Agnès Piednoir

Audrey Steinberger

Cécile Cottin-Bizonne

Catherine Barentin
email: catherine.barentin@univ-lyon1.fr

Apparent Non-Newtonian Behavior of Ionic Liquids

teaching and research institutions in France or abroad, or from public or private research centers.

Ionic liquids (ILs) have attracted great interest in the past few years. They have been seen as opening new frontiers in material science 1 from promising alternatives to traditional industrial solvents 2 and new lubricants. [START_REF] Palacio | A Review of Ionic Liquids for Green Molecular Lubrication in Nanotechnology[END_REF][START_REF] Lhermerout | Are Ionic Liquids Good Boundary Lubricants? A Molecular Perspective[END_REF] This has inspired a large number of experimental studies, simulations and theoretical studies in the recent years. [START_REF] Marsh | Room temperature ionic liquids and their mixtures-a review[END_REF][START_REF] Zhang | Ionic Liquids[END_REF][START_REF] Hayes | Structure and Nanostructure in Ionic Liquids[END_REF] Here, we focus on the mechanical properties of ILs. Their viscosities are classically measured by the falling sphere method, the Stabinger device, [START_REF] Tariq | Viscosity of (C2-C14) 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ionic liquids in an extended temperature range[END_REF] or flows in capillary tubes. [START_REF] Fitchett | 1-Alkyl-3-methylimidazolium Bis (perfluoroalkylsulfonyl) imide water-immiscible ionic liquids the effect of water on electrochemical and physical properties[END_REF] These methods allow the exploration only a small range of solicitations, whereas viscosity may depend on the applied stress or shear rate. [START_REF] Voeltzel | Rheology of an Ionic Liquid with Variable Carreau Exponent: A Full Picture by Molecular Simulation with Experimental Contribution[END_REF][START_REF] Spikes | Comment on: Rheology of an Ionic Liquid with Variable Carreau Exponent: A Full Picture by Molecular Simulation with Experimental Contribution[END_REF][START_REF] Safinejad | Atomistic reverse nonequilibrium molecular dynamics simulation of the viscosity of ionic liquid 1-n-butyl 3-methylimidazolium bis(trifluoromethylsulfonyl)imide [bmim][Tf2N][END_REF] A liquid whose viscosity is constant with the applied stress or shear rate is considered as Newtonian. Otherwise, the liquid is considered as a non-Newtonian fluid. A decrease (resp. increase) of the viscosity with the shear rate is called shear-thinning (resp. shear-thickening). Shear-thinning observed above a critical shear rate γc for complex fluids is generally understood as a consequence of an alignment of aggregates [START_REF] Larson | Flow-induced mixing, demixing, and phase transitions in polymeric fluids[END_REF][START_REF] Förster | Shear thinning and orientational ordering of wormlike micelles[END_REF] inducing a decrease in dissipation. Shear-thickening is often attributed to the solid friction between particles [START_REF] Seto | Discontinuous shear thickening of frictional hard-sphere suspensions[END_REF][START_REF] Wyart | Discontinuous shear thickening without inertia in dense non-Brownian suspensions[END_REF][START_REF] Clavaud | Revealing the frictional transition in shear-thickening suspensions[END_REF][START_REF] Comtet | Pairwise frictional profile between particles determines discontinuous shear thickening transition in noncolloidal suspensions[END_REF] inducing an increase in dissipation. Therefore the analysis of such non-Newtonian effects allows the exploration of some properties of the system such as the interaction between particles or the microstructure. Indeed, the knowledge of the relaxation time [START_REF] Förster | Shear thinning and orientational ordering of wormlike micelles[END_REF] associated with the critical shear rate γc gives access to the characteristic size of the microstructure.

Both shear-thinning and shear-thickening effects have been reported experimentally [START_REF] Burrell | Non-Newtonian viscous shear thinning in ionic liquids[END_REF][START_REF] Kulkarni | Comparison of Physicochemical Properties of New Ionic Liquids Based on Imidazolium, Quaternary Ammonium, and Guanidinium Cations[END_REF][START_REF] Jacquemin | Physico-Chemical Properties of Non-Newtonian Shear Thickening Diisopropyl-ethylammonium-Based Protic Ionic Liquids and Their Mixtures with Water and Acetonitrile[END_REF][START_REF] Shakeel | Rheology of pure ionic liquids and their complex fluids: A review[END_REF] for several ILs at shear rates above typically 10 2 -10 4 s -1 . In Burrel et al.'s work, [START_REF] Burrell | Non-Newtonian viscous shear thinning in ionic liquids[END_REF] a transition between both behaviors has even been shown for the same ionic liquid by varying the temperature. Shear-thinning has also been reported numerically [START_REF] Voeltzel | Rheology of an Ionic Liquid with Variable Carreau Exponent: A Full Picture by Molecular Simulation with Experimental Contribution[END_REF][START_REF] Safinejad | Atomistic reverse nonequilibrium molecular dynamics simulation of the viscosity of ionic liquid 1-n-butyl 3-methylimidazolium bis(trifluoromethylsulfonyl)imide [bmim][Tf2N][END_REF] for shear rates above typically 10 7 -10 9 s -1 . However, 5 orders of magnitude separate the experimental and numerical results for the shear-thinning effect, and there is no theoretical explanation of the observed shear-thickening behavior. Understanding this huge difference in critical shear rate for the shear-thinning, as well as the transition between shear-thinning and shear-thickening behaviors motivates further rheological experiments on IL.

In this work, we study the properties of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, denoted [C n mim][NTF2] with n = 1, 2, 4, 8, 10 indicating the length of the alkyl chain, purchased from Solvionic. The viscosity of [C n mim][NTF2] is measured with a controlled-stress rheometer (AntonPaar MCR301). The dependence of the viscosity on the shear rate, ranging from 5 to 5000 s -1 , is explored using different geometries (couette, cone-plate and plate-plate). The dependence of the viscosity on the temperature ranging from 273 to 333 K is also explored in the Couette geometry, the temperature being imposed by a Peltier device at the stator. The viscosity of a Newtonian fluid (water/glycerol mixture) in the same rheological conditions (shear rates, temperatures, geometry) is also measured as a benchmark to avoid any misinterpretation of the experimental data. [START_REF] Dontula | Does the Viscosity of Glycerin Fall at High Shear Rates?[END_REF][START_REF] Fardin | Flow instabilities in large amplitude oscillatory shear: a cautionary tale[END_REF] In the main text, we present only the results obtained with [C 10 mim][NTF2] whose molecular structure has been investigated theoretically. [START_REF] Pádua | Molecular solutes in ionic liquids: a structural perspective[END_REF][START_REF] Shimizu | Structure and Aggregation in the 1-Alkyl-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid Homologous Series[END_REF] In the first part, we measure the viscosity of [C 10 mim][NTF2] with a Couette geometry (100 µm gap) as a function of the shear rate γ and for several temperatures T . For any studied temperature from 273 K to 333 K, the viscosity displays a plateau at low shear rates, whose value is called zero-shear viscosity η 0 . The viscosity then decreases at higher shear rate, above a critical shear rate denoted γc . These viscosity decreases are well fitted by a Carreau law: η( γ)/η 0 = (1 + ( γ/ γc ) 2 ) -α , which allows to define the value of the critical shear rate γc for each curve (see the Supporting Information). The exponent α is of the order of 0.03, typically 1 order of magnitude smaller than the ones found with classical shear-thinning systems. [START_REF] Voeltzel | Rheology of an Ionic Liquid with Variable Carreau Exponent: A Full Picture by Molecular Simulation with Experimental Contribution[END_REF][START_REF] Larson | Flow-induced mixing, demixing, and phase transitions in polymeric fluids[END_REF] Concerning η 0 , its value is calculated by averaging all data on the plateau region corresponding here to γ/ γc ranging from 0.04 to 0.3. We represent in Figure 1a the normalized viscosities η/η 0 with respect to the normalized shear rate γ/ γc . Note that both the zero-shear viscosity η 0 and γc depend on temperature, as shown in Figure 1b. A viscosity decrease with the shear rate has already been observed on various ILs by Kulkarni et al. [START_REF] Kulkarni | Comparison of Physicochemical Properties of New Ionic Liquids Based on Imidazolium, Quaternary Ammonium, and Guanidinium Cations[END_REF] for measurements carried out using a controlled-stress rheometer, with a critical shear rate typically above γc = 200 s -1 . Burrel et al. [START_REF] Burrell | Non-Newtonian viscous shear thinning in ionic liquids[END_REF] have also measured critical shear rates γc above 1000 s -1 on several ionic ILs using a controlled shear-rate rheometer. Both authors have interpreted it as a signature of a shear-thinning effect in these ILs, associated with the alignment of IL aggregates [START_REF] Burrell | Non-Newtonian viscous shear thinning in ionic liquids[END_REF] or to the disruption of a network of hydrophobic blocks. [START_REF] Kulkarni | Comparison of Physicochemical Properties of New Ionic Liquids Based on Imidazolium, Quaternary Ammonium, and Guanidinium Cations[END_REF] However, other experiments based on ultrasound attenuation [START_REF] Makino | Viscoelastic properties of room temperature ionic liquids[END_REF] and on shear impedance spectroscopy [START_REF] Yamaguchi | Quantitative Analysis of Conductivity and Viscosity of Ionic Liquids in Terms of Their Relaxation Times[END_REF] found relaxation times corresponding to critical shear rates above 10 8 s -1 in several imidazolium-based IL. On the numerical side, shear-thinning has also been observed at similarly high critical shear rates (above 10 7 s -1 ) in simulations of IL. [START_REF] Voeltzel | Rheology of an Ionic Liquid with Variable Carreau Exponent: A Full Picture by Molecular Simulation with Experimental Contribution[END_REF][START_REF] Safinejad | Atomistic reverse nonequilibrium molecular dynamics simulation of the viscosity of ionic liquid 1-n-butyl 3-methylimidazolium bis(trifluoromethylsulfonyl)imide [bmim][Tf2N][END_REF] These values of critical shear rates are however more than 4 orders of magnitude higher than the ones found in refs [START_REF] Burrell | Non-Newtonian viscous shear thinning in ionic liquids[END_REF][START_REF] Kulkarni | Comparison of Physicochemical Properties of New Ionic Liquids Based on Imidazolium, Quaternary Ammonium, and Guanidinium Cations[END_REF] and in our rheological measurements.

From the knowledge of the critical shear rate, we can estimate a corresponding size a for the aggregates. Indeed, shear tends to align the aggregates, whereas thermal fluctuations tend to make them rotate. Shear-thinning is then observed when γ ≥ D rot = k B T 8πη 0 a 3 , where D rot is the Brownian rotational diffusion coefficient. We deduce that the typical aggregate

size is a = k B T 8πη 0 γc 1/3
. For a viscosity of η 0 = 0.1 Pa.s (typical of [C 10 mim][NTF2] at room temperature), a critical shear rate of γc ∼ 200 s -1 suggests an aggregate size a of around 20 nm, corresponding to the aggregation of about a few tens of thousands of IL molecules, which is surprisingly huge. The same aggregate size around 20 nm is deduced from critical shear rates for all temperatures and all ILs we have probed (see Figure 1b and Supporting Information). Similar sizes of aggregates with other IL can be extracted in the same way from Kulkarni et al [START_REF] Kulkarni | Comparison of Physicochemical Properties of New Ionic Liquids Based on Imidazolium, Quaternary Ammonium, and Guanidinium Cations[END_REF] and Burrel et al. [START_REF] Burrell | Non-Newtonian viscous shear thinning in ionic liquids[END_REF] The fact that this size is independent of the nature of IL is also unexpected. On the numerical side 12 much smaller aggregate sizes of around a 0.8 nm are found, in agreement with the nano-heterogeneities measured experimentally with small-and-wide-angle X-ray scattering (SWAXS) in [C n mim][NTF2]. [START_REF] Russina | Morphology and intermolecular dynamics of 1-alkyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ionic liquids: structural and dynamic evidence of nanoscale segregation[END_REF] These huge differences in aggregate sizes obtained on one side by rheological experiments and on the other side by simulations or SWAXS, ultrasound experiments and shear impedance spectroscopy raise doubts about the interpretation of the viscosity decrease observed in the rheological experiments in terms of shear-thinning.

In the following, we suggest to interpret this viscosity decrease not as a result of a shear-thinning effect but as a result of heating effects instead. Viscous heating is possible whenever the viscosity η 0 is temperature-dependent, [START_REF] Nahme | Beitrge zur hydrodynamischen Theorie der Lagerreibung[END_REF] which is the case for IL (see refs [START_REF] Tariq | Viscosity of (C2-C14) 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ionic liquids in an extended temperature range[END_REF][START_REF] Burrell | Non-Newtonian viscous shear thinning in ionic liquids[END_REF][START_REF] Kulkarni | Comparison of Physicochemical Properties of New Ionic Liquids Based on Imidazolium, Quaternary Ammonium, and Guanidinium Cations[END_REF] and Figure 1b). The temperature rise ∆T due to shear has been estimated several decades ago [START_REF] Bird | Viscous heating effects in a cone and plate viscometer[END_REF] and scales as ∆T

η 0 γ2 h 2 κ
with h the typical gap of the geometry and κ the thermal conductivity. This temperature rise induces, at first order, a viscosity decrease ∆η given by: 32

∆η η 0 = 1 η 0 ∂η 0 ∂T ∆T = β ∆T T (1)
where

β = T η 0 ∂η 0
∂T T is the thermal sensitivity. For [C 10 mim][NTF2], we measure |β| 16 at room temperature. From Equation 1, we expect the relative variation of viscosity to scale as:

∆η η 0 ∝ γ2 (2)
With the aim to test this dependence, we plot in Figure 2 of viscosity (typically smaller than 5-6%), we observe a power law dependence with the shear rate whose slope is between 1 and 2. It is slightly smaller than the slope of 2 predicted by Equation 2, which is not so surprising for two reasons. First, Equation 2 is only a first-order approximation that neglects the effect of the viscosity decrease on the magnitude of the viscous heating and is only valid for small relative viscosity variations. Second, the temperature increase is also limited by the thermal exchanges with the environment, including both the liquid bath and the Peltier element that controls the setpoint temperature. In particular, it is important to note that the volume of the sheared liquid in the 100 µm gap Couette geometry corresponds to only 1% to the total liquid volume, which plays the role of a temperature reservoir. Finally, a saturation is observed at higher relative viscosity variations (around 10%) which corresponds to a temperature increase of ∆T = T β ∆η η 0 = T 2 4660 ∆η η 0 comprised between 1.6 K (at 273 K) and 2.4 K (at 333 K). We think that it is due to the thermal regulation by the Peltier element. The fine modelisation of these thermal exchanges is however beyond the scope of this paper, since it is very dependant on the experimental geometry, the proportion of sheared liquid with respect to the non-sheared liquid, the details of the temperature control loop, and the position of the temperature sensor. To further check the validity of our viscous-heating assumption, we perform similar experiments with a Newtonian liquid, a water/glycerol (10/90) mixture. Its thermal sensitivity at room temperature |β| = 18 and its zero-shear viscosity are similar to the ones of [C 10 mim][NTF2]. We observe that the dependence of its viscosity on the shear rate (Figure 3a) is extremely similar to the one obtained with [C 10 mim][NTF2] (Figure 1a). In particular, the order of magnitude of the critical shear rate (∼ 100 s -1 ) is very similar. However, Newtonian liquids such as a water-glycerol mixture do not exhibit shear-thinning behavior at such a small range of shear rates at room temperature. [START_REF] Dontula | Does the Viscosity of Glycerin Fall at High Shear Rates?[END_REF] Therefore, the only reasonable explanation for this viscosity decrease is the temperature increase induced by the mechanical shear.

From the knowledge of β and from equation 1, we estimate the temperature increase in Note that geometry matters for the viscosity decrease. A dependence of the critical shear rate on the geometry has been reported a long time ago, [START_REF] Turian | Viscous heating effects in a cone and plate viscometer II[END_REF][START_REF] Sukanek | An experimental investigation of viscous heating in some simple shear flows[END_REF] and more recently 32 for Newtonian liquids. In all these studies, the viscosity drop was due to viscous heating. We have observed a similar dependence on the geometry for [C 10 mim][NTF2]. In a Couette geometry with a 100 µm gap, γc ∼ 50 s -1 at T = 278 K (see Figure 1a); whereas for a 1 mm gap, γc ∼ 500 s -1 at the same temperature (see Figure 4a). Such a dependence on the geometric setup rules out the explanation that the critical shear rate is due to intrinsic bulk properties of IL and thus confirms the viscous-heating assumption on a thin layer around the rotor.

Note that, interestingly, performing experiments on [C 10 mim][NTF2] using a Couette geometry with a 1mm gap and spanning temperatures, we could observe various behaviors of the viscosity with the shear rate (see Figure 4). At low temperature (278 K) and high η 0 (0.29 Pa.s), as previously described, viscosity decreases at high shear rates (Figure 4a). At intermediate temperature (303 K) and η 0 (0.085 Pa.s) the viscosity dependence on the shear rate is non-monotonic (Figure 4b). Finally, at high temperature (333 K) and low η 0 (0.027 Pa.s), the viscosity increases at high shear rate (Figure 4c). Such a viscosity increase for IL has already been observed in references [START_REF] Burrell | Non-Newtonian viscous shear thinning in ionic liquids[END_REF][START_REF] Jacquemin | Physico-Chemical Properties of Non-Newtonian Shear Thickening Diisopropyl-ethylammonium-Based Protic Ionic Liquids and Their Mixtures with Water and Acetonitrile[END_REF] in a cone plate geometry at high shear rate (above 1000 s -1 ), and for zero-shear viscosities below typically 50 mPa.s. In those two studies, the viscosity increase is interpreted as a shear-thickening effect occurring in IL. To test this interpretation, we performed experiments in the same geometries (Couette with 1 mm gap and cone-plate) with a Newtonian fluid: a water-glycerol (35/65) mixture, whose nominal viscosity is close to the ones of the IL used in. [START_REF] Burrell | Non-Newtonian viscous shear thinning in ionic liquids[END_REF][START_REF] Jacquemin | Physico-Chemical Properties of Non-Newtonian Shear Thickening Diisopropyl-ethylammonium-Based Protic Ionic Liquids and Their Mixtures with Water and Acetonitrile[END_REF] As shown on Figure 4d, a viscosity However, Newtonian fluids such as a water-glycerol mixture are not shear-thickening.

The only reasonable interpretation is that the rheometer measures a dissipation in addition to the viscous dissipation. Such an additional dissipation at high shear rates generally originates from instabilities. [START_REF] Fardin | Flow instabilities in large amplitude oscillatory shear: a cautionary tale[END_REF][START_REF] Ewoldt | Complex fluids in biological systems[END_REF] In the Couette geometry, this can be well explained by the Couette-Taylor instability. [START_REF] Larson | Flow-induced mixing, demixing, and phase transitions in polymeric fluids[END_REF] This instability generates secondary flows that increase the dissipation within the fluid. It occurs when the Taylor number T a becomes higher than 1700, where T a = ρ 2 γ2 h 5 η 2 0 R , with ρ the liquid density and R the radius of the inner cylinder. In our experimental conditions, T a 2000 for γ 2000 s -1 so that the Couette-Taylor instability can explain the viscosity increase observed in the Couette geometry (plain circle, Figure 4d).

In the cone-plate geometry, as well as in the plate-plate geometry (see the Supporting Information for more details), we interpret this apparent viscosity increase as an additional dissipation due to the fluctuations of the contact line and the radial movement of the liquid at the edges. For a typical range of shear rates from 2000 to 5000 s -1 , we observe oscillations of the liquid edges anchored on surface defects (see movies in the Supporting Information). We propose that this oscillating movement originates from the competition between centrifugal forces F g and capillary forces F γ . The centrifugal force is constant and set by the shear rate and the capillary force is assumed to vary between two extreme values (F min γ and F max γ ) depending on the shape of the meniscus, as sketched on the scheme 1. As F g > F γ , centrifugal forces expel the fluid out of the gap leading to a change of the meniscus shape and of the contact angles. This results in an increase of F γ . As F γ exceeds F g , the fluid is pulled back inside the gap. Note that such a competition between centrifugal and capillary forces has been previously reported to explain oscillations of the contact line in another geometry. [START_REF] Couvreur | The role of wetting heterogeneities in the meandering instability of a partial wetting rivulet[END_REF] In order to test our interpretation, we estimate the typical shear rate γ for which centrifugal force F g and capillary force F γ are the same order of magnitude. The centrifugal force integrated on the whole cone-plate geometry is given by:

F g = π 6 ρRh 3 γ2 (3)
where R and h stand for the rotor radius and the gap at the extremity of the cone-plate.

The capillary force exerted by the two walls on the meniscus, assuming a perfectly circular contact line at the two walls1 , is given by:

F γ = 4πRγ LV cos θ (4)
where γ LV and θ are respectively the surface tension of the air-liquid interface and the contact angle. Both forces are of the same order of magnitude for a shear rate:

γ = 24γ LV cos θ ρh 3 (5)
For classical values of surface tension γ LV = 60 mN/m and for θ = 0, h = 1 mm, ρ = 10 3 kg/m 3 , we found γ 1200 s -1 . This is in good agreement with the typical shear rates observed experimentally as shown in Figure 4d. This analysis in terms of force balance leads to a shear rate γ above which capillary instability is expected. Moreover, taking into account viscous dissipation sets a critical viscosity η above which this instability is damped (see the Supporting Information). Overall, this capillary instability is then observed for γ > γ and for η < η .

Finally, instabilities and viscous heating can also coexist and compete at high shear rates, and the relative importance of these two effects depends on the shear rate and the zero shear viscosity η 0 . Indeed, lower (resp. higher) η 0 promotes instabilities (resp. viscous heating).

Such a competition gives an explanation for a non-monotonic dependence of viscosity on the shear rate, as observed in Figure 4b.

To summarize, we measured the increase or decrease of the viscosity at high shear rates for IL and Newtonian fluids. Those behaviors cannot be simply attributed to shear-thickening or shear-thinning effects. Indeed, rheological measurements performed at high shear rate γ > 1000 s -1 have to be interpreted carefully, since many additional effects may occur. On one hand, viscous heating is at the origin of a viscosity decrease that can be misinterpreted as a shear-thinning. The resulting viscosity decrease is all the more important as the thermal sensitivity β and the viscosity η 0 are high. On the other hand, instabilities (Couette-Taylor or contact line fluctuations) induce a dissipation increase, which can be misinterpreted as shear-thickening. The instability thresholds depend both on the experimental geometry and the fluid properties. Both instabilities are favored at a low viscosity (η < 0.1 Pa.s in our experiments) and large gap. As a consequence, there is no evidence that the ILs mentioned here are non-Newtonian in the investigated range, since similar behaviors have been observed in known Newtonian liquids. A comparison with matching Newtonian liquids (similar viscosity and thermal sensitivity) should also be performed for the systems investigated in [START_REF] Burrell | Non-Newtonian viscous shear thinning in ionic liquids[END_REF][START_REF] Kulkarni | Comparison of Physicochemical Properties of New Ionic Liquids Based on Imidazolium, Quaternary Ammonium, and Guanidinium Cations[END_REF][START_REF] Jacquemin | Physico-Chemical Properties of Non-Newtonian Shear Thickening Diisopropyl-ethylammonium-Based Protic Ionic Liquids and Their Mixtures with Water and Acetonitrile[END_REF] or cited in the review of Shakeel et al., [START_REF] Shakeel | Rheology of pure ionic liquids and their complex fluids: A review[END_REF] in order to validate or invalidate the interpretation of the observed viscosity variations in terms of shear-thinning or shear-thickening. Moreover strong dependence of the rheological response on the geometry (couette, cone-plate, plate-plate) has to be considered carefully.

  Results obtained with other ILs [C n mim][NTF2] are collected in the Supporting Information and are similar to those of [C 10 mim][NTF2]. The first part of the paper is devoted to the study of the shear-thinning effect and the second part to the shear-thickening effect.

Figure 1 :

 1 Figure 1: a) Evolution of the normalized viscosity, η/η 0 , with the normalized shear rate γ/ γc for the ionic liquid [C 10 mim][NTF2] for various temperatures from 273 to 333K, in a Couette geometry with a 100 µm gap. b) For [C 10 mim][NTF2], critical shear rate γc (triangle) as a function of temperature T , and zero-shear viscosity η 0 (circle) including error bars (smaller than the symbol size) as a function of temperature T and fit with η 0 (T ) = 1.7 × 10 -8 exp(4660/T ). The relative error for zero-shear viscosity ranges from 0.1 to 0.8% depending on the temperature.

  the absolute relative variation of the viscosity of [C 10 mim][NTF2] as a function of the normalized shear rate, for various temperatures. The experimental uncertainty on the relative variation of viscosity (0.6% in average) sets the lower limit of the Y-axis. For intermediate values of the relative variations

Figure 2 :

 2 Figure 2: Evolution of the absolute relative variation of viscosity of [C 10 mim][NTF2] with the normalized shear rate γ/ γc at various temperatures ranging from T = 273 to 333 K.

Figure 3 :

 3 Figure 3: a) Evolution of the viscosity with shear rate for a water-glycerol (10/90) mixture at T=296 K, in a Couette geometry with a 100 µm gap; inset: evolution of η 0 with the temperature T and fit with η 0 (T ) = 1.8 × 10 -9 exp(5440/T ). b) Evolution of the absolute relative variation of viscosity of this water-glycerol mixture with the shear rate at T = 296 K; inset: calculated temperature increase ∆T as a function of shear rate for the water-glycerol mixture.

Figure 4 :

 4 Figure 4: Evolution of the viscosity with shear rate for [C 10 mim][NTF2], in a Couette geometry with a 1 mm gap, a) at T=278 K, b) at T=303 K; c) at T=333 K. d) Evolution of the viscosity with shear rate for a water-glycerol (35/65) mixture, with η 0 =14 mPa.s, in a cone-plate geometry (triangles) and in a Couette geometry with a 1 mm gap (circles), at T=295 K.

Scheme 1 :

 1 Scheme 1: Sketch of the oscillating process: competition between centrifugal and capillary forces exerted on a sheared liquid in a plate-plate geometry. The centrifugal force F g is constant for a given shear rate whereas the value of the capillary force F γ depends on the shape of the meniscus and oscillates between two extreme values.

Note that the exact value of the capillary forces depend on the exact position of the contact line and on the value of the contact angle.

Acknowledgement

The authors thank Laurent Joly for fruitful discussions and Margarida Costa Gomes and Agilio Padua for introducing them to ionic liquids.

Supporting Information Available

This material is available free of charge via the Internet at http://pubs.acs.org/.


R Larcher

Marc Pineton De Chambrun

Fanny Garnier

Emma Rubenstein

Julie Carr

Jonathan Charbit

Kevin Chalard

Marc Mourad

Matthieu Amalric

Laura Platon

One-year outcome of critically ill patients with systemic rheumatic disease: a multicenter cohort study

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.


Dr Géraldine Dantelle

Georges Boulon
email: georges.boulon@univ-lyon1.fr

Yannick Guyot

Denis Testemale

Malgorzata Guzik

Shunsuke Kurosawa

Dr Kei Kamada

Dr Akira Yoshikawa

Prof Assoc Georges Boulon

Prof Yannick Guyot

Prof Assoc Malgorzata Guzik

Prof Assoc Shunsuke Kurosawa

Prof Assoc Kei Kamada

Prof Akira Yoshikawa

Géraldine Da

Y Yokota

K Sugiyama

] K Kamada

Y Shoji

V Kochurikhin

A Nagu

J Pejchal

Y Ohashi

M Nikl

M Yoshino

S Yamamoto

Y Wu

F Meng

Q Li

] Y Wu

] M Nikl

V Babin

K Pilarova

E Mihokova

A Beitlerova

K Bartosiewicz

S Bartosiewicz

A Kurosawa

Kamada Yoshikawa

S Liu

J A Mares

X Feng

C D'ambrosio

C D'

Ambrosio S Sahi

M Groza

S R Rotman

H L Tuller

C Warde

M V Derdzyan

A G Petrosyan

C Dujardin

] K Bartosiewicz

] M Tella

M Auffan

L Brousset

E Morel

O Proux

C Chanéac

B Angeletti

Characterization of Ce 4+ in Ce 3+ , Mg 2+ -Co-Doped Gd 3 Al 2 Ga 3 O 12 Garnet Crystal

Keywords: fast scintillator, Ce 3+ and Ce 4+ dopants, garnet crystals, optical spectroscopy, XANES spectroscopy fast scintillator, Ce fast scintillator, Ce XANES spectroscopy density, stopping power growth technology

The presence of Ce 4+ ions in Ce 3+ -doped materials has always been a big issue.

The first precise work showing the presence of Ce 4+ has been published in 1992 by Rotmann on Ce 3+ , Ca 2+ -co-doped YAG. [28] Optically, it has been determined that the cerium can change valence state between Ce 3+ and Ce 4+ when annealed in an atmosphere of reducing and oxidizing gases, respectively. Under cathodoluminescence, the Ce 3+ ion emission is seen even when only Ce 4+ is present in the crystal and it was interpreted as an electron first entering a Ce 4+ in an upper excited 5d state, thereby converting it temporarily to Ce 3+ , followed by the standard 5d4f emission of Ce 3+ at 550 nm. Then, in 2013, Blahuta & al. proposed, a new scintillation mechanism under ionizing irradiation involving Ce 4+ ions in Ce 3+ -Mg 2+ /Ca 2+ -codoped LYSO single crystals. [7] Nowadays, in co-doped materials, the evidence of Ce 4+ is admitted for the sequential charge capture of an electron-hole pair by Ce 3+ according to: Ce 3+ + e -+ h +  Ce 4+ + e - (Ce 3+ )*  Ce 3+ + hν Then, there is a continuous process of capturing a hole from the valency band.

As a result, Ce 4+ ions are stabilized by the addition of Mg 2+ /Ca 2+ divalent ions. [14,18,20,21] Such centers create another fast radiative recombination pathway working in parallel with the classical mechanism based on the stable Ce 3+ centers. It means the skipping of the first hole trapping stage could result in acceleration of the decay by suppression of slow components. [29] This observation has to be combined with the favorable position of the shallow levels below the conduction band leading to slow components of the decay. The main purpose of the Mg 2+ /Ca 2+ -co-dopant as analyzed by Melcher's group was to limit the formation of vacancies (Vo) that were detrimental in term of afterglow level. [14,18 ] All previous results on Ce 3+ , Mg 2+ -co-doped GAGG are in favour of a promising scintillator for timing devices in highenergy physics and Time-of-Flight PET (TOF-PET) applications.

In this way, in the following part, we summarize the main spectroscopic properties of Ce 3+ -doped GAGG and Ce 3+ , Mg 2+ -co-doped GAGG garnet crystals. 3+ -doped Gd 3 Al 2 Ga 3 O 12 (GAGG) and Ce 3+ , Mg 2+ -co-doped GAGG garnet crystals Especially, the Ce 3+ -doped GAGG shown in Figure 1a and the Ce 3+ , Mg 2+ -co-doped GAGG shown in Figure 1b are single crystals developed by using the Czochralski method at IMR, Tohoku University. [15,21] The absorption spectra of 0.5% Ce 3+ -doped GAGG and 0.5% Ce 3+ , 0.5% Mg 2+ -co-doped GAGG crystals are presented in

Crystal growth and spectroscopic properties of Ce

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This article is protected by copyright. All rights reserved bands at roughly 450 and 340 nm, respectively, the Gd 3+ 4f 8 S 7/2  6 P J and 8 S 7/2  6 I J absorption lines and, especially in the co-doped crystal, the strong Ce 4+ charge transfer band (CTB) in UV below 320 nm. This CTB is assigned from the oxygen 2p orbitals of the valence band to the Ce 4+ 4f orbitals by analogy with Ce 3+ ,Mg 2+ /Ca 2+ -co-doped LYSO [7] and also with Ce 3+ , Mg 2+ -co-doped Lu 3 Al 5 O 12 (LuAG) ceramics. [26] The hole in the maximum of this broad absorption band at around 275 nm corresponds to the resonant energy transfer between the Gd 3+ 6 I J  8 S 7/2 emission lines and the Ce 4+ CTB broad absorption which corresponds to a nonradiative energy transfer since Ce 4+ centers do not emit. Finally, in Ce 3+ , Mg 2+ -co-doped GAGG there is two contributions to shortening of the decay: one due to the presence of Ce 4+ and another one associated with the energy transfer from Gd 3+ to Ce 4+ ions. A discussion on the energy migration and energy transfer processes in the set of un-doped and Ce 3+ activated multi-component garnet single crystals has ben recently reported. [30] Radioluminescence spectra under excitation by X-rays from CuKa of the Ce 3+ -doped GAGG and Ce 3+ , Mg 2+ co-doped GAGG are shown in Figure 3. The broad visible band is composed of the two usual transitions 5d4f ( 2 F 5/2 ) and 5d4f ( 2 F 7/2 ). It is important to mention the decreasing of the Ce 3+ emission intensity by Mg 2+ co-doping as a result of the creation of Ce 4+ in these crystals and then the reduction of Ce 3+ active centers. Scintillation decays of the 5d4f visible broad band of 1% Ce 3+ -doped GAGG (0% Mg) and 1% Ce 3+ ,0.1% Mg 2+ co-doped GAGG crystals under excitation by 137 Cs radio-isotope (662 keV) are shown in Figure 4. Decay profiles can be fitted by two exponentials, corresponding to the fast and slow radiative recombination of the Ce 3+ emitting centers.

The decay-time components of 1% Ce 3+ -doped GAGG (0% Mg) and 1% Ce 3+ ,0.1% Mg 2+ co-doped GAGG crystals and the corresponding percentage of the total scintillation output are listed in Table 1. The most prominent observation is the shortening of the scintillation decaytime after Mg 2+ co-doping. Without Mg 2+ ,1% Ce 3+ -doped GAGG is characterized by 92.2 ns decay time (81%) and in 1% Ce 3+ , 0.1% Mg 2+ co-doped GAGG 58% of the total decay by 45.2 ns decay time. The shortening was already noticed first in Ce 3+ , Mg 2+ /Ca 2+ -co-doped LYSO crystal [7] , in Ce 3+ , Ca 2+ -co-doped GAGG crystal [14] and Ce 3+ , Mg 2+ -co-doped LuAG ceramics. [25][26][27] Accepted Article

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The shortening was already noticed first in Ce LYSO crystal

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LYSO crystal [7] Accepted Article [7] , in Ce Accepted Article

, in Ce

This article is protected by copyright. All rights reserved Table 1 The decay-time components of 1% Ce 3+ -doped GAGG (0% Mg) and 1% Ce 3+ , 0.1% Mg 2+ co-doped GAGG crystals and the corresponding percentage of the total scintillation output.

Samples

Evidence and evaluation of stable Ce 4+ ions by XANES technique

in Ce 3+ , Mg 2+ -co-doped GAGG Previous articles considered that the charge transfer absorption band located at 260 nm is the probe of presence of Ce 4+ ions in Ce 3+ , Mg 2+ /Ca 2+ -co-doped LYSO single crystals [7] and Ce 3+ , Ca 2+ -co-doped GAGG [6,21] . This hypothesis seems in agreement with the creation of Ce 4+ ions due to the introduction of divalent ions as Ca 2+ and Mg 2+ . However, the quantitative values of the Ce 3+ /Ce 4+ concentration ratio in Ce 3+ , Mg 2+ -co-doped GAGG have never been measured. Therefore, our goal is firstly focused on the confirmation of the

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Previous articles consider

This article is protected by copyright. All rights reserved presence of Ce 4+ ions in this crystal and secondly on the evaluation of the Ce 3+ /Ce 4+ ratio measured by the X-ray Absorption Near Edge Spectroscopy (XANES) spectroscopy at the Ce L III threshold which is another effective technique able to distinguish unambiguously the Ce 3+

and Ce 4+ oxidation states. Measurements have been made at the European Synchrotron Radiation Facility (ESRF) in Grenoble performed in high energy resolution fluorescencedetected (HERFD) mode at the Fame-UHD beamline. The photon energy was scanned from 5.68 keV to 5.85 keV using a Si (220) double-crystal monochromator. A glove box filled with helium was used between the samples, the crystal analyzer spectrometer and the detector, to avoid partial beam absorption by the air. The signal was recorded with a five-Ge (331) crystal analyzer and a Vortex-Ex detector. The beam size was 300 x 100 µm 2 (horizontal x vertical FWHM). The energy calibration was done using the CeO 2 spectrum. The Ce 3+ /Ce 4+ ratio was determined by linear combination of XANES spectra of standards, i.e. nano-CeO 2 as a reference for Ce 4+ and another Ce 3+ -doped YAG single-crystal as a reference for Ce 3+ [31] .

The experimental data were analyzed using the Demeter/Athena software. [32] We also have evaluated the Ce 3+ /Ce 4+ ratio by withdrawing ratio dependence of XANES spectra corresponding to different values of this ratio and compared with the experimental ones.

The Ce L III threshold XANES spectra of Ce(acetate) 3 and Ce 3+ -doped YAG as Ce 3+ trivalent reference (characteristic peak at 5727 eV) and CeO 2 as Ce 4+ tetravalent reference (characteristic peak at 5738 eV) are shown in Figure 5. These two lines are enough dispersed to characterize each oxydation state. The XANES experimental data can be seen also for 0.5% Ce 3+ -doped YAG, 0.5% Ce 3+ -doped GAGG and 0.5% Ce 3+ ,0.5% Mg 2+ -codoped GAGG garnet single-crystals in Figure 6, Figure 7 and Figure 8, respectively.

It is clear that we do not observe any signal from Ce 4+ reference in 0.5% Ce 3+ -doped YAG and 0.5% Ce 3+ -doped GAGG and then, only the presence of Ce 3+ ions is detected in these crystals without any Mg 2+ , whereas in Ce 3+ , Mg 2+ -co-doped GAGG, Ce 4+ ions are clearly observed. The estimation of the Ce 3+ /Ce 4+ ratio in the spectra of 80/20 is the result of the linear combination fit method of the two Ce 4+ and Ce 3+ references. In Figure 7 and Figure 8, we choose to drawn the simulation of Ce 4+ concentration dependences in order to show, at a glance, the evolution of the XANES spectra. The Ce 3+ /Ce 4+ ratio of 82.5/17.5 mentionned in Figure 8 for Ce 3+ , Mg 2+ -co-doped GAGG garnet single-crystal confirms the role of stable Ce 4+ in the scintillation process and obviously the absence of stable Ce 4+ in the crystals without any Mg 2+ , as expected. Then, the two methods give the same order of magnitude of the Ce 3+ /Ce 4+ ratio value, by considering an error of roughly 10% on the Ce 4+ value. In 0.3%Ce 3+ , 0,3%Mg 2+ -co-doped LuAG transparent ceramics the Ce 3+ /Ce 4+ ratio is 75/25. [26] The behaviour of Ce 3+ , Ca 2+ -co-doped GAGG garnet single crystal is completely different since the evaluated Ce 3+ /Ce 4+ ratio is 4/96. [18] The segregation coefficient of Ce 3+ ions in garnets is far from one as we already analyzed it in melt crystal growth and ceramic processing for optical applications in YAG [33][34] and (Gd,Y) 3 Al 5 O 12 . [35] Here, we started from the nominal concentrations of 1mol.% Ce 3+ , 0.1 mol.% Mg 2+ -co-doped GAGG. and we need to give some values of the real concentrations in the crystals. The solidification fraction (g) described as :g = (mass of grown crystal)/(mass of starting materials in crucible) has been measured and was about 0.23. The composition distributions in grown crystals were monitored by the EPMA; JXA-8621MX, JEOL.

The average effective segregation coefficient of Ce 3+ is K eff = 0.23. As a result, the real concentration of Ce 3+ in the crystal is estimated between 0.15 and 0.38 mol.%. [16] Traces of Mg 2+ could not be detected by the EPMA but were analyzed by ICP-AES. Mg 2+ concentration at g = 0.1 and g = 0.2 were 16 and 25 wt. ppm (0.061 and 0.091 mol%), respectively.

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XANES spectra. In

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In Ce

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Ce combination fit has been applied and provided

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Indeed, we have always to take into account of the competition for scintillation

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Indeed, we have always to take into account of the competition for scintillation mechanisms between the creation of Ce

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This article is protected by copyright. All rights reserved thermo-stimulated luminescence intensity [22] and there is an improvement of fast performances from the creation of Ce 4+ . [20] 5. XANES data with Ce 3+ -doped LuAG, Ce 3+ , Li + -co-doped LuAG and Ce 3+ , Mg 2+ -co-doped LuAG Ce 3+ -activated LuAG single-crystal is another extensively studied fast and bright scintillator which is demanded in coming medical and industrial applications.

Especially, the development of Ce 3+ -doped Lu 3 Al 5 O 12 (LuAG)-based single crystal scintillators is a good example of materials prepared by the micro-pulling down method at the initial stage of material screening and by Czochralski or Bridgman methods to obtain higher quality and larger size single crystals afterward. [23] It has been observed that these materials gave rise to new class of ultra-efficient complex oxide scintillators, the light yield of which considerably exceeds the values achieved for the best Ce 3+ -doped orthosilicate scintillators.

The interpretation is rather based on trapping processes including the nature and role of material defects giving rise to shallow traps at small energies below the conduction band and Ce 4+ is not refered. Then, the Ce 3+ , Li + co-doped Lu 3 Al 5 O 12 single crystals were also prepared by micro pulling down method and, namely the scintillation decay and light yield have revealed the effect of the co-doping like the smooth Ce 4+ charge transfer absorption below 350 nm which is clearly enhanced with increasing concentration of Li + . [24] Again, the scintillation decays were accelerated and light yields were increased by Li + co-doping.

In Ce 3+ , Ca 2+ /Mg 2+ /Li + -co-doped LuAG crystals and ceramics, interesting similarities have been found with the previous analysis of Ce 3+ , Ca 2+ /Mg 2+ -co-doped GAGG, mainly by the role demonstrated of Ce 4+ ions. [25][26][27] Consequently, it is worthwhile finding out what makes these materials unique and what they all have in common.

Single crystals have been grown also at IMR, Tohoku Unversity, and co-doped not only with Mg 2+ but also with Li + ions, with the objective to compare results to those of the above GAGG crystals. As for segregation phenomenon, measurements have been done only for Ce 3+ , Li + -co-doped GAGG but not for Ce 3+ , Li + -co-doped LuAG. From the nominal concentration 0.15mol.% Li + ,1mol.% Ce 3+ -co-doped GAGG we have found the average effective segregation coefficient K eff of Ce 3+ , Ga 3+ and Al 3+ ions showed K eff = 0.28, 0.94 and The interpretation is rather based on trapping processes including the nature and role of

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The interpretation is rather based on trapping processes including the nature and role of material defects giving rise to shallow traps codopant. We have also compared the XANES spectrum with the weak XANES spectra analysed in Ce 3+ , Mg 2+ -co-doped LuAG transparent ceramic [26] . It makes sense to compare the results of both, crystal (0.5% Mg 2+ ) and transparent ceramics (0.3% Mg 2+ corresponding to the maximum of light output at 25070 ph/MeV). The Ce 4+ concentration of 25% has been evaluated in this transparent ceramics and clearly increased to 37% of Ce 4+ ion when the Mg 2+ concentration was increased from 0.3% to 0.6%. As a result, we can conclude that Ce 4+ ion plays the same role with the same concentrations in scintillation mechanisms of Ce 3+ , Mg 2+co-doped LuAG garnet, either for crystal or for transparent ceramics.

On the other hand, a major feature has to be mentioned: XANES spectra does not show any trace of Ce 4+ in Ce 3+ , Li + -co-doped LuAG. It means, although interesting scintillation data have been reported, the real role of Ce 4+ remains questionable [23][24] . As an example, an increase of the absorption below 350 nm that could be related to formation of Ce 4+ oxidation state has been recorded in Ce 3+ , Li + -co-doped LuAG grown by micro-pulling and compared to Ce 3+ -doped LuAG, the scintillation decay was accelerated and the high light yield of 24,000 photon/MeV was obtained with 0.2% Li (200 ppm) co-doped samples. [31] It has been suggested that Li + ions in Ce 3+ , Li + -co-doped LuAG do substitute Lu 3+ sites with charge balance attained in part by conversion of Ce 3+ to Ce 4+ and creation of O-hole centers, whereas in Ce 3+ , Li + -co-doped YAG, Li + goes in interstitial position with charge compensation attained by reduction of anion vacancies. [22,29] The behaviour of these Lu-based

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LuAG, the scintillation decay was accelerated and the high light yield of This article is protected by copyright. All rights reserved garnet scintillators is then different from other Y or Gd-based garnets and should be more deeply analysed. The absence of detectable Ce 4+ ion from these XANES spectra, as compared with those of Ce 3+ , Mg 2+ -co-doped GAGG and Ce 3+ , Mg 2+ -co-doped LuAG, induces some questions on the scintillation mechanism in Ce 3+ , Li + -co-doped LuAG.

Conclusion

In this work, we were dealing with the most advanced oxide scintillator Ce 3+

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Conclusion

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Conclusion

In this work, we were dealing with the most advanced oxide scintillator Ce

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In this work, we were dealing with the most advanced oxide scintillator Ce

Gd

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Garnet single crystal: 2 inch size 1%Ce single crystal grown by the CZ method at IMR, Tohoku University, Sendai, Japan.

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single crystal grown by the CZ method at IMR, Tohoku University, Sendai, Japan.

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Figure 1b

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Scintillation decay profiles at room temperature of the 5d

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Scintillation decay profiles at room temperature of the 5d the 1%Ce

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This article is protected by copyright. All rights reserved Figure 5 The XANES spectroscopy at the Ce L III threshold. X-ray absorption spectra of Ce 3+ and Ce 4+ are different. CeO 2 is chosen as a reference for Ce 4+ (characteristic peak at 5738 eV) and Ce(acetate) 3 and Ce 3+ -doped YAG are chosen as a reference for Ce 3+ (characteristic peak at 5727 eV).

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The XANES spectroscopy at the Ce L

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The XANES spectroscopy at the Ce L and Ce

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(characteristic peak at 5727 eV).

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This article is protected by copyright. All rights reserved Figure 6 Comparison of the experimental XANES spectrum of the 0.5% Ce 3+ -doped YAG single-crystal (black, with thickness=0,53mm) with the spectrum obtained by a linear combination fit method. Another Ce 3+ -doped single-crystal is used as a reference for Ce 3+ (red). We observe the presence of only Ce 3+ ions in this sample.

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Comparison of the experimental XANES spectrum of the 0.5% Ce

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Comparison of the experimental XANES spectrum of the 0.5% Ce crystal (black,

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ion. Thickness of samples=0,53mm.

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Thickness of samples=0,53mm.

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Figure 2 .

 2 We clearly see the two first Ce 3+ 4f5d absorption Accepted Article when only Ce Accepted Article when only Ce 4+ Accepted Article 4+ is present in the crystal and Accepted Article is present in the crystal and in an upper excited 5d state, thereby converting it temporarily to Ce Accepted Article in an upper excited 5d state, thereby converting it temporarily to crystals admitted for the sequential charge capture of an electron Accepted Article admitted for the sequential charge capture of an electron Ce another fast radiative recombination pathway working in parallel with the Accepted Article Such centers create another fast radiative recombination pathway working in parallel with the classical mechanism based on the Accepted Article classical mechanism based on the trapping stage could result in acceleration of the decay by suppression of slow components Accepted Article trapping stage could result in acceleration of the decay by suppression of slow components This observation has to be combined with the favorable position of the shallow levels Accepted Article This observation has to be combined with the favorable position of the shallow levels below the conduction band leading to slow components of the de Accepted Article below the conduction band leading to slow components of the de 2+ as analyzed by Melcher's group was to limit the formation of vacancies Accepted Article dopant as analyzed by Melcher's group was to limit the formation of vacancies (Vo) that were detrimental in term of afterglow level Accepted Article (Vo) that were detrimental in term of afterglow level doped Accepted Article doped GAGG are in favour of a

Indeed, we have

  always to take into account of the competition for scintillation mechanisms between the creation of Ce 4+ and the presence of the shallow electron traps and deeper traps in the crystal lattice of garnet. These traps capture electrons from the conduction band and contribute to extend component in scintillation decay. It is difficult at this point to evaluate each contribution of Ce 4+ ion and traps to the fastest component of scintillation response. It is only clear that Ce 3+ , Mg 2+ -co-doped GAGG shows lower afterglow and

FIGURESFigure 1a

 1a FIGURES

Figure 1b

 1b Figure 1bGarnet single crystal: 3 inch diameter of 1%Ce, 0.1%Mg-co-doped Gd 3 Al 2 Ga 3 O 12 (GAGG) garnet single crystal grown by the CZ method at IMR, Tohoku University, Sendai, Japan.

Figure 1a

 1a Figure 1aGarnet single crystal: 2 inch size 1%Ce

Figure 1b

 1b Figure 1bGarnet single crystal:Accepted ArticleGarnet single crystal: (GAGG) garnet single crystal grown by the CZ method at IMR, Tohoku University, Sendai,Accepted Article(GAGG) garnet single crystal grown by the CZ method at IMR, Tohoku University, Sendai,

Figure 4

 4 Figure 4Scintillation decay profiles at room temperature of the 5d4f visible broad band of the 1%Ce 3+ -doped GAGG (0% Mg 2+ ) and 1%Ce 3+ , 0.1%Mg 2+ -co-doped GAGG crystals under excitation by 137 Cs radio-isotope (662 keV).

  and Ce(acetate) Accepted Article 5738 eV) and Ce(acetate)(characteristic peak at 5727 eV).

Figure 7 Figure 8

 78 Figure 7Experimental Ce L III -edge XANES spectra of Ce 3+ -doped GAGG garnet single crystal (black curve). The XANES spectra of the Ce 4+ (CeO 2 ) and Ce 3+ (Ce 3+ : YAG) mentioned here by 0.0 Ce 4+ , are used as references. Ratio dependence of XANES spectra have been simulated for different Ce 4+ concentrations. Any trace of Ce 4+ has been detected and so the sample contains only Ce 3+ .

Figure 9

 9 Figure 9 Experimental Ce L III -edge XANES spectra of Ce 3+ -doped LuAG (red), Ce 3+ , Li + -co-doped LuAG (blue) and Ce 3+ ,Mg 2+ -co-doped LuAG (black), respectively. Only Ce 3+ , Mg 2+ -co-doped LuAG shows a weak noisy signal at 5738 eV and then the presence of Ce 4+ ion. Thickness of samples=0,53mm. presence of Ce Accepted Article presence of Ce 4+ Accepted Article 4+ ion.

Figure 9

 9 Figure 9 Experimental Accepted Article Experimental + Accepted Article + -Accepted Article -co

Table 1

 1

	absorption band at Accepted Article absorption band at around 275 nm Accepted Articlea round 275 nm  Accepted Article  8 Accepted Article 8 S Accepted Article S 7/2 Accepted Article 7/2 emission lines and the Ce Accepted Article emission lines and the Ce radiative energy transfer since Ce Accepted Article radiative energy transfer since Ce GAGG there is two Accepted Article GAGG there is two contributions to shortening of the decay: Accepted Article contributions to shortening of the decay: and another Accepted Article and another one associated with the energy transfer from Gd Accepted Article one associated with the energy transfer from Gd the energy migration and energy transfer processes in the set of un Accepted Article the energy migration and energy transfer processes in the set of un component garnet single crystals has ben recently Accepted Article component garnet single crystals has ben recently Radioluminescence spectra under excitation by X Accepted Article Radioluminescence spectra under excitation by X GAGG and Ce Accepted Article GAGG and Ce 3+ Accepted Article 3+ , Accepted Article , Mg Accepted Article Mg 2+ Accepted Article 2+ composed of the two usual transitions 5d Accepted Article composed of the two usual transitions 5d mention the decreasing of the Accepted Article mention the decreasing of the creation of Ce Accepted Article creation of Ce 4+ Accepted Article 4+ in these crystals and then the reduction of Ce Accepted Article in these crystals and then the reduction of Ce Scintillation decays Accepted Article Scintillation decays Ce Accepted Article Ce 3+ Accepted Article 3+ , Accepted Article ,0.1% Accepted Article 0.1% Mg Accepted Article Mg Accepted Article (662 keV) Accepted Article (662 keV) are shown in Accepted Article are shown in corresponding to the fast and slow radiative recombination of the Ce Accepted Article corresponding to the fast and slow radiative recombination of the Ce The decay Accepted Article The decay-Accepted Article -time components of Accepted Article time components of doped GAGG crystals Accepted Article doped GAGG crystals

Accepted Article Table 1. The most prominent observation is the shortening of the scintillation decay Accepted Article . The most prominent observation is the shortening of the scintillation decay time after Mg

	Accepted Article time after Mg 2+ Accepted Article 2+ co Accepted Article co-Accepted Article -doping. Without Mg Accepted Article doping. Without Mg decay time (81%) and in Accepted Article decay time (81%) and in ns decay time. Accepted Article ns decay time. The shortening was already noticed first in Ce

Light output % Fast decay time (ns)/ ratio (%) Slow decay time (ns)/ ratio (%) Timing resolution/ps

	0 % Mg	100	92.2/81	296/19	452
		79	45.2/58	135/42	196
		93.5	39.6/53	132/47
	In Ce 3+ -doped Gd 3 Al 2 Ga 3 O 12 single crystals, we should add the scintillation decays were
	accelerated by both Ca 2+ and Mg 2+ co-dopants. Comparing to Ca 2+ co-doping,
	the Mg 2+ co-doped samples showed much faster decay and comparatively smaller light output
	decrease with increasing Mg 2+ dopant concentration but of enough intensity to be applied.
	The best sample corresponds to 100 at. ppm Mg 2+ with performances mentioned in Table1.
	Finally, Ce 3+ , Mg 2+ -co-doped GAGG shows improvement of both, shortening of the decay
	-time (45.2 ns on 58% of the decay), a fast timing resolution (196 ps, close to 180 ps,
	the value of the LYSO crystal) and a high light yield (44000 photon/MeV), corresponding to
	only 79% of the Ce 3+ -doped Gd 3 Al 2 Ga 3 O 12 light output, so that is really a promising
	scintillator for application as positron emission tomography.

Evidence and evaluation of stable Ce Accepted Article Evidence and evaluation of stable Ce

	Accepted Article scintillation output. Accepted Article scintillation output. Samples Light output Accepted Article 3+ Accepted Article 3+ -Accepted Article -doped Gd Accepted Article doped Gd 3 Accepted Article 3 accelerated by both Accepted Article accelerated by both co Accepted Article co-Accepted Article Accepted Article decrease with increasing Mg best sample corresponds to Accepted Article best sample corresponds to Accepted Article Finally, Accepted Article Finally, Ce Accepted Article Ce 3+ Accepted Article 3+ , Accepted Article , Mg Accepted Article Mg 2+ Accepted Article 2+ 45.2 Accepted Article 45.2 ns on 58% of the decay) Accepted Article Accepted Article only 79% of the Accepted Article only 79% of the Ce Accepted Article Ce scintillator for application as Accepted Article scintillator for application as , Mg Accepted Article Mg 2+ Accepted Article 2+ -Accepted Article -co Accepted Article co-Accepted Article -doped doped Accepted Article Previous articles consider

Samples

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-doped samples showed much faster decay and comparatively smaller light output Accepted Article doped samples showed much faster decay and comparatively smaller light output decrease with increasing Mg ns on 58% of the decay) the value of the LYSO crystal) and Accepted Article the value of the LYSO crystal) and 3+ Accepted Article 3+ , Accepted Article

  It is worth mentioning an absolute quantitative estimation with XANES needs to know the absorption cross sections for both, the Ce 3+ and Ce 4+ transitions. If the 4f5d electric dipole allowed transition cross section is known as high in different materials, the Ce 4+ charge transfer absorption band is not known. However, a roughly comparison can be done from Figure2in Ce 3+ , Mg 2+ -co-doped GAGG garnet single-crystal when comparing the absorption areas of the first Ce 3+ 4f5d absorption band at 450 nm with the Ce 4+ charge transfer absorption band in the 225-325 nm UV spectral range after deducing both Gd 3+ absorption lines and the second Ce 3+ 4f5d absorption band at 340 nm. At a glance, areas shows values of the same order of magnitude giving some sense at the Ce 3+ /Ce 4+ ratio deduced from XANES spectra. In Ce 3+ , Mg 2+ -co-doped LYSO and Ce 3+ , Ca 2+ -co-doped LYSO7 , the linear combination fit has been applied and provided 80/20 and 65/35 ratios, respectively, so that, with Mg 2+ co-doping the same amounts of Ce 4+ are observed in GAGG and LYSO.

	Accepted Article detected (HERFD) Accepted Article detected (HERFD) mode at the Fame Accepted Articlem ode at the Fame 5.68 keV to 5.85 keV using a Si Accepted Article 5.68 keV to 5.85 keV using a Si helium was used between the samples, the crystal analyzer spectrometer and the detector, to Accepted Article helium was used between the samples, the crystal analyzer spectrometer and the detector, to avoid partial beam absorption by the air. The signal was recorded with a five Accepted Article avoid partial beam absorption by the air. The signal was recorded with a five analyzer and a Vortex Accepted Article analyzer and a Vortex FWHM). The energy calibration was done using the CeO Accepted Article FWHM). The energy calibration was done using the CeO determined by linear combination of XANES spectra of standards, Accepted Article determined by linear combination of XANES spectra of standards, reference for Ce Accepted Article reference for Ce 4+ Accepted Article 4+ and Accepted Article and experimental data were analyzed using the Accepted Article experimental data were analyzed using the evaluated the Ce Accepted Article evaluated the Ce 3+ Accepted Article 3+ /Ce Accepted Article /Ce corresponding to different Accepted Article corresponding to different The Ce L Accepted Article The Ce L III Accepted Article III threshold Accepted Article threshold trivalent reference Accepted Article trivalent reference (characteristic peak at 5738 eV) Accepted Article (characteristic peak at 5738 eV) dispersed to characterize each oxydation state. The Accepted Article dispersed to characterize each oxydation state. The 0.5% Ce Accepted Article 0.5% Ce 3+ Accepted Article 3+ -Accepted Article -doped YAG, 0.5% Accepted Article doped YAG, 0.5% GAGG Accepted Article GAGG garnet Accepted Article garnet t is clear that we do not observe any signal from Ce Accepted Article t is clear that we do not observe any signal from Ce Ce Accepted Article Ce 3+ Accepted Article 3+ Accepted Article -Accepted Article -doped Accepted Article doped crystals without any Mg Accepted Article crystals without any Mg The estimation Accepted Article The estimation Accepted Article linear combination fit method of the two Accepted Article linear combination fit method of the two choose to Accepted Article choose to drawn the simulation of Ce Accepted Article

drawn the simulation of Ce

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  .1 and 3.2 wt ppm (0.023 and 0.014 mol%), respectively. Li concentration was decreased with increasing solidification fraction. This indicates evaporation of Li oxide during the crystal growth. Then, the real composition was 0.023-0.014mol.% Li + , Ce 0.25-0.5 mol.% Ce 3+ -co-doped GAGG. We can only say that the real concentration with respect of the nominal concentration in LuAG may vary in the same way as for GAGG due to the same conditions of crystal growths.Figure9are the first spectra ever recorded for these crystals. First, we note it make sense that Ce 3+ -doped LuAG does not contain any trace of Ce 4+ , as well non-co-doped Ce 3+ -doped YAG and Ce 3+ -doped GAGG. Only Ce 3+ , Mg 2+ -co-doped LuAG shows a weak noisy signal at 5738 eV and then shows the presence of traces of Ce 4+ ion consistently with Ce 3+ , Mg 2+ -co-doped GAGG. To confirm this Ce 4+ signal, let's remind Nikl & al19 have reported the Ce 4+ charge transfer absorption band clearly observed and enhanced with increasing concentration of Mg 2+

	Accepted Article material defects giving rise to shallow traps is not refered. Then, the Ce Accepted Article Accepted Article revealed the effect of the co nm which is Accepted Article nm which is clearly enhanced with increasing concentration of Li Accepted Article clearly enhanced with increasing concentration of Li scintillation decays were accelerated and light yields were increased by Li Accepted Article scintillation decays were accelerated and light yields were increased by Li Ca Accepted Article Ca 2+ Accepted Article 2+ /Mg Accepted Article /Mg 2+ Accepted Article 2+ /Li Accepted Article /Li und with the previous analysis of Ce Accepted Article und with the previous analysis of Ce role demonstrated of Ce Accepted Article role demonstrated of Ce these materials unique and what they all have in common. Accepted Article these materials unique and what they all have in common. Single crystals have been grown also at IMR, Tohoku Unversity, and co Accepted Article Single crystals have been grown also at IMR, Tohoku Unversity, and co 2+ Accepted Article 2+ but also with Li Accepted Article but also with Li GAGG crystals. Accepted Article GAGG crystals. As for segregation phenomenon, measurements have been done Accepted Article As for segregation phenomenon, measurements have been done doped Accepted Article doped GAGG The Ce L III -edge XANES spectra of 0.5% Ce 3+ -doped Accepted Article 0.5% Ce 3+ ,0.5% Li + -co-doped LuAG and 0.5% Ce 3+ , 0.3% Mg 2+ -co-doped LuAG shown in LuAG,

is not refered. Then, the Ce by micro pulling down method and, namely the scintillation decay and light yield have Accepted Article by micro pulling down method and, namely the scintillation decay and light yield have revealed the effect of the co GAGG This article is protected by copyright. All rights reserved were 5

  , Mg 2+co-doped Gd 3 Al 2 Ga 3 O 12 (GAGG) garnet host, grown at the Tohoku University, Sendai, for applications which require fast timing resolution and high light yield such as PET.The co-doping of Ce 3+ ion by Mg 2+ one has revealed the creation of Ce 4+ ion, assigned from the charge transfer absorption band in UV, due to charge compensation effect, occuring in the scintillation mechanisms, in addition of the usual Ce 3+ ion, like in Ce 3+ , Ca 2+ -co-doped GAGG and Ce 3+ , Mg 2+ -co-doped LYSO oxydes. The presence and then the role played by the stable Ce 4+ ion in this garnet for the scintillation mechanisms has been demonstrated by XANES spectroscopy at the Ce L III threshold of the European Synchrotron Radiation Facility (ESRF) in Grenoble. In addition, the estimation of the Ce 3+ /Ce 4+ ratio measured by a linear combination fit method is 80/20 and 82.5/17.5 from the simulation of Ce 4+ curves. XANES analysis also reveals the creation of Ce 4+ in Ce 3+ , Mg 2+ -co-doped LuAG crystal, another important scintillator, like it was shown in transparent ceramics of the same composition and confirms the absence of stable Ce 4+ in crystals like Ce 3+ -doped GAGG, Ce 3+doped LuAG and Ce 3+ -doped YAG, respectively, without any Mg 2+ ion. It means that the scintillation mechanisms are probably the same in Ce 3+ , Mg 2+ -co-doped GAGG and Ce 3+ , Mg 2+ -co-doped LuAG garnet crystals with the occurrence of stable Ce 4+ ion. On the other side, any trace of Ce 4+ ion has not been detected in Ce 3+ , Li + -co-doped LuAG, in which Mg 2+ has been substituted by Li + . Nevertheless, this crystal is considered as efficient for scintillation. Despite some discussion in literature on the nature of luminescence centers, the real mechanisms of scintillation with Li + ions have to be more clearly understood.

J Ce 4+ CTB absorption Ce 3+ 4f g 5d absorption Gd 3 Al 2 Ga 3 O 12 :Ce 3+ 0.5% Mg 2+ 0.5%, d=0.87 mm Gd 3 Al 2 Ga 3 O 12 :Ce 3+ 0.5%, d=0.57 mm Absorbance (a.u.) Wavelength (nm) Gd 3+ absorption lines

  Absorption spectra at room temperature of the 0.5% Ce 3+ -doped GAGG and 0.5% Mg 2+ , 0.5% Ce 3+ -co-doped GAGG crystals, at room temperature.

		Absorption spectra
	8 S 7/2 g	6 I J
	Figure 2

8 S 7/2 g 6 P

0,0 0,2 Accepted Article 0,2 0,4 Accepted Article 0,4 0,6 Accepted Article 0,6 0,8 Accepted Article 0,8 1,0 Accepted Article 1,0

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