed previously, the condensation scale Λ 3 = e -8π 2 k 3 S 7

is not-gauge invariant anymore due to the U(1) X SU(3) 2 anomaly:

U(1) X SU(3) 2 : C 3 = 1 4π 2 × 1 3 + 2n , U(1) X SU(2) 2 : C 2 = 1 4π 2 × 8n 3 , (40)
while the nonperturbative superpotential is:

W eff = λ φ M P 2n 3 X 1 + 2Λ 7 3 X 3 . (41)
The first term in ( 41) is a perturbatively generated operator if we assume that n is a multiple of 3 2 . If Λ 3 ≪ V , analogously to the model in the previous section this axion is essentially a combination of a S and a φ . The axion decay constant will be determined as before and is therefore naturally of the order of the unification scale.

Gauged flavor symmetry and axion couplings to matter

We now identify the U(1) X discussed in the preceding sections with a flavor symmetry [8,14], since those are naturally anomalous due to the structure of fermion masses and lead to the GS mechanism [15]. Doing this, we will see that we generate axionic couplings for the light physical axion of the theory. Since the explicit examples discussed so far were supersymmetric, we focus on the Minimal Supersymmetric Standard Model (MSSM) in what follows.

We then charge the different MSSM superfields such that the Yukawa terms, as well as the µ-term, now explicitly involve φ:

W MSSM =λ u,ij φ M P Xq i +Xu j +X hu Q i U j H u + λ d,ij φ M P Xq i +X d j +X h d Q i D j H d + λ e,ij φ M P X l i +Xe j +X h d L i E j H d + µ φ M P X hu +X h d H u H d . (42)
A clever choice of U(1) X charges for the MSSM fields then allows to account for, or at least soften, the mass hierarchies and the µ-problem of the MSSM. We note that the U(1) X charge of φ makes it possible to choose most, if all, of the MSSM charges to be positive, consistently with the GS conditions (12). Starting from this superpotential, one can work out the couplings of the physical axion to the MSSM fields. Triangle loop diagrams combined with the GS term give for instance the coupling of the axion to QCD gauge fields:

L ⊃ i (2X q i + X u i + X d i ) 64π 2 a P Q f a T r(G G) = C 3 16
a P Q f a T r(G G) , (43)
where a P Q is given by the expression in (23) 6 , its decay constant f a in (24) and C 3 is the SU(3) gauge anomaly coefficient which appears in (12). Note that the domain wall number N DW = i (2X q i + X u i + X d i ) can be chosen equal to 1 with a consistent choice of charges for the Higgs doublets. This expression can be understood as a modification of the QCD kinetic function ( 13) when the quarks are integrated out:

f 3 = k 3 S - C 3 2 ln φ M P , (44)
which displays clearly the two canceling contributions to the U(1) X SU(3) 2 anomaly. Similar expressions hold for the other factors of the MSSM gauge groups.

We can deduce from this an interesting prediction of such models if we embed them in unified theories. Indeed, in such a case the anomaly coefficients are linked at the unification scale. For instance, for SU(5) unification, the MSSM gauge couplings verify g 2 3 = g 3 2 = 5 3 g 2 Y , while the fact that S determines all the gauge kinetic functions gives

g 2 Y k Y = g 2 2 k 2 = g 2 3 k 3 and the GS conditions impose C 3 k 3 = C 2 k 2 = C Y k Y .
All this can be combined to get C 3 = C 2 = 3 5 C 1 . Thus, the ratios of the couplings of the axion to the MSSM gauge fields are determined: for instance we get that the ratio (at the GUT scale) between the electromagnetic coupling and the gluons coupling is

E N = 8 3 . (45)
We stress that (45) is valid not only in flavor models of the type (42), but in any anomalous U(1) model in which SU( 5) unification of gauge couplings is imposed. Indeed, ( 45) is enforced uniquely by unification and the kinetic function ( 44), determined by gauge invariance.

There are also couplings of the axion to the spin of fermions arising from (42):

∂ µ a f a (ψ L,I X L,I γ µ ψ L,I + ψ R,I X R,I γ µ ψ R,I ) . ( 46
)
Their strength is given by the U(1) X charges of the MSSM fields, so the lighter generations are more coupled than the heavier ones. Besides, once expressed in terms of mass eigenstates, those couplings can be off-diagonal in flavor space, leading to possible flavor-changing currents [16]. However, if the axion dynamics lies at the string/GUT scale, all those effects are very much suppressed and evade current constraints. Still, since the couplings to the first generation of the MSSM are not specifically suppressed, recently proposed experiments [10] could have the sensitivity to probe such string scale decay constants in the near future.

6 Comments on moduli stabilization and intermediate scale decay constants

Moduli stabilization and axions in string models with anomalous U(1) were studied in various papers [17] and the issue of axion mass and decay constant in string theory in various works, see e.g. [18,19].

In the context of models of the type discussed in our note, the value of the gravitino mass is highly correlated to the stabilization of the moduli. One should distinguish the case where the coupling to supergravity lifts the axion mass, like in the model in Section 3, from the case where it does not, like in Section 4. In the first case, there is a strong correlation between the values of the gravitino mass and the axion mass such that keeping the axion light requires very small values of the gravitino mass. It was shown that in minimal models the requirement of "uplifting" the vacuum energy to zero is only compatible with large values of the gravitino mass [20]. In more sophisticated models with several charged scalars the gravitino mass can be reduced to the TeV range [21], but still far from the small values needed to keep the axion light enough. In other stabilization schemes, it is still possible to keep the axion light enough with more realistic values of the gravitino mass, see e.g. [19]. On the other hand, for models in which coupling to supergravity does not lift the axion mass, like in our Section 4, the scale of supersymmetry breaking is completely decorrelated from the axion mass, which then only gets a mass from QCD nonperturbative effects.

The moduli stabilization in Sections 3 and 4 was also enforcing a high-scale axion decay constant due to the U(1) X D-term expression (11). This can be relaxed in models where the moduli sector is slightly more complex. For example, let us consider a model of two moduli and a charged superfield:

K = - 3 2 ln(T 1 + T 1 -δ 1 V X ) - 3 2 ln(T 2 + T 2 + δ 2 V X ) + φ † e -2V X φ , (47)
on which the anomalous U(1) X symmetry acts as follows:

δV X = Λ + Λ , δφ = 2φΛ , δT 1 = δ 1 Λ , δT 2 = -δ 2 Λ . (48)
The U( 1)

X D-term potential V D = g 2 X 2 (|φ| 2 + 3δ 2 4(T 2 +T 2 ) -3δ 1 4(T 1 +T 1 )
) 2 now allows for a high scale stabilization of the moduli with a small or intermediate scale φ. To illustrate this, we furthermore assume that there are two hidden strong sectors 1 and 2, with gauge kinetic functions given by:

f 1 = T 1 4π , f 2 = n 2 T 1 + n 1 T 2 4π
, where n i = πδ i are integers ,

such that the group 1 is anomalous with respect to U(1) X whereas f 2 is gauge invariant.

Strong dynamics can then generate couplings of the type: 7 W = W 0 + Aφ n 1 e -2πT 1 + Be -2π(n 2 T 1 +n 1 T 2 ) .

(50)

In order to compute the vacuum of the theory, we assume that the uplift of the vacuum energy does not depend on the axions (e.g. à la KKLT [22]). Thus, as far as the axions are concerned we look at first order for the supersymmetric vacuum: 8

D φ W ≡ W φ + K φ W = An 1 φ n 1 -1 e -2πT 1 + φW = 0 D T 1 W = -2πAφ n 1 e -2πT 1 -2πn 2 Be -2π(n 2 T 1 +n 1 T 2 ) - 3 2(T 1 + T 1 ) W = 0 D T 2 W = -2πn 1 Be -2π(n 2 T 1 +n 1 T 2 ) - 3 2(T 2 + T 2 ) W = 0 , (51)
and we solve this set of equations given the value of m 3/2 = |W |e K/2 M 2 P and assuming that W ≈ W 0 and |φ| 2 ≪ T -1 1,2 , which we eventually check to be valid:

T 2 + T 2 T 1 + T 1 = n 2 n 1 , 2πn 2 (T 1 + T 1 )e -2πn 2 (T 1 +T 1 ) = 3W 0 2B , |φ| = W 0 e π(T 1 +T 1 ) n 1 A 1 n 1 -2 . (52)
If we choose for instance m 3/2 = 10 GeV, n 1 = 3 and n 2 = 1, we numerically get T 1 + T 1 = 3(T 2 + T 2 ) ≈ 6M P and |φ| ≈ 10 11 GeV, which implies an intermediate scale for the physical axion. However, in this setup the axion mass is tied to the supersymmetry breaking scale and cannot be light enough to provide a proper QCD axion. To cope with this, one can for instance implement the configuration (47) within the 3-2 model of Section 4. This amounts to consider the following superpotential (where all fields are those defined either above or in Section 4):

W = W 0 + λ φ M P 2n 3 X 1 + 2Λ 7
3 X 3 + Be -2πk 2 (n 2 T 1 +n 1 T 2 ) , with Λ 3 = e -2πk 1 T 1 7

. (53) 7 Those nonperturbative effects have periodicity T i = T i + 1 and are called stringy instanton effects. The other option is to use fractional instanton effects, like in Section 3, which would be, with the present section notations, of the type e -2πTi/N where N ∈ N. 8 Those three equations can be combined to check that the D-term potential vanishes.

There is as expected a massless axion in the low-energy limit, and its associated decay constant can be of intermediate scale: for instance, choosing n 1 = n 2 = 1, n = 6, k 1 = 17, k 2 = 4 and φ ≈ 10 12 GeV, one finds X 1/3 1 ≈ 10 12 GeV and a gravitino mass of ≈ 10 -4 eV (consistent with gauge mediation of supersymmetry breaking).

We consider the minimal case of one anomalous abelian symmetry, like in the original context it was studied[4], the perturbative heterotic string. Our arguments do however apply to other string models as well, in particular orientifold models, by relabeling appropriately the modulus field, as in our Section 6.

In case of additional U (1) gauge symmetries, anomalous or not, the counting may be different but a similar result always applies.

It can be checked that the argument below does not change if some of the fields in the scalar potential appear with a complex conjugation.

We use here the same convention as in[7] to define charges of chiral superfields.

The anomalies with respect to U (1) X can be canceled by fields from other sectors, for example (MS)SM fields.

This assumes that the axion is mostly carried by a φ and a S , which requires that every other dynamics breaking the PQ symmetry in the hidden sector or in the MSSM happens at a much lower energy.

Acknowledgments

The authors acknowledge partial support from the ANR Black-dS-String, and thank Michele Cicoli and Stefan Pokorski for enlightening discussions.


Barbara Padalino
email: barbara.padalino@uniba.it

Lydiane Aubé

Meriem Fatnassi

Davide Monaco

Touhami Khorchani

Mohamed Hammadi

Giovanni Michele Lacalandra

Aube Lydiane

Meriem ´2

Fatnassi

Could Dromedary Camels Develop Stereotypy? The First Description of Stereotypical Behaviour in Housed Male Dromedary Camels and How It Is Affected by Different Management Systems

Could dromedary camels develop stereotypy?

The first description of stereotypical behaviour in housed male dromedary camels and how it is affected by different management systems

Introduction

Animal behaviour is influenced by the prevailing environment, and behavioural modifications are used to assess the impact of different kinds of management on animal welfare [START_REF] Inglis | How an animal's behavioural repertoire changes in response to a changing environment: a stochastic model[END_REF]. Animals housed in artificial habitats are confronted by a wide range of potentially provocative environmental challenges, and animals in captivity can develop stereotypical behaviours [START_REF] Haskell | Onfarm assessment of the effect of management and housing type on behaviour and welfare in dairy cattle[END_REF], i.e. repetitive, unvarying and apparently functionless behaviour patterns [START_REF] Mason | Stereotypy: a critical review[END_REF]. Since these behaviours have usually been associated with suboptimal living conditions [START_REF] Mills | Repetitive movement problems in the horse[END_REF], they have often been used to assess animal welfare in different species (e.g. [START_REF] Broom | Animal welfare: concepts and measurement[END_REF][START_REF] Mason | Can't stop, won't stop: is stereotypy a reliable animal welfare indicator?[END_REF][START_REF] Normando | Variables affecting the prevalence of behavioural problems in horses. Can riding style and other management factors be significant?[END_REF][START_REF] Hausberger | Could Work Be a Source of Behavioural Disorders? A Study in Horses[END_REF]). Thus, Mason & Latham [START_REF] Mason | Can't stop, won't stop: is stereotypy a reliable animal welfare indicator?[END_REF] suggested that stereotypy should always be taken seriously as a warning sign of potential suffering. Stereotypy can take a wide range of different forms (e.g. locomotor or oral; [START_REF] Rushen | A decade-or-more's progress in understanding stereotypic behaviour Stereotypic Animal Behaviours[END_REF]) and the causes of these abnormal behaviours have been the subject of much discussion [START_REF] Mason | Stereotypy: a critical review[END_REF][START_REF] Broom | Animal welfare: concepts and measurement[END_REF][START_REF] Editors | Abnormal behaviours: stereotypies[END_REF]. The animal's lack of control over its environment, frustration, threat, fear, and lack of stimulation have all been mentioned as the main causes leading to the development of abnormal behaviour [START_REF] Mason | Stereotypy: a critical review[END_REF][START_REF] Broom | Animal welfare: concepts and measurement[END_REF]. One of the reasons why animals develop stereotypies is that endorphins are released when performing them, producing a form of pleasure that can help the animal to cope with the various captivity stressors, which in turn may positively reinforce the behaviour (in sows [START_REF] Cronin | Endogenous opioids are involved in abnormal stereotyped behaviours of tethered sows[END_REF]; in macaques [START_REF] Crockett | Beta-endorphin levels in longtailed and pigtailed macaques vary by abnormal behaviour rating and sex[END_REF]).

Therefore, intensive management systems which do not allow the animal to express its behavioural needs could lead to the development of repetitive and functionless behaviours. Cooper and McGreevy [START_REF] Cooper | Stereotypic Behaviour in the Stabled Horse: Causes, Effects and Prevention without Compromising Horse Welfare In: The Welfare of Horses, Animal Welfare[END_REF] reported that, in horses, stereotypies were related to a number of management factors, such as concentrate feeding or social isolation, and that the form of stereotypy usually depended on the constraints to which the animals were exposed. Nicol [START_REF] Nicol | Understanding equine stereotypies[END_REF] also suggested that oral stereotypies in horses (e.g. cribbiting, wood-chewing) may develop in response to a low-forage diet, because these behaviours may increase salivary flow, reducing gastric tract acidity and speeding up the transit of ingested feed. Locomotor stereotypy in horses (e.g. weaving) may derive from some frustrated attempt to move or escape from the stable [START_REF] Nicol | Understanding equine stereotypies[END_REF].

Moreover, different studies have shown negative correlations between enclosure size and the prevalence of pacing in different species (red deer [START_REF] Pollard | The effects of pen size on the behaviour of farmed red deer stags confined in yards[END_REF]; giraffe and okapi [START_REF] Bashaw | A survey assessment of variables related to stereotypy in captive giraffe and okapi[END_REF]).

The husbandry of male camels has been changing recently to more intensive management systems where they are kept isolated in a box or pen and used for programmed mating [START_REF] Rahim | Studies on the sexual behaviour of the dromedary camel[END_REF] or artificial insemination [START_REF] Skidmore | Artificial insemination in dromedary camels[END_REF][START_REF] Monaco | The experimental semen collection centers for dromedary camels in Egypt and Tunisia: Current situation and future developments[END_REF]. One of the major problems of male camel rearing is that during the breeding season bulls can become very aggressive towards other males or humans and for this reason they are kept in a single box or tethered with ropes [START_REF] El-Whishy | Reproduction in the male Dromedary (Camelus dromedarius): a review[END_REF]. In an individual box, the animals are isolated and it is known that social isolation can create stress [START_REF] Kim | Social isolation in animal models of relevance to neuropsychiatric disorders[END_REF] and may lead to stereotypical behaviour. McGreevy et al. [START_REF] Mcgreevy | The prevalence of abnormal behaviours in dressage, eventing and endurance horses in relation to stabling[END_REF] have shown that the time spent in a single box was positively correlated with an increased risk of abnormal behaviour in horses. Camels are also social animals and in feral conditions usually live in herds and spend most of the day walking to pasture [START_REF] Gauthier-Pilters | The camel. Its evolution, ecology, behaviour, and relationship to man[END_REF], so captivity could affect their behaviour, as already reported in other feral animals housed in artificial habitats [START_REF] Morgan | Sources of stress in captivity[END_REF]. Thus, our hypothesis was that, as in other species, confinement stressors such as restricted movement, reduced retreat space, forced proximity to humans, reduced feeding opportunities and maintenance in abnormal social groups, could also lead to the development of stereotypical behaviour in male dromedary camels. Since stereotypical behaviours have not yet been reported in dromedary camels, the aim of this study was to identify and describe them for the first time in males housed in single boxes.

While there are several studies on the effects of different housing systems on the behaviour of cattle, horses, hens, pigs and other domestic animals [START_REF] Haskell | Onfarm assessment of the effect of management and housing type on behaviour and welfare in dairy cattle[END_REF][START_REF] Mcgreevy | The prevalence of abnormal behaviours in dressage, eventing and endurance horses in relation to stabling[END_REF][START_REF] Lay | Hen welfare in different housing systems[END_REF], few studies have been carried out to assess the effects of captivity in camels. Therefore, it was thought of interest to study what effects different forms of husbandry could have on their behaviour, in an attempt to suggest how to optimize camel breeding techniques in the future.

Materials and Methods

Animals and management systems

Four clinically healthy male dromedary camels (Camelus dromedarius), ranging in age from 5 to 8 years, with a mean body weight of 526625 kg and good body condition score (3.560.25 arbitrary units; from 0 to 5 accordingly with Faye et al. [START_REF] Faye | Body condition score in dromedary camel: A tool for management of reproduction[END_REF]), were used for this study. All animals were identified by ear tags (#808, #514, #515, #504). The camels had been reared at the Arid Lands Institute's experimental station in Me ´denine, Tunisia (33u309N, 10u409E), 18 m above sea level.

In summer, the bulls are kept in a single open-air paddock shaded by trees whereas, starting from October, they were put into single boxes (Height = 3 m, Length = 5 m and Width = 3 m) with sand floors. They were tethered with a rope on the fetlock of the foreleg and were able to walk around inside the box. The boxes were located far from the females' pen, preventing them from seeing and touching any dams; the gates of the stable pointed eastwards, facing an open-air paddock and with a small window on the opposite wall. The gates of the stable were made by bars; camels were able to put their head outside the box through the bars or the window.

The male dromedary camels were tested in three different management systems: i) housed in single boxes for the whole day (H24; their usual and traditional method), ii) housed in the same box for 23 hours, adding 1 hour free in a paddock from 2 p.m. to 3 p.m. (H23) or iii) housed in the same box for 22 hours and 30 min with 1 hour of freedom again in the paddock from 2 p.m. to 3 p.m. and 30 minutes from 8.00 a.m. to 8.30 a.m. in boxes placed in a little pen adjacent to the female herd's pen (ExF). The paddock lies in front of the stable where the boxes are located and measures 250 square metres. Female herd's pen is bordered by a 130 cm-high wall dividing the two pens, but females were free to move and reach the males.

Each experimental condition lasted 7 days and was preceded by a habituation week, so the whole trial took six weeks (three weeks for the habituation period and three weeks of experimental situations) from February to March 2013, from the middle to the end of the breeding season, starting with the traditional husbandry form (H24) and ending with the exposure to the female rearing system (ExF).

The camels were fed with 5 kg oat hay at 9.00 a.m., and 3 kg concentrate supplement based on barley (60%), wheat bran (17.5%), olive cake (17.5%) and a mineral and vitamin complex (5%) at 3.00 p.m. The chemical composition of the oat hay was: Dry Matter (DM) = 90%, Crude Protein (CP) = 6.81%, Ash = 7.9%. Dry matter content of the concentrate was 90.9% and its chemical composition was CP = 11.4%; Acid-detergent fibre (ADF) = 13.2%; Neutral-detergent fibre (NDF) = 31.6% and Ash = 8.1%. The feeding quantity and quality remained constant during the experiment. The diet met the maintenance requirements as set by Laudadio et al. [START_REF] Laudadio | Nutritional composition of three fodder species browsed by camels (Camelus dromedarius) on arid area of Tunisia[END_REF], and water was available once every two days.

During the trial, the bulls were used for semen collection twice weekly. They were well accustomed to this practice and to the traditional husbandry system, so we changed only the management system in accordance with the experimental protocol.

Ethics Statement

The experiments were conducted according to the protocols approved by the Italian Ministry for Scientific Research in accordance with EC regulations. No special permission for behavioural research on wild animals such as this study is required in Italy.

Behavioural parameters

In each management system, the four males were filmed in their single box by a video-camera (Sony Camcorder digital video) from 8.00 to 8.30 a.m. every morning for 7 days in each experimental condition, without being disturbed by the operator. The videos were analysed by an expert ethologist, who filled out a focal animal sampling ethogram, defined as the sampling method whereby the recorder chooses one individual and records all behaviours performed by the individual in a specified time window (one bull in his single box, located far or adjacent to the females' paddock for 30 minutes) [START_REF] Altmann | Observational study of behaviour: sampling methods[END_REF].

The duration of the subsequent behavioural states was noted down: rumination, resting, standing, walking, looking outside and stereotypy. On the basis of the ethogram, the average time spent on these behavioural activities during the 30-min observation periods was calculated for each management system.

The videos were then studied again and after accurate analysis the presence of the stereotypical behaviours was identified and on the basis of their nature, were split into two categories:

Locomotor stereotypes. Head-shaking: the camel raised his head to the vertical with a very fast movement (this behaviour included a movement of the head by up to 90u). This stereotypy was considered as punctual behaviour because it lasted only about one second.

Pacing in a circle: the camel walked to the other side of his box (stopped and tried to look through a small window in the wall), and walked back to his initial position (in doing so, the camel always followed the same path which described a circle). The camel repeated this movement several times without any clear motivation: this stereotypy was considered as a state, because it always lasted more than 10 seconds.

Oral stereotypy. Self-biting or self-mutilation: the camel bit different parts of his own forelegs (right or left) from the shoulders to the feet. This stereotypical behaviour was considered as a stateindeed the camel could bite his legs for a variable length of time, ranging from just a few seconds to several minutes.

Bar-mouthing: licking, biting or playing with the lips on the bars of box's gate. This stereotypy was considered as punctual behaviour because it lasted only a few seconds.

Thus, a behavioural sampling ethogram, in which the observer notes all the durations and frequencies of a specific behaviour [START_REF] Altmann | Observational study of behaviour: sampling methods[END_REF]) was filled out. The duration of the following behavioural states were calculated: locomotor and oral stereotypical behaviour; the total duration of stereotypical behaviours was calculated as the sum of the duration of the two categories (locomotor+oral). The frequency of the following behavioural events (punctual behaviours) was also recorded: locomotor and oral stereotypical behaviour; the total frequency of stereotypical events was also calculated as the sum of locomotor+oral. Moreover, the frequencies of putting the head outside the box and of scratching were recorded, so as to measure how many times they were stimulated by the situation outside their box and how many times they scratched, which could be a sign of boredom in captivity.

Statistical Analyses

All behavioural parameters were subjected to repeated-measures analysis of variance using the Generalized Linear Model procedure (SAS, version 9, 1999). Independent variables were the management system (H24, H23 and ExF), the periodof observation (from Monday to Sunday), and the interaction between those variables. Data were normally distributed. Tukey's post hoc test was used to perform statistical multiple comparisons. The p-level was set at 0.05. All data were expressed as quadratic mean and mean standard error.

Results

The average time spent in behavioural activities during 30-min observation periods while in the single box in the three different management systems (H24, H23, ExF) is reported in Fig. 1.

Three of the four male camels showed stereotypical behaviours, each differing from the others', while one of the males showed two types of locomotor stereotypy (Table 1).

The effect of the management system was significant (df = 2; F (2,6) = 3.86; P = 0.02) on the frequency and the duration of the stereotypical behaviours, whereas no significant difference was observed in period (from the first to the seventh day of the week) (df = 6; F (2,6) = 0.99; P = 0.44) nor in the interaction between management system and period(df = 12; F (2,6) = 0.80; P = 0.64). Consequently, only the effect of the three different management systems on the behavioural parameters was considered.

The duration (in sec) of stereotypical behaviours recorded during the thirty-minute observation period every morning was highest in H24 and decreased progressively from H24 to ExF. Figure 2 shows that total stereotypical duration decreased during the weeks with one hour free in the paddock, and was significantly lower when camels were in the box adjacent to the female herd (H24 vs. ExF: 186.8649.9 vs. 0.164.9 s/30 min; P = 0.03). The number of times these behavioural patterns occurred followed the same trend. When the camels were housed in H24, there was a stereotypical behaviour frequency of 12.761.4 in 30 min, significantly higher than for camels in systems H23 (P = 0.002) and ExF (P,0.0001); in addition, the value for H23 was also significantly higher than for ExF (P = 0.038) (Fig. 3).

The frequency of camels putting their heads outside their box was higher in the third housing system, when the camels were stimulated by the female herd, compared with the other two management systems (P,0.001). By contrast, the frequency of scratching behaviour was very low when they were in the pen adjacent to the female herd 0.660.5, and was significantly lower than for those allowed to roam free in the paddock for one hour (H23 3.060.9; P = 0.02) or kept in a box (H24 3.360.5; P = 0.003) (Fig. 4).

Discussion

Three out of four male camels showed abnormal repeated behaviours corresponding to the general definition of stereotypical behaviours (i.e. repetitive, unvarying and apparently functionless behaviour patterns, [START_REF] Mason | Stereotypy: a critical review[END_REF]). As different kinds of stereotypical behaviour were observed in the three camels, a distinction was made between oral and locomotor stereotypies were distinguished as in other species [START_REF] Bashaw | A survey assessment of variables related to stereotypy in captive giraffe and okapi[END_REF][START_REF] Rushen | A decade-or-more's progress in understanding stereotypic behaviour Stereotypic Animal Behaviours[END_REF][START_REF] Mcgreevy | Equine behaviour: a guide for veterinarians and equine scientists[END_REF][START_REF] Dallaire | Stress and behaviour in domestic animals: Temperament as a predisposing factor to stereotypies[END_REF]. Mason et al. [START_REF] Mason | Why and how should we use enviromental enrichment to tackle stereotypic behaviour?[END_REF] assessed that animals could develop different repertoires of stereotypical behaviour and broke down stereotypy by taxon, to show that different orders of mammals typically favour different types of abnormal repetitive behaviour (locomotor, oral or non-locomotor body movements). This analysis revealed that stereotypical carnivora systematically prefer locomotor movements, while ungulates display oral forms. Accordingly with the data reported for ungulates, these camels developed both oral and locomotor stereotypies, showing a preference for oral ones. Indeed, two of the four camels performed different oral stereotypies: self-biting and bar-mouthing; while one of the four males performed two different locomotor stereotypies: head-shaking and pacing in a circle. The camel which exhibited head-shaking also exhibited circling, so this individual developed two different kinds of locomotor stereotypy. This finding agrees with previous observations in horses [START_REF] Mills | Factors associated with the prevalence of stereotypic behaviour amongst Thoroughbred horses passing through auctioneer sales[END_REF]: a horse already showing one locomotor stereotypy is more likely to develop a second than horses either performing oral stereotypy or expressing no form of stereotypy.

The camels in this experiment spent about 10% of the observation period stereotyping in H24; the range reported for cows is from 1% to 38% of a 24-h period before, during and after grazing [START_REF] Redbo | Changes in duration and frequency of stereotypies and their adjoining behaviours in heifers, before, during and after the grazing period[END_REF], but in another study, where animals were reared in better conditions, this figure dropped to 1-2% [START_REF] Bolinger | The effects of restraint using self locking stanchions on relation to behavior, feed intake, physiological parameters, health, and milk yield[END_REF]. A horse housed in a single box can spend up to 8 h crib-biting each day [START_REF] Mcgreevy | Prevention of crib-biting: a review[END_REF], whereas one female captive giraffe could spend more than 40% of the night licking and tongue-playing [START_REF] Baxter | The effect of increasing dietary fibre on feeding, rumination and oral stereotypies in captive giraffe (Giraffa camelopardalis)[END_REF]. The latter stereotypical behaviours were related to poor management, i.e. diets with low fibre, thus confirming the effect of husbandry on the prevalence of stereotypy.

It could therefore be supposed that the traditional housing system (H24) in which camels showed the greatest incidence of stereotypies was a sub-optimal management system for this species, in agreement with studies carried out by Mills in horses [START_REF] Mills | Repetitive movement problems in the horse[END_REF], and that the presence of stereotypical behaviour in these individuals was a sign of poor welfare, as inferred by Mason & Latham [START_REF] Mason | Can't stop, won't stop: is stereotypy a reliable animal welfare indicator?[END_REF] who suggested that stereotypy could be a sign of suffering. Thus, in H24 the camels were probably frustrated, lacking stimulation and control of their environment and could not therefore exhibit natural behaviours (e.g. it would be impossible for them to perform any social interaction), because this housing system did not satisfy the behavioural needs of this species.

These four stereotypies have already been reported in other species and different explanations have been suggested concerning the cause of these abnormal behaviours [START_REF] Cooper | Stereotypic Behaviour in the Stabled Horse: Causes, Effects and Prevention without Compromising Horse Welfare In: The Welfare of Horses, Animal Welfare[END_REF][START_REF] Nicol | Understanding equine stereotypies[END_REF][START_REF] Bashaw | A survey assessment of variables related to stereotypy in captive giraffe and okapi[END_REF][START_REF] Mcgreevy | The prevalence of abnormal behaviours in dressage, eventing and endurance horses in relation to stabling[END_REF]. The three major constraints were limited space, lack of stimulation (especially social contact) and controlled feeding. The development of stereotypy in these camels could also be explained by one of these three constraints or by their cumulative effect.

One of the four camels exhibited an unusual kind of oral stereotypy, i.e. bar-mouthing, which consisted in biting, licking or playing with the lips on the bars of the cage. The development of this stereotypy in a camel housed in a box was not surprising, indeed, these kinds of abnormal oral behaviours have also been reported in other captive animals (bank voles, [START_REF] Garner | Evidence for a relationship between cage stereotypies and behavioural disinhibition in laboratory rodents[END_REF]; pigs, [START_REF] Lawrence | A review of behavioural factors involved in the development and continued performance of stereotypic behaviours in pigs[END_REF]). Rebdo [START_REF] Redbo | Changes in duration and frequency of stereotypies and their adjoining behaviours in heifers, before, during and after the grazing period[END_REF] suggested that feeding frustration could facilitate oral stereotypy; in our study, the camels were fed with 5 kg of oat hay and 3 kg of concentrate and did not have the opportunity to forage on pasture. Therefore, these camels may have felt feeding frustration, which could explain why one individual had developed this kind of stereotypy. This hypothesis agrees with previous studies in gilts where the time spent performing oral stereotypies (e.g. bar-chewing) was negatively correlated with their feed allowance (review by Lawrence & Terlouw, [START_REF] Lawrence | A review of behavioural factors involved in the development and continued performance of stereotypic behaviours in pigs[END_REF]). Nicol [START_REF] Nicol | Understanding equine stereotypies[END_REF] also suggested that low-forage diets could be the main cause of the onset of oral stereotypy in horses. Moreover, Rebdo [START_REF] Redbo | Changes in duration and frequency of stereotypies and their adjoining behaviours in heifers, before, during and after the grazing period[END_REF] has shown that heifers exhibited no abnormal behaviours when at pasture. Camels are also herbivorous animals and in natural conditions usually graze for 8-12 hours per day [START_REF] Gauthier-Pilters | The camel. Its evolution, ecology, behaviour, and relationship to man[END_REF]; therefore, as has been proposed for other species, the lack of pasture may have been the trigger for this oral stereotypy.

Self-biting or self-mutilation was performed by one of the four camels during our observations. In captive-reared rhesus monkeys, the absence of physical contact with conspecifics negatively affected their behaviour and the prevalence of self-biting was positively correlated to the number of years spent in a single stall [START_REF] Lutz | Stereotypic and self-injurious behaviour in rhesus macaques: A survey and retrospective analysis of environment and early experience[END_REF]. Camels are social animals and, while old males can occasionally be solitary, camels usually live in herds made up of males, females and young, or females and young without a male, or males and females without young or only one male, with an average of 25 individuals per herd [START_REF] Gauthier-Pilters | The camel. Its evolution, ecology, behaviour, and relationship to man[END_REF]. Therefore, it could be supposed that social deprivation in this species may lead to the development of self-biting. McDonnell [START_REF] Mcdonnell | Practical review of self-mutilation in horses[END_REF] suggested that social and/or feeding distraction could reduce the prevalence of selfmutilation in horses.

Pacing has been reported in a wide range of captive animals (cats, dogs, hens and horses, review by Dallaire, [START_REF] Dallaire | Stress and behaviour in domestic animals: Temperament as a predisposing factor to stereotypies[END_REF]; okapi and Table 1. Description of the stereotypy shown by each camel while in their single box.

Camel Stereotypy

808

''Bar-mouthing'': Licking, biting or playing with the lips on the bars 514 None 515 ''Self-biting'': the camel bit different parts of his own forelegs (right or left) from the shoulders to the feet.

504

''Head-shaking'': the camel raised his head to the vertical with a very fast movement (this behaviour included a movement of the head up to 90u). ''Pacing in a circle'': The camel walked to the other side of his box (and sometimes stopped and looked through the window), and walked again until he was back in his initial position (in doing so, the camel always followed the same path which described a circle) doi:10.1371/journal.pone.0089093.t001 giraffe, [START_REF] Bashaw | A survey assessment of variables related to stereotypy in captive giraffe and okapi[END_REF]; red deer stags, [START_REF] Pollard | The effects of pen size on the behaviour of farmed red deer stags confined in yards[END_REF]; arctic fox, polar bear, American mink, and lion, [START_REF] Clubb | Animal welfare: captivity effects on wide-ranging carnivores[END_REF]; bears, [START_REF] Vickery | Stereotypic behaviour in Asiatic black and Malayan sun bears[END_REF]) and has been related to confinement-specific stressors [START_REF] Morgan | Sources of stress in captivity[END_REF]. In natural conditions, camels usually walk a lot during the day, grazing 8-12 hours daily and walking at an average speed of 2 km/h, but if necessary, they can walk 150 km per day in the desert [START_REF] Gauthier-Pilters | The camel. Its evolution, ecology, behaviour, and relationship to man[END_REF]. In our study, the camels were housed in single boxes, so it is to be presumed that this area was unable to fulfil the camels' needs to walk as much during the day as they would do under natural conditions, which is probably why one camel developed this locomotor stereotypy. This hypothesis is in agreement with different studies showing a negative correlation between enclosure size and the prevalence of pacing in different species (red deer, [START_REF] Pollard | The effects of pen size on the behaviour of farmed red deer stags confined in yards[END_REF]; giraffe and okapi, [START_REF] Bashaw | A survey assessment of variables related to stereotypy in captive giraffe and okapi[END_REF], monkeys, [START_REF] Mason | Stereotypy: a critical review[END_REF], carnivores, [START_REF] Clubb | Animal welfare: captivity effects on wide-ranging carnivores[END_REF]). Moreover, Nicol [START_REF] Nicol | Understanding equine stereotypies[END_REF] suggested that locomotor stereotypy in horses may derive from some frustrated attempt to move or escape from the stable, and Lawrence & Terlouw [START_REF] Lawrence | A review of behavioural factors involved in the development and continued performance of stereotypic behaviours in pigs[END_REF] supposed that the development of pacing could be based on escape behaviour. Therefore, as has been proposed for other species, providing camels with a bigger enclosure would help improve their welfare. However, Morgan and Tromborg [START_REF] Morgan | Sources of stress in captivity[END_REF] concluded that increasing the space available to the animal did not always have a positive effect on its welfare, particularly for prey animals, because it may well be that it is not the quantity of space available to the animal which is important but rather its quality, and what it gives the animals in the way of behavioural opportunity. Head-shaking was observed in one individual. This or similar forms of stereotypical behaviour (head-tossing, bobbing, nodding or shaking) has been reported in a broad range of species (in horses, [START_REF] Cook | Bit-induced pain: a cause of fear, flight, fight and facial neuralgia in the horse[END_REF]; okapi and giraffe, [START_REF] Bashaw | A survey assessment of variables related to stereotypy in captive giraffe and okapi[END_REF]; bears, [START_REF] Vickery | Stereotypic behaviour in Asiatic black and Malayan sun bears[END_REF]; humans, [START_REF] Singer | Motor stereotypies[END_REF]; rats, [START_REF] Holmgren | Spontaneous and amphetamine induced head-shaking in infant rats[END_REF], cats, [START_REF] Dallaire | Stress and behaviour in domestic animals: Temperament as a predisposing factor to stereotypies[END_REF]; elephant, [START_REF] Friend | Behaviour of picketed circus elephants[END_REF]). The causes of such stereotypical behaviours have been poorly investigated. According to Crowell-Davis [START_REF] Crowell-Davis | Understanding Behavior: Head Shaking[END_REF], head-shaking can have a great variety of causes. As for the other stereotypies, we can suppose that the lack of stimulus, space and social contact may have led to the development of this stereotypy. Cooper et al. [START_REF] Cooper | The effect of increasing visual horizons on stereotypic weaving: implications for the social housing of stabled horses[END_REF] found that increasing the visual horizon significantly decreased the prevalence of head-shaking in horses housed in single boxes (i.e. increasing visual contact between neighbours and towards the environment allows horses to monitor the environment and to interact with other horses).

Our study showed the impact of the different kinds of management on the duration and frequency of stereotypy, with more frequent stereotypical behaviour among camels kept in a box for 24 h than among those allowed 1 hour free. Therefore, we could suggest that allowing camels to walk for 1 hour daily would be a good way of improving their living conditions, rather than keeping them in a single box around the clock. Time spent stereotyping also tended to decrease between camels in groups H24 and H23, which could suggest that 1 hour free has a positive impact (or at least not a negative one) on camel welfare but it would seem to be insufficient. The duration of locomotor stereotypy decreased between H24 and ExF and this could be explained by the fact that these animals had 30 minutes more to spend in an area adjacent to the female herd where the dams could walk around as much as they wanted, stimulating the bulls.

Overall, the frequency and duration of stereotypical behaviours were higher in H24 and H23 than ExF. This suggests that exposure to females in the pen could be a better environment for male camels because it more closely matches the needs of this species (i.e. more time to walk around and more chance for social contact).

The frequency of oral stereotypy (bar-mouthing and self-biting) decreased from H24 and H23 to ExF. This is not surprising and is in accordance with our hypothesis as well as with that of McDonnell [START_REF] Mcdonnell | Practical review of self-mutilation in horses[END_REF]: self-injuries decreased among stallions when they were placed in pasture with mares because it provided plenty of distraction and allowed the animals to perform social behaviour. In our study, camels were not placed directly with females but they could interact with and touch them (with an average of 35.9 touching events in 30 minutes), if females came near the wall and put their neck and head in the male's area, allowing contacts. Consequently, in ExF, the males put their heads outside the box (through the window or between the bars of the gate) more often than in the other groups, showing that they were monitoring their environment more when the females were close by. This could be explained by the fact that during ExF, they had a larger area of view (a wider horizon) of a more interesting environment around them than in their box, where they could only look at an empty space (poor of stimuli). Scratching was also influenced by the management system; indeed, its frequency was higher in H24 and H23 compared with ExF. According to Maestripieri et al. [START_REF] Maestripieri | A modest proposal: Displacement activities as an indicator of emotions in primates[END_REF] scratching could be a sign of stress, frustration or anxiety.

Moreover, Basset et al. [START_REF] Bassett | Effects of Training on Stress-Related Behavior of the Common Marmoset (Callithrix jacchus) in Relation to Coping With Routine Husbandry Procedures[END_REF] measured the frequency of selfscratching as an indicator of stress in the common marmoset (Callithrix jacchus) and considered an increase in this behaviour as a sign of reduced welfare. Thus, in this study, the lower frequency of scratching in ExF compared to boxed conditions could be interpreted as an improvement in their welfare needs, i.e. exposure to females could be a good way of providing male camels with stimulation and the opportunity to perform social and sexual behaviours instead of stereotypical ones.

On the basis of our preliminary findings, the traditional husbandry system of male dromedary camels reared under intensive management systems should be changed, by integrating it with at least one hour thirty minutes daily of walking around in paddocks, spending more time feeding (decreasing the concentrate/forage ratio), opening a window between the boxes which would allow the camels to have visual contact with their neighbours and spend some time near females.

Conclusion

Male dromedary camels may develop abnormal behaviour, just as other animals do, if they live in sub-optimal conditions, and this trial was the first step in identifying locomotor and oral stereotypies in male dromedary camels housed in single boxes. Overall, this preliminary study suggests that the traditional husbandry method could be improved by allowing free movement and social contact, both of which had positive impacts on the incidence of stereotypy. Further studies are needed to identify the behavioural needs of camels reared under intensive management systems and to optimize dromedary camel welfare and breeding techniques.

Figure 1 .

 1 Figure 1. Average percentages of time spent in behavioural activities during 30 min observation periods of camels while in their single box in the three different management system: housed in single box for 24 hours (H24), housed in single box for 23 hours and one hour in paddock (H23), housed in single box for 22 hours and 30 minutes, one hour in paddock and 30 minutes of female exposition (ExF). doi:10.1371/journal.pone.0089093.g001

Figure 2 .

 2 Figure 2. Effect of three different management systems (housed in single box for 24 hours (H24), housed in single box for 23 hours and one hour in paddock (H23), housed in single box for 22 hours and 30 minutes, one hour in paddock and 30 minutes of female exposition (ExF) on the duration (s/30 min) of stereotypical behaviour shown by male dromedary camels while in their single box. Oral stereotypies: self-biting or self-mutilation and bar-mouthing; Locomotor stereotypies: pacing in a circle; Stereotypical behaviour: sum of oral and locomotor stereotypies. doi:10.1371/journal.pone.0089093.g002

Figure 3 .

 3 Figure 3. Effect of three different management systems (housed in single box for 24 hours (H24), housed in single box for 23 hours and one hour in paddock (H23), housed in single box for 22 hours and 30 minutes, one hour in paddock and 30 minutes of female exposition (ExF) on the frequency (n/30 min) of stereotypical behaviour shown by male dromedary camels while in their single box. Oral stereotypies: self-biting or self-mutilation and bar-mouthing; Locomotor stereotypies: head-shaking and pacing in a circle; Stereotypical behaviour: sum of oral and locomotor stereotypies. doi:10.1371/journal.pone.0089093.g003

Figure 4 .

 4 Figure 4. Effect of three different management systems (housed in single box for 24 hours (H24), housed in single box for 23 hours and one hour in paddock (H23), housed in single box for 22 hours and 30 minutes, one hour in paddock and 30 minutes of female exposition (ExF) on the frequency (n/30 min) of scratching and putting the head outside the box shown by male dromedary camels while in their single box. doi:10.1371/journal.pone.0089093.g004

PLOS ONE | www.plosone.org

February 2014 | Volume 9 | Issue 2 | e89093

Acknowledgments

The authors are grateful to the Tunisian authorities and thank all of the technical staff at the Laboratory of Livestock and Wildlife for their valuable help during this study. The authors would also like to thank Dr. Julian Skidmore and Dr. Anthony Green for kindly suggesting stylistic improvements to the text and CIRAD (Centre de Coope ´ration Internationale en Recherche Agronomique pour le De ´veloppement) for coordination and administrative support.

This document has been produced with the financial assistance of the European Union through the ''PROCAMED'' Project: Promotion des syste `mes camelins innovants et des filie `res locales pour une gestion durable des I.B/1.1/493). collection and analysis, decision to publish, or preparation of the manuscript.

Author Contributions


A El

M Mesnaoui
email: mesnaoui@ucam.ac.ma

M Maazaz

J. -J Videau

PHYSICAL PROPERTIES AND CHEMICAL DURABILITY OF ZINC-LEAD-IRON PHOSPHATE GLASSES

Keywords: Phosphate glasses, Physical properties, Chemical durability, Toxic heavy metal

This study provides experimental data on the physical properties and the chemical durability of zinc lead-iron phosphate glasses with the general composition (100-x) (50P 2 O 5 -15ZnO-35PbO)-x.Fe 2 O 3 . The physical properties show that the addition of Fe 2 O 3 results in a high reticulation of the glass network in the composition range O � x � 10. As a consequence, a drastic improvement of the chemical durability was noticed and the dissolution mechanism was found to be controlled by a hydrolysis reaction with a non congruent dissolution.

Introduction

Phosphate glasses typically have a poor chemical durability in aqueous solutions, so glass technologists have often avoided their use in many applications. However recent studies show that it is possible to increase the chemical durability of phosphate glasses by replacing oxygen with nitrogen partially [1,2]. Another approach to improve the chemical durability is to add various oxides such as A}iO 3 , In 2 O 3 , Sc 2 O 3 [3,4]. Other studies showed that the addition of F e 2 O 3 to phosphate glasses drastically increases the resistance to the corrosion in aqueous solutions and suggested the use of these glasses as storage media for nuclear wastes [5,6].

Nowadays, pollution with heavy metal becomes a crucial problem for human beings. Lead and other heavy elements are considered to have cumulative taxie effects. In order to develop a matrix able to immobilise these elements, the system P 2 O 5 -Znü-PbO was recently studied for the case oflead [7]. In this paper, we studied the effect of the addition of Fe 2 Ü 3 to the 50P 2 Os-15ZnO-35PbO glass matrix. The relationship between the chemical durability of these glasses and their physical properties is discussed as well.

Experimental

Glasses with the composition of (100-x) (50P 2 O 5 -15ZnO-35PbO)-x.Fe 2 O 3 have been prepared as described previously [8] and the kenetic study have been realized according to the protocol adopted earlier [9].

Results

-Physical properties

Figure 1 shows the effect of the addition of Fe 2 O 3 on the density (p) and the molar volume (VM) of the (100-x) [50P 2 O 5 -15ZnO-35PbO] x.Fe 2 O3 glasses. The first increases from 4.16 (x = 0) to 4.28 g/cm 3 (x = 10), while the second decreases from 38.7 (x = 0) to 37 cm 3 /mol (x = 10) with increasing the iron concentration. The density increase may result from a strengthening of the chemical bonds within the glass network [10]. However, the molar volume decrease can be seen as a result of the glass network compactness. The glass transition temperature and Vickers microhardness are shown in figure 2. The initial addition ofFe 2 O 3 oxide causes a slight decrease of the Tg values to reach a minimum at x. = 2 (350 °C). This behaviour could originate from a slight decondensation of the metaphosphate glass network. For x > 4, the increase of the Tg values reflects somewhat the pronounced reticulation character of the glass network. AB for the Hv values, we observed a steady increase as x increases which can be assi gn ed to the strengthening of the glass network.

-Chemical durability

• Figure 3 shows the evolution of the initial dissolution rate (DR) as a function of the Fe 2 O 3 content. The glasses, with x values of 8 and 10, exhibit a very high resistance to corrosion in aqueous solution at 50 °C, consequently no data points can be reported for these glasses.

This result confirms the previous reports on the significant improvement of the chemical durability of iron phosphate glas ses in aqueous media [ 5,6]. For example, the dissolution rate of iron free glass Our data show that the dissolution rate decreases with the increase of the 0/P ( or increasing M+Fe/P) ratios, which is in good agreement with the results reported by Reis et al [10]. The later showed that the DR is higher when the 0/P ratio is close to 3 (metaphosphate case), and it is smaller when this ratio is close to 3.5 ( diphosphate case). In our case, the chemical durability of the glasses with x = 10 is comparable to that of window glass, soda lime and borosilicate glasses [10].

In the context of this study, we selected the glasses (x = 6 and 10) for a leaching test at 90 °C. The analysis of the leachant samples shows the presence of all cations constituting these glasses.

The corresponding weight variations of these cations in form of oxides and norrnalised to the glass area (mg cm-2 ) with the pH variation versus time (hours) are represented in fi gur e 4.

For x=6 (Fig. 4), the dissolution curve is not linear and the glass weight loss is moderate for t<50h. Between 50 and 300h, the glass dissolution is enhanced. After 300h, the corrosion curve indicates a slowdown. These characteristics were previously observed for zinc-alurninophosphate glasses containing lead and cadmium, and for alkali phosphomolybdate glasses [9,11]. For x = 10, the released quantities of the constituents are considerably reduced and the profile of the dissolution is linear. The pH for x = 6 exhibits an abrupt fall to reach a value close to 3, and increases slightly (0.2-0.3) when the dissolution was slowed down. For x = 10 the pH decreases progressively to reach a value close to 5 at the end of the leaching test. In both cases (x = 6, 10), we observe a limited quantity of Pb in the leaching solutions compared to other cations.

Figure 5 shows the SEM pictures of corroded glass surface of two samples. The micrograph (a) depict a rough and cracked surface for the sample x = 6. However, the glass surface (micrograph b) did not show much changes for the sample x = 10. The microanalysis of glass surfaces was carried out and the data are reported in table 1 in form of oxides together with the O/P ratio values corresponding to the analysed pristine glass and layer surface compositions. No significant change in the composition and the O/P ratio value are noted for x = 10 glass, which is in agreement with the SEM views. On the contrary, the layer composition of x = 6 glass is impoverished in phosphorus and zinc but richer in lead and iron than the pristine glass composition. The corresponding O/P ratio close to 3. 7 shows that these layer has a composition between diphosphate and monophosphate.

Nominal compositions mol. %) of glasses Analysed surface compositions (mol%)

Discussion

In the case of x = 6, the corrosion mechanism can be described by considering 3 stages. For this glass, the kenitic curves show three slope changes and may be correlated to the pH variation during the leaching process. In the 0<t<50h range, there is a transitional period associated with the pH decrease. After stabilization, the pH reaches the region of strong acidity (pH=3) and seems to be responsible to the exaltation of the glass corrosion in the 50h<t<300h period. Beyond 300h, there is a saturation of the leachant which is due to the formation of a protective layer which may prevent the diffusion through the surface of the glass. This layer, mainly formed by a diphosphate, is likely resulting either from the precipitation of insoluble phosphate species onto the glass surface induced by the ionic strength increase of the leachant [12], or from or the non-dissolution of short phosphate chains that are linked through more covalent bonds. For the glass x = 10, the evolution observed for 480 h of corrosion may be assigned to a transitional period since the pH is not yet stabilized. The duration increase of this domain (transitional period) can be interpreted by a better glass durability.

In general, the dissolution of phosphate glasses can be governed by a hydration process as descirebd by Buncker et al [13] and Delahaye et al [12,14] for alkaline and alkalino-earthy phosphates. This mechanism leads to a disruption of the modifier cations ionic bonds cross-linking the phosphate chains and hence are released into solutions. In our case, and because of the glass network reinforcement, the non congruent dissolution and the fast pH decrease lead us to assume that the corrosion of theses glasses was governed by hydrolysis reactions. In this case of hydrolysis, the pH lirnit reaches to the lower value (pH close to 3 for x = 6 at 90 °C). This can be interpreted as the acido-basic reaction in relation to the acidic sites of the middle chain polyphosphate groups for which the pKa value is between 0-3 [START_REF] Van Wazer | Phosphorus and its compounds[END_REF]. In this pKa range, the medium acidity of the leachant is mainly controlled by the presence ofH 2 PO 4 " anions.

Conclusion

Zinc-iron phosphate glasses have been investigated. The increase of the Fe 2 O3 content induces an increase of the glass transition temperature, density and Vickers microhardness; however, the molar volume decreases. The trend of theses physical properties shows a high reticulation of the glass network. These results are in good agreement with the improvement of the chemical durability especially for the glass with the ratio O/P = 3.27 (x = 10) which is comparable to window glasses (8.10-8 -5.10-9 g cm• 2 min" 1 ).

Because the dissolution rate of Pb is only 5 10-10 g cm• 2 min• 1 , this glass can be considered as a good candidate for storing this toxic heavy metal.

Figure 1 :Figure 2 :

 12 Figure 1 : Density(p) and molar volume (V M) versus x. Line was drawn as a guide for eye.460 560 440

Figure 3 :

 3 Figure 3 : Dissolution rate at 50 °C and 90 °C versus x. Lines are drawn as a guide for the eye.

Figure 4 :

 4 Figure 4 : Variations of weight losses of glasses and weight of glass constituents in leachante and pH versus time at 90 °C for glass x = 6 and x = 10.

Figure 5 :

 5 Figure 5 : SEM micrographs of the corroded glass surface.

Table 1 :

 1

	X 6	P 2 O s 48,4	PbO 33,6	ZnO 12,6	Fe 2 O3 P 2 O s 5,44 38,2	PbO 40.8	ZnO 7	Fe2Ü3 O/Pin O/Pac i 14 3,14 3,68
	10	46,7	31,5	12,5	9,3	44,1	32,9	12,2	10,8	3,26	3,37

Comparison of compositions (in form of oxides) and O/P ratio of pristine glasses ( analysed composition, O/Pini) and glass surface layers (O/Pac) obtained from SEM study at the end ofleaching test.

Acknowledgements

The authors are grateful for :financial support from French-Moroccan committee (through contract N ° : 99/187/SM) and PROTARS III (D 14/69). Gratitude is also extended to A. Outzoughit for his help in SEM Analysis.


Abdelouahab El Hadrami

Mohamed Mesnaoui
email: mesnaoui@ucam.ac.ma

Mohamed Maazaz

Christine Labrugère

Alain Wattiaux

Jean-Jacques Videau

A Wattiaux2

J-J Videau

Structure of zinc-lead-iron phosphate glasses

Keywords: Phosphate glasses, Raman spectroscopy, XPS, Môssbauer spectroscopy, EPR

Introduction

In our previous study [l], we proposed zinc iron phosphate glasses as a good storage medium for toxic heavy metals because of their high stability under extreme leaching conditions. The composition dependence of density, glass transition temperature, Vickers microhardness and chemical durability were fully described. The addition of Fe 2 O 3 to 50 P 2 O 5 -15 ZnO -35 PbO glass have shown a significant variation in physical properties and in chemical durability. T o undersand the se variations a structural approch seems to be interesting.

Raman scattering has been successfully utilised to give structural informations of glasses [2][3]. Their structure is dependent upon the covalente bonding between PO4 tetrahedra forming a chains or rings via bridging oxygen. The chains are linked together by cross bonding between modifier cations and non bridging oxygens. The chemical bonding change was studied through X-ray photoelectron spectroscopy (XPS) by fitting O 1 s spectra and thus quantitatively measure percentage of bridging and non bridging oxygen [ 4,5].

In order to get more information about iron valence and coordination in glass other investigations [6,7] have been carried out using Mossbauer spectroscopy. Furthermore, the electronic paramagnetic resonance (EPR) has been used to give some structural informations. In fact, iron coordination has been provided using this technique [8].

Experimental procedures

Raman spectra oflead-zinc-iron phosphate glasses were measured in the stokes spectral range from 200 to 1400 cm-1 at room temperature. The samples were eut and polished to form parallel faces. The 514.5 nm wavelength of an argon-ion laser source was used as an excitation wavelength. The spectra were recorded using a double grating monochrometer Jobin-Yvon UlOOO driven by a microcomputer. The slit widths were set to obtain a resolution of2.5 cm-1 • XPS data were collected using a VG Scientific 220i-XL Escalab. Mg Ka X-ray line :from the non monochromatized source (1253.6 eV ) was used to acquire Ols small areas at a constant pass energy of 6 eV. The Ols spectra of the samples were carefully examined taking into account the contribution of C--0 and C--C bonds resulting a surface pollution. The Binding energies were referenced to the Cls energy at 284.6 eV. All XPS peaks were simulated based on a Gaussian-Laurentzian hybrid peak fitting using a VG Scienti:fic software. The half-width of the Ols fitting components was :fixed at 1.6-1.8 range for the various compositions of glas ses. T o prevent a long term surface contamination, the XPS analyses were performed on pieces that were immediately placed under vacuum after being tak:en apart from freshly broken glasses.

Môssbauer spectra were measured at room temperature using a Halder spectrometer with a constant acceleration and 57 Co source (rhodium matrix). The Samples were prepared from glass powders mixed with a resin in order to get 10 mg/cm 2 of iron. The Môssbauer spectra were simulated with broadened paramagnetic lorentzian doublets.

The EPR spectra were measured at room temperature with a Bruker EMX 8/12 spectrometer operating at an X band of 9 .45 GHz and magnetic field ranging from O to 10000 G.

Results and interpretations -Raman Spectroscopy

It is known that the structure of phosphate glasses is based on PO 4 tetraedra that are linked to each other by bridging oxygen atoms to form phosphate chains or rings [9]. The metal cations are cross linked to the non bridging oxygen atoms of each PO4 tetrahedra. These bonds are essential the linkage between the phosphate chains. The Raman Spectra of (100-x) (50P 2 Os-15ZnO-35PbO)-xFe 2 O 3 glassy samples recorded between 200 and 1400 cm-1 are reported in the figure 1. We noticed that the spectrum of the lead-zinc metaphosphate is very common for metal metaphosphate glasses [10]. Two characteristic peaks were observed at about 700 cm-1 and 1160 cm-1. These two peaks were attributed to the syrnmetric stretching mode of the P-0--P bridging bonds and to the symmetric stretching (PO 2 )" of the PO 4 tetraedra, respectively, by analogy with other studies [2][3][4][5][6][7][8][9][10][11]. The band observed at 1240 cm-1 has been assigned to the asymmetric stretching of (PO 2 r. The low frequency peak at about 340 cm-1 originates from the bend mode of the phosphate tetrahedra. The very weak shoulder observed at about 1070 cm-1 is due to the motion of the non bridging oxygen of PO 3 end groups [12]. The addition ofFe 2 O 3 dramatically affects the trend of the Raman spectra. W e clearly noticed that the intensity of the band observed at about 700 cm• 1 decrease when x increases, and its frequency shifts from 700 cm• 1 to 730 cm• 1 as shown in figure 2. These evolutions could result from the new interaction of divalent (Zn 2 +, Pb 2 J and trivalent (Fe 3 y cations with bridging oxygen atoms, which is likely going to affect the orientation and strength of the P-0--P bonds. The dominant band frequency shifts from 1160 cm• 1 to 1106 cm• 1 (figure 2). This decrease is attributed to the shortening of the length of the phosphate chains. Moreover, the appearance of a shoulder at 930 cm• 1 for the higher Fe 2 O 3 content can be attributed to the (PO 4 ) 3 -isolated monomers, whereas the shoulders at 980 and 1070 cm• 1 can be attributed to (P0 3 f end units in the polyphosphate entities such as [(P20 1 ) 4 -] diphosphate, [(P 3 0 10 ) 5 " ] triphosphate or [(P 4 0 13 ) 6 -] tetraphosphate groups in agreement with the literature [12][13]. The frequency of the band located at 1070 cm• 1 decrease as x increases and its intensity increases especially for x � 6. In low frequency range, the new bands can be assigned to the cation motion and chain conformations [ 14].

V POP(cm-1 )

�- ---------------, -X-ray photoelectron spectroscopy (XPS)

It is known that bridging and non-bridging oxygen atoms in phosphate glasses can be distinguished by fitting the Ols peak from XPS data. Figure 3 presents the XPS O 1 s spectra of the glasses as a function of x. As reported elsewhere [13,15], the two components at smaller and higher energies are assigned to the non-bridging oxygen atoms (NBO) and bridging oxygen atoms (BO), respectively. The intermediate component (ABO) can be attributed to the oxygen atoms that are linked to phosphoras and to divalent or trivalent cations forming a more covalent bond than that formed with ordinary modifier cations. These assignments are in good agreement with the results reported by Liu et al. [ 16].

The evolution of the three components area fractions are shown in figure 4. We mainly noticed that the fractions of BO and NBO decrease while that of ABO increases with x increase. �50 "' . -=, ,�-------'---�-�---'---c'--------='

Blnd.n9En•91{•V)
'"'' .

B1ndln9Enw9y(•V) 532 Bindln11Ener,y{aV)
o < 0 .4 7 and 1 < 8 < 1.3 correspond respectively to the presence of Fe 3+ and Fe 2+ cations in octahedral sites [6].

Velocity (mms-1 )

x = lO Velocity (mms-1 )

x = 8

Velocity (mms-1 ) The hyperfine parameters and the fractions of each ion are listed in table 1. Even though the iron is entirely in form of Fe 2 0 3 in the starting compositions, the glasses clearly contain Fe 2+ cations for which the content decreases with x. The ferrous ions origin is the reduction of Fe 3+ ions by NH 3 coming from CNH4) 2 HP0 4 decomposition in the batch. In the similar preparation procedure the content of reduced Fe 3+ cations depend in the iron concentration, consequently the NH 3 concentration, in the melt and may explain a high Fe 2 + content for the higher CNH.1) 2 HP04 percentage. This beh. aviour is in agreement with previous works [7]. The origin of ferrous ions can be also, attributed to the 0 2 -activity in the melt which may affect the Fe 3+ /Fe 2+ redox couple equilibrium. Indeed, the increase of Fe 2 0 3 content in the glass results in an increase of the oxygen activity, which may explain the large concentration ofFe 3 + cations [17]. -Electronic paramagnetic resonance (EPR) Figure 6 shows the evolution of the EPR spectra with the concentration of iron. The spectra are mainly characterised by two lines centred around effective values of g = 4.3 and g = 2. Since the Fe 2+ cation is known to be sensitive to EPR spectra only at low temperatures, the features of these spectra, obtained at room temperature, are related only to the trivalent iron. So the resonance at g = 4.3 and g = 2 are respectively assigned to Fe 3+ cations in low and higher symmetry as suggested by Kukjian and Sigety [8]. The existence of some distorted FeO 4 tetrahedra could explain

x = 6
x = lO x = 8 x = 6 x = 4 x = 2
2000 the resonance at g = 4.3. This hypothesis seems realistic and would correspond to the introduction of small quantities of isolated Fe 3+ cations into the phosphate chains. The ferric tetrahedra would share corners with phosphorus tetrahedra. These FeO 4 polyhedra would disappear when iron content increases in agreement with the vanishing of g = 4.3 line. For

x > 6, the width of g = 2 line increases. This phenomenon is due to the appearance of magnetically exchange coupled pairs of Fe 3+ cations. For higher iron content (x = 10) the widening could be related to the short range magnetic interactions as reported by Menil et al [7].

Structure-properties correlation

Due to the fact that the Raman spectrum for the iron-free glass (x = 0) shows the presence of PO 3 end groups, we logically conclude that there is a shortening of the zinc-lead metaphosphate chains. This tendency is in good agreement with other studies [12,18], in which the 31 P NMR analysis indicates the presence of Q 1 sites when ZnO is substituted for PbO in metaphosphate glasses [18]. For x = 0 glass, the 31 P NMR study reveals the presence of small quantities of Q 3 in addition to Q 2 and Q 1 that can be generated from the disproportionation reaction Q 2 ➔ Q 1 +Q 3 . This tendency was confirmed by our XPS study in which the percentage of BO is slightly smaller than the theoretical value (33.33%) in metaphophate glasses.

The addition of F e 2 O 3 in the zinc-lead metaphosphate glass results in a shortening of the average phosphate chain length. The later can approximately reach 4 phosphorus for x = 10 (experimental O/P ratio equal to 3.27). The general effect of the Fe 2 O 3 addition can be summarized as follows: 1) A shift of the symmetric PO 2 band towards lower frequencies, and the decrease in the intensity of the P-0--P band along with the appearance of new bands corresponding to the change of the chain ends and existence of monomer units detected the Raman technique.

2) A decrease of the BO fraction and an increase of ABO fraction of O 1 s spectra. The role iron atoms can be interpreted in different ways according to its oxidation state. The Fe 2+ cations could be considered as a network modifier in octahedral environ.ment. Its relatively high percentage for a low Fe 2 O 3 content does not clearly affect the physical properties expect for the glass transition Tg which reaches a minimum for x close to 2 [1 ].

According to the Mossbauer results, the Fe 3+ cations are mainly in octahedral coordination while the EPR spectra can be interpreted in term of the coexistence of two kinds of trivalent iron sites. The ferric cations are probably used as branching agents to reticulate the short polyphosphate groups ( average P 4 0 13 6 for x = 10) assuming the compensation of the depolymerisation effect. The increase in the PO--M bond strength (intermediate component in XPS O 1 s spectra) contributes in the reticulation increase and leads to a higher vitreous stability (Tg increase) and a greater hardness (Hv) [l].

In general, the phosphate glass dissolution can be govemed by a hydration process. It leads to a disruption of the modifying cations ionic bonds cross-linking the phosphate chains that are a:fterward released into solutions. For the more durable phosphate glasses, the hydrolysis process results in a cleavage of P-0--P bonds which namely controls the corrosion.

The improvement in the chemical durability, as reported in our previous study [1], can be related to strengthening not only of the P-0-Fe bonds as reported in other work [19] but also of the P O-P bridging bond with the shortening of the phosphate chains when x increases. In this case, the hydration of the phosphate network becomes difficult. The ion exchange process due to the hydration becomes thermodynarnically less probable and the degradation of the glass surface is mainly realised by hydrolysis in good agreement with pH fall [ 1] and high reticulation of the glass network. At the beginning of the glass corrosion, a preferential attack could happen at the end of the short polyphosphate chains by hydrolysis of P-0--Zn bonds keeping out the more covalent bonds such as P--0-Fe and P-0--Pb in agreement with the weak presence of these cations in the leachant [1]. Latter on, the acidic medium could involve a corrosion enhancement with partial hydrolysis of P--0-Pb and P-0-Fe bonds.

Conclusion

Zinc-iron phosphate glasses have been investigated using different techniques. Physical properties show a high reticulation of glass network [ 1]. These results are in good agreement with the improvement of the chemical durability especially for glass with the ratio O/P = 3.27 (x = 10). This improvement is related to the formation of P-0-M (M = Pb and Fe) and more resistant P-0-P bonds. The XPS study shows the decondensation of the phosphate network and the formation of P-0-M (Pb and Fe) bonds. Raman spectra indicate the presence of terminal groups PO 3 related to the formation of short chains resulting from Fe 2 O 3 addition. Môssbauer's data indicate the mixed valence of iron in phosphate glasses and octahedrally coordination for Fe 2 + and Fe 3+

. Complementary informations are taken by EPR study and indicate that Fe 3 + is in two different sites. For the higher Fe 2 O 3 glasses, the ferric cations play the role of branching agent to reticulate the short phosphate groups (P 4 0 13 6 -) assuming the compensation of the phosphate network decondensation effect.

Figure 1 :

 1 Figure 1: Raman spectra at room temperature of(l00-x)[50P 2 O 5 -15ZnO-35PbO]-xFeiO 3 glasses.

4 10 12 Figure 2 :

 4122 Figure 2 : Frequencies variations of vs(POP) and vs(PO 2 ) modes vs Fe 2 O 3 content in the (100-x)[50P 2 O 5 -15ZnO-35PbO]-xFeiO 3 glasses.

Figure 3 :

 3 Figure 3 : XPS Ols spectra of (100-x)[50P 2 O 5 -15ZnO-35PbO]-xFe 2 O 3 glasses.

Figure 4 :Figure 5 2 •

 452 Figure 4: Experimental area percentages ofvarious components of the Ols core level electrons vs Fe 2 O 3 content.

Figure 5 :

 5 Figure 5: Mossbauer spectra at room temperature of(l00-x)[50P 2 O 5 -15ZnO-35PbO]-xFe 2 O 3 glasses.

Figure 6 :

 6 Figure 6 : EPR spectra at room temperature of (1 0O-x)[50P2O5-15ZnO-35PbO]-xFe203 glasses.

Table 1 :

 1 Hyperfme parameters at 22 °C and the valence and site iron in (100-x)[50P 2 O 5 -15ZnO-35PbO]-xFe 2 O 3 glasses

	Fe2O3 10% 8% 6% DIS : distribution, ô : isomer shift, 11 : quadrupole splitting, r : linewidth, Oh : octahedral site DIS f(mm.s-1 ) 11 (mm.s-1 ) % population ô (rnm.s-1 ) 111 1.20 0.28 2.13 14.7 112 0.40 0.28 0.68 85.3 111 1.20 0.28 2.16 39.8 0.40 0.28 0.66 60.2 112 111 1.20 0.28 2.20 50.0 112 0.40 0.28 0.54 50.0	Site Fe 2+ o h Fe 3+ � Fe 2+ oh Fe 3 + o h Fe 2+ o h Fe 3+ 0 h

Acknowledgements

The authors are grateful for financial support from French-Moroccan committee (through contract N °: 99/ 187/SM) and PROTARS III (D14/69). Gratitude is also extended to J. M. Bassat for his help in EPR measurement