Victor A Drits

Bruno Lanson
email: bruno.lanson@obs.ujf-grenoble.fr

Catherine Bougerol-Chaillout

Anatoli I Gorshkov

Alain Manceau

Structure of heavy-metal sorbed birnessite. Part 2: Results from electron diffraction

dimensional periodic distribution of interlayer Me located above and below vacant octahedra sharing three corners with Mn layer octahedra (TC sites). Tetrahedral coordination of these Me cations in TC sites, as in ZnBi, leads to the formation of strong H-bonds between adjacent layers. A similar incommensurate effect occurs in one layer hexagonal MeBi if octahedrally coordinated Me cations periodically distributed along the a axis are located above and/or below empty tridentate cavities sharing three edges with Mn layer octahedra ( VI TE sites, PbBi). SAED patterns of type V contain only sub-cell reflections and were observed mostly for PbBi and CuBi micro-crystals. Three different conditions can lead to the absence of supercell reflections: (1) a low amount of sorbed Me (PbBi); (2) the presence of Me having a similar scattering power as that of Mn on a single side of vacant layer sites (CuBi); or (3) a random distribution of interlayer species.

ABSTRACT

Selected-area electron diffraction (SAED) and energy dispersive analysis were used to study the structure of synthetic heavy-metal sorbed birnessites (MeBi). Samples were prepared by equilibrating a suspension of Na-rich buserite (NaBu) at pH4 in the presence of various heavy metal cations (Me), including Pb, Cd, Zn, and Cu.

Five main types of SAED patterns were observed. Types I and II were observed only for ZnBi micro-crystals, and they both consist of two super-cell reflection networks related by a mirror plane parallel to the a * c * plane. In direct space, these twinned networks correspond to the hexagonal supercells with A H = B H = 7 b/ 3 , and A H = B H = 7 b, for ZnBi type I and II, respectively. In the two varieties, the supercells result from an ordered distribution of vacant layer octahedra capped by interlayer Zn in ZnBi layers. This distribution is described by a hexagonal cell with A H = 7 b. In ZnBi micro-crystals of type I, interstratified twinned right-and left-handed fragments are similar to chalcophanite (ZnMn 3 O 7 -3H 2 O -Wadsley 1955; [START_REF] Post | Chalcophanite, ZnMn 3 O 7 -3H 2 O: New crystal-structure determinations[END_REF], and distributions of vacant layer octahedra from adjacent layers are regularly shifted with respect to each other by 1/3 of the long diagonal of the hexagonal layer unit cell. In ZnBi micro-crystals of type II, distributions of vacant layer octahedra are not regularly shifted from one layer to the adjacent one. SAED patterns of types III and IV occur for PbBi, ZnBi, and CdBi micro-crystals and contain super-cell reflections distributed parallel to [100] * with a periodicity which is not commensurate with that of the MeBi sub-structure (a * /2.15 and a * /5.25, respectively). The super-cell reflections result from the ordered distribution within MeBi layers of vacant layer sites capped by Me as pairs along the a axis. Within each pair, vacant sites are separated by 2a for type III, and by 5a for type IV. In one-layer monoclinic structures, the apparent incommensurability arises from the +a/3 shift between adjacent layers having a similar one-

INTRODUCTION

Buserite and birnessite are layered hydrous manganese oxides, the layers of which are built up of edge-sharing Mn octahedra. Hydrated cations are present in their interlayer spaces to compensate the negative layer charge that arises from the presence of heterovalent Mn and/or vacant sites in the octahedral layers. Because buserite and birnessite can fix heavy metals (Co, Pb, Cd, Cu, Zn, …) in contaminated soils and aqueous systems [START_REF] Taylor | The association of trace elements with maganese minerals in Australian soils[END_REF][START_REF] Mckenzie | The sorption of cobalt by manganese minerals in soils[END_REF]McKenzie , 1980;;[START_REF] Burns | Mineralogy of ferromanganese nodules[END_REF][START_REF] Bendell-Young | The relative importance of manganese and iron oxides and organic matter in the sorption of trace metals by surficial lake sediments[END_REF]Manceau et al. 2000 and references therein), it is essential to determine the structure of heavy-metal sorbed birnessites (MeBi).

Selected-area electron diffraction (SAED) has often been used, sometimes to complement X-ray diffraction (XRD), to study the structure of birnessite, and especially to reveal the super-cell periodicities in these phyllomanganates [START_REF] Post | Crystal structure determinations of synthetic sodium, magnesium, and potassium birnessite using TEM and the Rietveld method[END_REF]Manceau et al. 1992;[START_REF] Kuma | Crystal structures of synthetic 7 Å and 10 Å manganates substituted by mono-and divalent cations[END_REF][START_REF] Drits | The structure of monoclinic Na-rich birnessite and hexagonal birnessite. Part 1. Results from X-ray diffraction and selected area electron diffraction[END_REF][START_REF] Drits | Sub-and super-structure of four-layer Ca-exchanged birnessite[END_REF]. However, the origin of super-cell reflections observed in SAED patterns of natural and synthetic birnessites remained unclear for a long time. [START_REF] Giovanoli | Über Oxidhydroxide des vierwertigen Mangans mit Schichtengitter. 1. Mitteilung: Natriummangan(II,III)manganat(IV)[END_REF] described super-cell reflections of high-pH Na-rich synthetic birnessite, hereafter referred to as NaBi, and attributed them to the ordering of vacant layer octahedra capped by interlayer cations as in chalcophanite [START_REF] Wadsley | The crystal structure of chalcophanite, ZnMn 3 O 7 .3H 2 O[END_REF][START_REF] Post | Chalcophanite, ZnMn 3 O 7 -3H 2 O: New crystal-structure determinations[END_REF]. The same interpretation was proposed by [START_REF] Chukhrov | Birnessite characterization[END_REF] for a Carich birnessite sample, and by [START_REF] Kuma | Crystal structures of synthetic 7 Å and 10 Å manganates substituted by mono-and divalent cations[END_REF] for synthetic NaBi samples exchanged with different monovalent and divalent cations. On the contrary, [START_REF] Post | Crystal structure determinations of synthetic sodium, magnesium, and potassium birnessite using TEM and the Rietveld method[END_REF] suggested that high-pH synthetic birnessites consist of vacancy-free layers and, therefore, that super-cell reflections more likely result from an ordered distribution of interlayer cations. This conclusion was supported by the contrasting distributions of super-cell reflections observed for NaBi and Mg-exchanged birnessite. However, in their structural model, Mg 2+ cations are located almost above/below layer octahedral sites [START_REF] Post | Crystal structure determinations of synthetic sodium, magnesium, and potassium birnessite using TEM and the Rietveld method[END_REF], and [START_REF] Manceau | X-ray absorption spectroscopic study of the sorption of Cr(III) at the oxide-water interface[END_REF] showed that this position may be occupied only if underlying octahedral sites are vacant.

New insight into this problem was provided by [START_REF] Drits | The structure of monoclinic Na-rich birnessite and hexagonal birnessite. Part 1. Results from X-ray diffraction and selected area electron diffraction[END_REF] and [START_REF] Silvester | The structure of monoclinic Na-rich birnessite and hexagonal birnessite. Part 2. Results from chemical studies and EXAFS spectroscopy[END_REF]. Using XRD, SAED, extended X-ray absorption fine structure (EXAFS) spectroscopy, and solution chemistry, these authors demonstrated that NaBi consists of vacancy-free layers, and that super-cell reflections observed in its SAED patterns result from an ordered distribution of interlayer cations. In contrast, low-pH synthetic birnessites (HBi) resulting from the low-pH equilibration of Na-rich buserite (NaBu) contain significant numbers of vacant layer sites capped by interlayer Mn cations (Mn interlayer ), but their SAED patterns are either devoid of super-cell reflections or contain extremely faint super-cell reflections [START_REF] Drits | The structure of monoclinic Na-rich birnessite and hexagonal birnessite. Part 1. Results from X-ray diffraction and selected area electron diffraction[END_REF][START_REF] Manceau | Structural mechanism of Co 2+ oxidation by the phyllomanganate buserite[END_REF][START_REF] Silvester | The structure of monoclinic Na-rich birnessite and hexagonal birnessite. Part 2. Results from chemical studies and EXAFS spectroscopy[END_REF]. Indeed, systematic location of Mn interlayer either above or below vacant layer sites induces a low electrostatic potential contrast between occupied and vacant octahedral layer sites, and the intensity of super-cell reflections is strongly decreased even if vacant layer sites are regularly distributed [START_REF] Drits | The structure of monoclinic Na-rich birnessite and hexagonal birnessite. Part 1. Results from X-ray diffraction and selected area electron diffraction[END_REF][START_REF] Lanson | Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH[END_REF].

In the present article, SAED and energy dispersive analysis (EDA) were used to determine the origin of the different super-cell reflection distributions observed for individual micro-crystals of heavy-metal sorbed birnessites equilibrated at pH 4. The sub-structure of these MeBi samples, and more especially their layer stacking modes, the nature and content of stacking faults, the amounts of vacant layer sites and the location, coordination and amounts of interlayer cations (Mn interlayer and Me) were determined by XRD and EXAFS spectroscopy (Table 1). The combined use of XRD (Lanson et al. 2002a), SAED, EDA (present article), and EXAFS spectroscopy [START_REF] Manceau | Structure of heavy metal sorbed birnessite. Part 3: Results from powder and polarized EXAFS spectroscopy[END_REF] provides new insight not only into the structural and chemical features of these MeBi, but also into the structural mechanism of formation as a result of the equilibration of high-pH NaBu at low pH in the presence of heavy metal cations.

EXPERIMENTAL

The birnessite samples studied in the present article are those described in the two companion papers (Lanson et al. 2002a;[START_REF] Manceau | Structure of heavy metal sorbed birnessite. Part 3: Results from powder and polarized EXAFS spectroscopy[END_REF], and their chemical compositions are given in Table 1 of Lanson et al. (2002a). In the following, they will be denoted according to their Me/Mn ratios (e.g., ZnBi 122 standing for a 0.122 Zn/Mn ratio).

Aliquots from diluted suspensions were dried onto carbon-coated copper or aluminum grids.

The samples were then mounted on a tilting sample holder. SAED patterns were recorded using either a JEOL JEM100c equipped with a Kevex X-ray spectrometer and operated at 100 kV or a Philips CM300ST equipped with a Kevex spectrometer system for energydispersive analysis (EDA) and operated at 300 kV. The standards used for the semiquantitative EDA were SrCuO 2 , Y 2 BaCuO 5 , LaMnO 3 , La 2 CuO 4 , and BaPbO 3 .

RESULTS

Features of SAED patterns

The different types of SAED patterns observed for the various birnessite samples are shown in Figures 12345 All super-cell reflections of the twinned supercells may be described using a common supercell with A or = 7a, B or = 7b and γ = 90°, or A H = B H = 7b and γ = 120° in direct space.

SAED patterns of type III.

In contrast to all other SAED patterns, SAED patterns of type III contain weak sub-cell reflections with h + k = 2n + 1 (Fig. 3a), which indicates that these crystals have a primitive rather than a base-centered unit cell. In addition, along the a * axis there is a weak super-cell reflection between any pair of sub-cell reflections, both intense and weak ones. These weak super-cell reflections are located at almost equal distance from the two sub-cell reflections, and a careful measurement has shown that they are regularly spaced by a * /2.15 (Fig. 3b), meaning that the periods of the sub-and super-cells are incommensurate along the a axis. The super-cell parameters are A = 2.15 a, B = b and γ = 90°i n direct space.

SAED patterns of type IV. These patterns are also characterized by an incommensurate distribution of super-cell reflections along the a * axis. The interval between two successive sub-cell reflections is divided into five almost equal intervals by four super-cell reflections (Fig. 4a). A careful measurement of these intervals has shown that the distance between two nearest super-cell reflections is slightly smaller than 2a * /5 and is actually equal to 2a * /5.25.

All super-cell reflections can be described using a supercell with A or * = a * /5.25, B or * = b * and γ * = 90°, corresponding to the A or = 5.25 a, B or = b and γ = 90° base-centered unit cell in direct space. Because the value of A or is close to 5a, sub-and super-cell reflections coincide at low h values, whereas at higher h values positions of these reflections are further away resulting in elongated spots (Fig. 4b).

SAED patterns of type V. SAED patterns of this type contain only sub-cell reflections distributed according to hexagonal symmetry (Fig. 5).

Occurrence of the different types of SAED patterns

Individual particles of each sample, depending mostly on the identity and amount of Me, have different SAED patterns. For example,

EDA results

Relative proportions of Me, Mn, and Na, as determined from EDA in each type of microcrystal previously identified by SAED, are reported in Table 3. As one may note in this table, the type of SAED pattern depends on the Me/Mn ratio for ZnBi, CdBi, and PbBi microcrystals. In particular, this ratio decreases in the sequence of SAED type II > I > IV > III in ZnBi micro-crystals, V > IV > III in CdBi micro-crystals and IV > V in PbBi micro-crystals.

The Me/Mn ratios obtained from EDA are close to those calculated from chemical analysis (Lanson et al. 2002a) Lanson et al. 2002a;[START_REF] Manceau | Structure of heavy metal sorbed birnessite. Part 3: Results from powder and polarized EXAFS spectroscopy[END_REF].

SAED patterns of type I

This type of SAED pattern was observed only in ZnBi 69 (Table 2). To determine the positions of Zn in the interlayer of this structure, let us consider the 2D distribution of lattice nodes in one of the two twinned networks. According to XRD (Lanson et al. 2002a), adjacent layers in ZnBi are predominantly shifted with respect to each other by +a/3. Figure 6 To reconstruct the three-dimensional (3D) distribution of Zn in ZnBi 69, let us consider three adjacent interlayers formed by four successive layers labeled from 1 to 4 (Fig. 7). In the first interlayer (Figs. 6, 7a), the Zn cations linked to layers 1 and 2 have coordinates (0,0) and (A/3, 2B/3), respectively. To determine the position of Zn above layer 2, one must take into account the fact that Zn has to be located above and below vacant layer sites, in agreement with EXAFS and XRD results obtained for ZnBi 122 and 128 (Table 1). Furthermore, because Zn and Mn have similar scattering powers, the presence of Zn as pairs on each side of vacant layer sites is necessary to increase the electronic density of these sites as compared with that of occupied Mn layer sites, and thus to induce significant intensity for super-cell reflections. As a consequence, Zn above layer 2 is also at (A/3, 2B/3), Zn cations coordinated to each side of layer 3 have coordinates (2A/3, B/3), and the position of Zn cations linked to the fourth layer is (0,0), the same as those linked to layer 1 (Fig. 7c). Only such a distribution of Zn in successive interlayers allows preservation of the same unit cell for the whole crystal.

This distribution implies a three-layer periodicity with A = B = 7.536 Å, C = 3c = 21.18 Å, α = β = 90° and γ = 120°. In projection along the c * axis, Zn cations from successive interlayers occupy sites with coordinates (0, 0), (A/3, 2B/3), and (2A/3, B/3) with equal probability, resulting in the reduced cell deduced from SAED patterns. This structure, corresponding to one of the twin networks, is identical to that of chalcophanite, which has a similar three-layer unit cell with A = B = 7.53 Å, C = 3•6.93 = 20.79 Å, γ = 120°, and space group R3bar [START_REF] Post | Chalcophanite, ZnMn 3 O 7 -3H 2 O: New crystal-structure determinations[END_REF]. In this mineral, successive layers are shifted with respect to each other by 1/3 of the unit cell long diagonal.

However, in contrast to the chalcophanite structure described above, these ZnBi crystals are twinned, most likely as a result of a specific distribution of vacant octahedra in adjacent layers. When the vacant site of the lower layer is set at the unit cell origin, and the vacant site of the upper layer has coordinates (A/3, 2B/3), vacant sites of the upper layer may be described by either one of the two hexagonal unit cells connected by a mirror plane ac passing through the vacant site (A/3, 2B/3 -Fig. 8). The right-handed cell (regular dashed line) has the same orientation as that of the lower layer, whereas the left-handed cell (irregular dashed line) is rotated with respect to the right-handed one by 21.79° around a common origin (A/3, 2B/3), so that the A axes of both cells form the same 10.89° angle with the a axis (Fig. 8). As a result, the complete distribution of super-cell reflections is described by the unique supercell with A = B = 7b and γ = 120°, which is common to both right-and left-handed structural fragments. In addition to the 7 vacant layer sites from the lower layer (large solid circles), this supercell contains 7 vacant layer sites from the upper layer distributed according either to the right-handed (large open circles) or to the left-handed (large shaded circles) cell (Fig. 8). On Figure 8, note that the right-handed distribution of vacant layer sites in the upper layer allows the presence of Zn above and below each layer vacancy, as in this case Zn sites within the same interlayer are all far enough from each other to permit the formation of isolated Zn octahedra (large solid and open circles -Fig. 8).

In the alternative distribution of vacant octahedra described by the left-handed unit cell (large shaded circles -Fig. 8), three out of the seven available TC sites from the upper layer are too close to those from the lower layer for Zn to occupy the two sites simultaneously (Fig. 8). In addition to the short Zn-Zn distance (~3.1 Å), pairs of Zn cations located simultaneously in these two sites would share edges formed by H 2 O molecules coordinated to Zn (H 2 O Zn ) that cannot screen the electrostatic repulsion between the two Zn cations. in ZnBi 69 interlayers will be described below.

SAED patterns of type II

This type of SAED pattern is overwhelmingly present in ZnBi 122 and 128 crystals and occurs less frequently in ZnBi 69 (Table 2). Figure 9 shows the mutual arrangement of Mn layer in two adjacent ZnBi layers in projection along the c * axis and the lattice nodes of the A H = B H = b 7 = 7.535 Å and γ = 120° unit cell describing super-cell reflections of one of the two networks. The origin of the unit cell is set to coincide with the center of a vacant octahedron from the lower layer (large solid circles -Fig. 9). As for crystals of type I in ZnBi 69, the distribution of Zn located in TC sites in individual layers of type-II crystals is similar to that described for chalcophanite. In contrast, the distribution of vacant sites in adjacent layers of these type-II crystals differs substantially from that of chalcophanite. In the chalcophanite structure, the specific distribution of vacant sites in adjacent layers (Fig. 7) reduces the periodicity of the 2D lattice from b 7 to b 7 / 3 (Fig. 6). On the contrary, to preserve the 2D b 7 periodicity in projection along the c * axis for the distribution of Zn, the vacant octahedron of the upper layer can be located in one of the three sites having coordinates (0.476, 0.381), (0.905, 0.524), and (0.619, 0.095) in the hexagonal cell (sites 1, 2 and 3, respectively -Fig. 9), in addition to the (1/3, 2/3) site. These sites are far enough from the nearest octahedral sites of the lower layer not to induce significant electrostatic repulsion of Zn cations coordinated to the two layers, and vacant layer octahedra may be capped by Zn on their two sides.

In such a structure, each layer has the highest possible symmetry in projection along the c axis. However, the superposition of two successive layers dramatically decreases their common symmetry, as the unit cell loses all elements of symmetry except for the translation along A H and B H (Fig. 9).

The structural model used to describe SAED patterns of type I may also be invoked to explain the twinning observed in SAED patterns of type II, and for a given distribution of vacancies in the lower layer, there are two possibilities for the vacancy distribution in the upper layer (Fig. 9). If vacancies of both layers possess the same azimutal orientation (large solid and open circles, solid and regular dashed lines -Fig. 9), then Zn may systematically coordinate both sides of the vacant layer sites. In contrast, if vacancies of the lower and upper in ZnBi samples (Table 1); the possible distribution of Zn and Mn interlayer in these samples will be described below.

SAED patterns of types III and IV

To combine the observed incommensurate periodicity along the a axis with a periodic distribution of vacant layer sites capped by Me in a MeBi structure, two conditions should be fulfilled. The first is a proper mutual arrangement of adjacent layers. For example, it will be shown that a relative displacement of adjacent layers with respect to each other by ±a/3 is favorable for this incommensurate periodicity along the a axis. The second condition includes the periodic distribution of vacant sites within layers and from one layer to the adjacent one, with some specific limitations described hereafter.

As an example, one may consider two adjacent layers shifted with respect to each other by +a/3. To obtain an average periodicity of Me ordering equal to 2.25a, vacant octahedra may form pairs along the a axis within each layer, with vacant sites capped by Me being separated by 2a in each pair (Fig. 10). Vacancy pairs belong alternatively to the upper layer and to the lower layer and are distributed periodically along the a axis. If the first vacancy pair belongs to the lower layer (bottom of Figure 10a), the next one along the a axis belongs to the upper layer and is separated from the first one by 2.33a in projection along the c * axis (Fig. 10). The third vacancy pair belongs again to the lower layer, and the distance between the nearest vacancies of the second and third vacancy pairs is 2.67a (Fig. 10). In such a distribution, the distance between the first and third vacancy pairs for the lower layer, as well as between the second and fourth pairs for the upper one, is (2 + 2.33 +2 + 2.67)a = 9a, corresponding to four intervals between successive Me cations (Fig. 10). As a result, the average periodicity of the Me distribution along the a axis is equal to 2.25a. Similar diffraction effects can occur for 1H MeBi structures (e.g., PbBi 58 -Table 2) if vacant layer sites are distributed periodically along the a axis, assuming that Me is not located in TC sites but rather is above or below empty tridentate cavities, sharing three edges with neighboring Mn layer octahedra (TE sites, position (-0.333, 0) -Fig. 9 in Lanson et al. 2002a).

For PbBi 58, the presence of Pb in this alternative position is consistent with the results obtained from XRD and EXAFS (Lanson et al. 2002a;[START_REF] Manceau | Structure of heavy metal sorbed birnessite. Part 3: Results from powder and polarized EXAFS spectroscopy[END_REF]. Figure 11 shows two adjacent layers in which pairs of Me cations located in TE sites are distributed along the a axis with A S = (2 + 2.67 + 2 + 2.33)a = 9a periodicity in projection along the c * axis, yielding an average 2.25a periodicity. This period may be reduced to match the experimental one as for the 1M layer sequence.

A similar approach may be used to describe the A = 5.25 a periodic distribution of Me along the a axis (SAED patterns of type IV) for 1M micro-crystals. Within each layer, pairs of vacant sites capped by Me are separated by 5a to match the observed period (Fig. 12).

Because of the +a/3 relative displacement between adjacent layers, the distance between the nearest vacancies of the first and the second vacancy pairs is 5.33a whereas that between the second and third vacancy pairs is 5.67a, leading to an average 5.25a distance between Me cations (Fig. 12). To Finally, the A = 5.25 a periodic distribution of Me is also encountered in some CdBi crystals that have a 1H layer stacking. The distribution of Cd in these crystals will be discussed below, as XRD showed that Cd is systematically present in VI TC sites (Table 1).

SAED patterns of type V

All crystals of CuBi, PbBi 6, and some of the other MeBi crystals exhibit SAED patterns of type V. Three hypotheses may account for the absence of super-cell reflections in these micro-crystals. The first is the presence of Me having a scattering power similar to that of Mn located either above or below vacant layer octahedra. In this case, in projection along c * , vacant and occupied octahedra have similar scattering power, and super-cell reflections are extremely weak or absent, even if Me cations are periodically distributed. This hypothesis is supported by the extremely weak super-cell reflections observed in the SAED patterns of HBi, in spite of the ordered distribution of vacant layer sites [START_REF] Drits | The structure of monoclinic Na-rich birnessite and hexagonal birnessite. Part 1. Results from X-ray diffraction and selected area electron diffraction[END_REF]). This may be the case for CuBi 156, because this sample contains a large amount of sorbed metal, a condition which seems likely to induce a periodic distribution of these cations within individual interlayers.

The second hypothesis is a low Me content. This hypothesis can account for the absence of super-cell reflections in PbBi 6, because the low concentration of Pb (Pb/Mn = 0.006) is not conducive to long-range interactions. This hypothesis is supported by the SAED and EDA analysis of PbBi 58, which showed that micro-crystals having a Pb/Mn ratio equal to 0.067 are characterized by type-IV SAED patterns whereas micro-crystals with a lower Pb/Mn ratio (Pb/Mn = 0.043) exhibit type-V SAED patterns.

According to the third hypothesis, type-V SAED patterns result from a random distribution of Me cations, so that their average periodicity coincides with that of the layer, as in the case of CdBi. As the scattering power of Cd is significantly higher than that of Mn, strong super-cell reflections should arise from an ordered distribution of Cd, even if these cations were located either above or below vacant layer sites. This is the case for crystals of types III and IV, which are characterized by super-cell reflections along the a axis in spite of a low Cd content (Cd/Mn = 0.070-0.081). The absence of super-cell reflections in SAED patterns of crystals having a higher content of Cd (Cd/Mn = 0.154) can be considered as evidence for the random distribution of vacant sites among all available octahedral layer sites.

The results of SAED pattern interpretation are summarized in Table 4 to show relationships between the structural diversity of the MeBi samples and the identity of Me.

DISCUSSION

Low-pH transformation of NaBu as a function of Me

In NaBu synthesized at high pH following the protocol of [START_REF] Giovanoli | Über Oxidhydroxide des vierwertigen Mangans mit Schichtengitter. 1. Mitteilung: Natriummangan(II,III)manganat(IV)[END_REF], the distribution of heterovalent Mn layer is ordered, as Mn 3+ cations are segregated in Mn 3+ -rich rows parallel to [010] and separated from each other along the a axis by two Mn 4+ rows.

Because Mn 3+ octahedra are elongated along the a axis, this ordered distribution violates the hexagonal layer symmetry and results in the formation of the A = 3a supercell [START_REF] Drits | The structure of monoclinic Na-rich birnessite and hexagonal birnessite. Part 1. Results from X-ray diffraction and selected area electron diffraction[END_REF][START_REF] Silvester | The structure of monoclinic Na-rich birnessite and hexagonal birnessite. Part 2. Results from chemical studies and EXAFS spectroscopy[END_REF]Lanson et al. 2002b). These authors, together with [START_REF] Lanson | Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH[END_REF], showed that the transformation of NaBu to its low-pH form (HBi) involves two successive reactions. At first, one third of Na interlayer is replaced by solution protons and one third of Mn 3+ layer disproportionates to form Mn 4+ layer and Mn 2+ solution , resulting in the formation of vacant layer sites. The second reaction includes the exchange of protons for remaining Na and the migration of half of remaining Mn 3+ layer from layer to interlayer. As a result, the initially vacancy-free Mn 3+ -rich rows consist of regularly alternating vacant and occupied layer octahedra in the HBi structure. In addition, at pH 4 about 0.04 Mn 2+ re-adsorb into the interlayer [START_REF] Silvester | The structure of monoclinic Na-rich birnessite and hexagonal birnessite. Part 2. Results from chemical studies and EXAFS spectroscopy[END_REF], thus leading to the formula H + 0.324 Mn 3+ 0.123 Mn 2+ 0.043 (Mn 4+ 0.722 Mn 3+ 0.111 0.167 )O 2 . Equilibration of a NaBu suspension at pH 4 in the presence of aqueous Co 2+ is also accompanied by a layer-to-interlayer migration of Mn 3+ and by the subsequent replacement of Mn 3+ by Co 3+ [START_REF] Manceau | Structural mechanism of Co 2+ oxidation by the phyllomanganate buserite[END_REF]).

These results consistently indicate that during equilibration of NaBu under acidic conditions, Mn 4+ is immobile and unreactive. In contrast, interpretation of SAED data has

shown that, at least in the presence of aqueous Pb, Cu, Zn, and Cd, the low-pH transformation of NaBu induces the migration of Mn layer into the interlayer from sites initially occupied by Mn 3+ and Mn 4+ . As an example, the distribution of Mn 3+ and Mn 4+ in the initial NaBu layer and that of Zn according to the chalcophanite unit cell (SAED types I and II -Figs. 8,9) are superimposed in Figure 13. In this case, only one-third of the octahedral layer sites linked to Zn were initially occupied by Mn 3+ layer . Moreover, the chalcophanite distribution of Zn prevents any further migration of Mn 3+ layer from the initial Mn 3+ -rich rows. Indeed, such a migration of Mn 3+ layer would lead to the formation of a pair of edge-sharing vacant octahedra (not shown) and of a pair of adjacent interlayer octahedra sharing a common edge (irregular dashed lines -Fig. 13).

Two alternative NaBu-to-ZnBi transformation mechanisms may account for such a distribution of vacant layer octahedra. In the first, both Mn 3+ layer and Mn 4+ layer can migrate to the interlayer with similar probability, inconsistent with the higher mobility of Mn 3+ layer observed by [START_REF] Manceau | Structural mechanism of Co 2+ oxidation by the phyllomanganate buserite[END_REF] and [START_REF] Lanson | Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH[END_REF]. In addition, the migration of Mn 4+ layer seems unrealistic in the presence of a significant amount of Mn 3+ layer , because the departure of Mn 3+ layer releases the steric strain within the layers. The alternative hypothesis assumes an electron transfer from Mn 3+ layer to the adjacent Mn 4+ layer , and the subsequent migration of the reduced Mn 3+ layer into the interlayer. According to this hypothesis, the NaButo-ZnBi transformation may be described as a four-step reaction. The first step is the diffusion of Zn in the interlayer, the second is the migration of electrons from Mn 3+ layer to the Mn 4+ layer to generate the distribution of Mn 3+ layer according to the chalcophanite unit cell. The third step is the migration of Mn 3+ layer , resulting in a regular distribution of vacant layer octahedra. The last step involves the sorption of Zn on the two sides of vacant layer sites, with a partial substitution of Zn for Mn interlayer .

In crystals having type-III and type-IV SAED patterns, Me cations are distributed as pairs and are separated from each other by 2a, or 5a, respectively, along the a axis. Such a distribution may be achieved only by changing the initial A = 3a super-cell periodicity of Mn 3+ layer ordering.

Interlayer structure and layer stacking mode as a function of Me

XRD results (Table 1) showed that the +a/3 shift between adjacent layers prevails for ZnBi varieties, whereas 1H layer stacking is predominant in PbBi and CdBi. Consistently, examination of Table 4 shows that additional structural features of MeBi, such as the In ZnBi crystals of types III and IV (Figs. 10,12), adjacent layers are also shifted by +a/3 but the 1D ordering of Zn is totally different from the ordered chalcophanite-like distribution.

To account for this apparent contradiction, one may note that together with VI Mn 3+ and VI Zn 2+ , a significant amount of IV Zn 2+ is present in ZnBi interlayers (Table 1). Because VI Zn should prevail in micro-crystals with a chalcophanite-like distribution of Zn, it is possible to assume that 1D distributions of super-cell reflections observed for ZnBi micro-crystals (type-III and -IV SAED patterns) correspond to 1M layer pairs that contain IV Zn or a mixture of VI Zn and IV Zn as interlayer cations. In this case, the short O layer -H 2 O Zn distances permit the formation of strong H-bonds (Fig. 10c).

Similarly, two different configurations are possible for Pb located in TC or TE sites, and the ordered distribution along the a axis of Pb located in VI TE sites is likely responsible for the incommensurate 1D distributions of super-cell reflections observed for 1H PbBi 58 micro-crystals (Fig. 11). Such a location for Pb increases the distance between Mn layer of one layer and Pb of the adjacent one, thereby decreasing their electrostatic repulsion as compared with the location of Pb in TC sites.

Finally, the origin of the 1D periodicity observed for 1H CdBi micro-crystals may be linked to the relatively high proportion of 1M layer pairs in the 1H stacking (Table 1). As a consequence, the 1M stacking mode may prevail in some CdBi crystals leading to the observed 1D super-cell periodicity (SAED patterns of type IV) if Cd 2+ is distributed in VI TC sites as shown in Figure 10b.

Structural meaning of Me/Mn ratios

The Me/Mn ratios determined by EDA for individual micro-crystals having a 1D supercell periodicity along the a axis may be used to calculate the frequency of the 1D Me sequences along the b axis. If Nbarb is the average separation between two adjacent sequences of Me, the primitive unit cell with A = 2.15a, B = Nbarb and γ = 90° contains 4.30Nbar layer octahedra. One out of these 4.30 N layer sites is vacant and capped by one Me. As a consequence:

Me/Mn = 1/(4.3Nbar-1).

(1)

If the vacant octahedron is capped by Me on each side, then Me/Mn = 2/(4.3Nbar-1).

(2)

For the base-centered supercell with A = 5.25a, B = Nbarb and γ = 90°, similar considerations transform Equations 1 and 2 to:

Me/Mn = 2/(10.5Nbar-2) and Me/Mn = 4/(10.5Nbar-2), respectively. ( 3), ( 4)

From Equation 1, Nbar is equal to 3.7 for PbBi and CdBi micro-crystals with A = 2.15a

and Me/Mn ≈ 0.07, that is Me sequences ordered along the a axis are repeated along the b axis every 3-4b. If Me cations coordinate the two sides of a vacant layer octahedron then Nbar ≈ 7. Such an interval between two adjacent Me sequences is too long to provide a noticeable contribution to the intensity of observed super-cell reflections.

Similar conclusions may be drawn for CdBi micro-crystals with A = 5.25a and Cd/Mn = 0.08. In this case, Equation 3 leads to Nbar ≈ 2.5, that is the Cd sequences are repeated every 2-3b along the b axis. Again, location of Cd on only one side of a vacant layer octahedron looks preferable, as the presence of Cd on the two sides of the same vacant layer site would again lead to long distances between successive ordered Cd sequences. However, according to the structural formulae obtained from chemical and XRD results, Cd is located above and below vacant layer sites (Lanson et al. 2002a).

Application of Equations 1 and 3 to ZnBi micro-crystals of types III and IV also leads to very long averaged intervals (~6.5b and ~4.2b) between successive ordered Zn sequences along the a axis regardless of whether Zn is sorbed on one or on two sides of vacant layer octahedra. Furthermore, because of the similar scattering powers of Zn and Mn, the presence of strong super-cell reflections in ZnBi SAED patterns with A = 2.15a supports the occurrence of Zn on the two sides of vacant layer sites, in agreement with XRD results (Lanson et al. 2002a).

As shown above, the distance between Me sequences in MeBi crystals having type-III and -IV SAED patterns is systematically too long to induce the observed strong super-cell reflections. A possible explanation for this apparent contradiction arising from the results obtained from chemistry, XRD, EDA, and SAED lies in the existence of a dual reaction, within the same micro-crystals, during the NaBu-to-MeBi transformation. The first reaction leads to the formation of MeBi fragments with an apparent periodicity A = 2.15 a, or A = 5.25 a. In turn, the second reaction leads to the formation of HBi fragments containing mostly Mn 3+ in interlayers. The coexistence within the same micro-crystals of these MeBi and HBi structural fragments is consistent with all of the observations.

Chemical composition and structural models of chalcophanite-like ZnBi crystals

Chalcophanite and ZnBi have contrasting chemical compositions, chalcophanite layers being built up of Mn 4+ octahedra only [START_REF] Post | Chalcophanite, ZnMn 3 O 7 -3H 2 O: New crystal-structure determinations[END_REF], whereas ZnBi layers contain both Mn 3+ and Mn 4+ , together with vacant octahedra. This difference leads to contrasting layer charge between the two species, and, as a result, to contrasting cation composition of their respective interlayers; chalcophanite interlayers contain only VI Zn, whereas Mn 3+ , Zn 2+ , and H + are present in ZnBi interlayers. To better assess both layer and interlayer cation compositions of these ZnBi crystals, one may assume that only Mn 3+ have migrated from the layer as discussed above. As a result, 7 out of the 49 layer sites of the supercell common to the right-and left-handed chalcophanite unit cells (A = B = 7b and γ = 120°) are vacant (Fig. 13), the other layer octahedra being occupied by Mn 4+ (32.67) and Mn 3+ (9.33). As vacant layer sites are capped on each side by Zn, the ideal composition of this supercell is Zn 2+ 14 (Mn 4+ 32.67 Mn 3+ 9.33 7 )O 88.67 (OH) 9.33 , or Zn 2+ 0.286 (Mn 4+ 0.667 Mn 3+ 0.190 0.143 )O 1.810 (OH) 0.190 . The resulting 0.333 Zn/Mn ratio is significantly higher than those determined experimentally (0.12 and 0.083 for type-II and type-I crystals, respectively -Table 3), and several hypotheses may be considered to decrease this ratio. According to the second hypothesis, two competing reactions take place in NaBu crystals during their low-pH transformation to ZnBi. These two reactions lead to the formation of either chalcophanite-like fragments, or HBi fragments with Mn 3+ as the unique interlayer cation. The coexistence of these two reactions is likely as, on the one hand, Mn 3+ has a strong tendency to migrate from Mn 3+ -rich rows at low pH to decrease steric strains and on the other hand, the migration of electrons within the layers allows Mn layer to migrate from layer sites initially occupied both by Mn 4+ layer and Mn 3+ layer . According to this second hypothesis, the Zn/Mn ratio is a function of the relative proportions of the two structural variants. For example, the coexistence in equal proportions of HBi and chalcophanite-like ZnBi fragments (respectively Zn 2+ 0.043 Mn 3+ 0.123 (Mn 4+ 0.722 Mn 3+ 0.111 0.167 )O 1.676 (OH) 0.324 , and Zn 2+ 0.286 (Mn 4+ 0.667 Mn 3+ 0.190 0.143 )O 1.810 (OH) 0.190 ) leads to an average composition Zn 2+ 0.164 Mn 3+ 0.062 (Mn 4+ 0.694 Mn 3+ 0.150 0.155 )O 1.744 (OH) 0.256 and a 0.181 Zn/Mn ratio, closer to the one experimentally determined for ZnBi micro-crystals of type II (0.122).

The third hypothesis assumes that interlayer Zn and Mn are distributed randomly over the seven vacant octahedra of the supercell, Zn being located above and below vacant layer sites, whereas Mn interlayer is located either above or below these sites. In such a distribution, the numbers of vacant layer sites coordinated by two Zn cations (N) can be calculated from Zn/Mn = 2N/(49-N). Consequently, the numbers of vacant sites coordinated by interlayer Zn and Mn, respectively, should be 2.82 and 4.18 for micro-crystals of type II (Zn/Mn = 0.122 -Table 3) and 1.95 and 5.05 for micro-crystals of type I (Zn/Mn = 0.083 -Table 3). The following structural formulae may be calculated for these ZnBi micro-crystals: Zn 2+ 0.115 Mn 3+ 0.085 (Mn 4+ 0.667 Mn 3+ 0.190 0.143 )O 1.724 (OH) 0.276 (type II) Zn 2+ 0.080 Mn 3+ 0.103 (Mn 4+ 0.667 Mn 3+ 0.190 0.143 )O 1.708 (OH) 0.292 (type I)

In this case, the interlayer cation composition of micro-crystals of type II determined from SAED and EDA data is similar to that derived from the simulation of the experimental XRD pattern of ZnBi 122 (Zn 2+ 0.116 Mn 3+ 0.094 ) for the same 0.122 Zn/Mn ratio. However, the amount of Mn 4+ determined above for micro-crystals of type II (0.667) is lower than that derived from XRD results for ZnBi 122 (0.722). The latter value originates from the structural model proposed by [START_REF] Silvester | The structure of monoclinic Na-rich birnessite and hexagonal birnessite. Part 2. Results from chemical studies and EXAFS spectroscopy[END_REF] for the NaBu-to-HBi transformation, which includes the initial partial disproportionation of Mn 3+ in Mn 3+ -rich rows and the subsequent migration from layer to interlayer of Mn 2+ and Mn 3+ . In the supercell with A = B = 7a and γ = 120° (Fig. 13) determined for ZnBi micro-crystals of type II, which contain 32.67

Mn 4+ layer and 9.33 Mn 3+ layer , 2.33 of the 7 vacant layer sites were initially occupied by Mn 3+ . If the initial disproportionation step is taken into account, the latter Mn cations may be reduced, leading to the migration of 2.33 Mn 2+ to the interlayer and to an increased amount of Mn 4+ layer (32.67 + 2.33 = 35). The structural formula is then Both layer and interlayer cation compositions of this formula are very close to those obtained from the simulation of XRD data. However, one may note that the systematic distribution of Zn pairs above and below vacant layer sites from initial Mn 3+ -rich rows would

In both varieties, the ordered distribution of vacant layer sites in ZnBi layers strongly constrains their mutual arrangement, and is compatible only with the +a/3 1M stacking mode determined from XRD data. Furthermore, SAED patterns analysis allows to determine a 2D distribution of vacant layer sites, interlayer Zn and Mn 3+ in agreement with the structural formula determined by Lanson et al. (2002a) for the same ZnBi samples. It is possible to demonstrate that sorption of Zn is associated with a significant modification of the layers that includes 1) migration of electrons from Mn 3+ layer to the nearest Mn 4+ layer to form Mn 3+ -Mn 3+ pairs oriented along the A and B axes; 2) their subsequent disproportionation; 3) the migration of newly formed Mn 2+ from layer to interlayer to generate the distribution of vacant sites according to the observed supercell. Such modification of heterovalent Mn cation distribution within birnessite layers contrasts with the very stable distribution determined in all previous studies on high-and low-pH birnessite varieties [START_REF] Drits | The structure of monoclinic Na-rich birnessite and hexagonal birnessite. Part 1. Results from X-ray diffraction and selected area electron diffraction[END_REF][START_REF] Manceau | Structural mechanism of Co 2+ oxidation by the phyllomanganate buserite[END_REF][START_REF] Drits | Sub-and super-structure of four-layer Ca-exchanged birnessite[END_REF][START_REF] Lanson | Structure of H-exchanged hexagonal birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH[END_REF]. In these studies, Mn 3+ layer were systematically segregated in Mn 3+ -rich rows parallel to [010] and separated from each other along the a axis by two Mn 4+ rows, and migration of Mn cations from layer to interlayer was restricted to these Mn 3+ -rich rows during low-pH equilibration.

In addition, ZnBi as well as PbBi and CdBi micro-crystals also present SAED patterns characterized by an incommensurate super-cell periodicity along the a axis. This apparent incommensurability results from the combination of the periodic distribution of Me along the a axis with a specific layer stacking mode. For the 1M stacking mode with a +a/3 shift between adjacent layers determined for ZnBi, location of tetrahedrally coordinated Zn above and below vacant octahedra sharing three corners with Mn layer octahedra ( IV TC sites) is favorable for this apparent incommensurate periodicity along the a axis. For 1H PbBi, a similar effect occurs if octahedrally coordinated Pb is located above and/or below empty tridentate cavities sharing three edges with Mn layer octahedra ( VI TE sites). All symbols as in Figure 1. octahedra to form a triple-corner surface complex (Fig. 7 in Lanson et al. 2002a). In TE sites Me is located above/below empty tridentate cavities sharing three edges with Mn layer octahedra (Fig. 9 in Lanson et al. 2002a).

In the latter sites, coordination of Pb is always octahedral ( VI TE sites), whereas Me may be octahedrally or tetrahedrally coordinated in TC sites. Add. indicates additional position for Me.

  Fig.2b). Along each of these directions super-cell reflections are distributed with a period

  shows the mutual arrangement of Mn cations belonging to two adjacent layers in projection along the c * axis. Taking the origin of the hexagonal unit cell in a Mn site of the lower layer, the nodes of the hexagonal supercell (A H = B H = 7 b/ 3 = 4.350 Å and γ = 120° -Regular dashed line, Fig.6) coincide with three structurally different octahedral sites as the A H axis is rotated with respect to the a axis by 10.89° in agreement with the mutual orientation of A H * and a * (Fig.1). Two of these sites (large solid and open circles -Fig.6) coincide with positions of layer Mn cations (Mn layer ) from the upper and lower layers, whereas the third one is located in the center of a hexagon formed by Mn layer of the upper and lower layers (irregular dashed line -Fig.6). As a consequence, within each interlayer Zn may occupy only two of the three available lattice nodes, because XRD and EXAFS showed that Zn forms triple-corner surface complexes (TC sites -Fig.7inLanson et al. 2002a) above/below vacant Mn layer sites by sharing three layer oxygen atoms (O layer ). The first occupied node is located above a vacant octahedron of the lower layer (large solid circle -Fig.6), the second below a vacant octahedron of the upper layer (large open circle -Fig.6), and the third possible node is not occupied by Zn. As a result, within a given interlayer, Zn positions related to each layer define a hexagonal cell having A = B = b 7 = 7.536 Å and γ = 120° as in chalcophanite[START_REF] Wadsley | The crystal structure of chalcophanite, ZnMn 3 O 7 .3H 2 O[END_REF][START_REF] Post | Chalcophanite, ZnMn 3 O 7 -3H 2 O: New crystal-structure determinations[END_REF]. Because of the superposition of Zn belonging to adjacent layers, the overall periodicity of Zn distribution does not coincide with the periodicity of Zn distribution within an individual layer described above.

  Furthermore, in such a configuration, H 2 O molecules coordinated by two Zn cations would be strongly oversaturated by positive charges because of the short Zn-H 2 O Zn distance (2.05-2.10 Å). To avoid such an unfavorable distribution of Zn in interlayers formed by successive layers having different distributions of vacant layer sites, Zn must be located either in the sites above the lower layer or in those below the upper one. In this case, solution protons may be sorbed to provide local charge compensation in vacant octahedra that are capped by Zn only on one side. In such interlayers the amount of Zn is decreased from 14 to 11, as three pairs of Zn positions cannot host Zn cations simultaneously. This modification of vacant layer site distribution between adjacent layers is likely not to be systematic, and right-and lefthanded chalcophanite structural fragments probably alternate at random along the c * axis.Each fragment may consist of several layers with the chalcophanite-like distribution of layer vacancies, Zn, and H 2 O Zn .If adjacent layers having different distributions of vacant layer sites occur with the same probability in any micro-crystal, then the average thicknesses of right-handed and left-handed chalcophanite structural fragments are similar, and their SAED patterns contain two equivalent super-cell reflection networks related by a mirror plane. However, according to simulation of XRD patterns and to chemical data(Lanson et al. 2002a), the above structural model is only a simplified scheme of the actual structure of ZnBi 69 crystals of type I, as Mn also occurs as an interlayer cation together with Zn. The possible distribution of Zn and Mn

  layers are respectively described by right-and left-handed cells (large solid and shaded circles, solid and irregular dashed lines -Fig. 9), possible TC sites for Zn are separated from each other either by short or by long distances. Interlayer sites separated by short distances cannot simultaneously host Zn because of electrostatic repulsion. As for SAED patterns of type I, the proposed model does not account for the significant amount of Mn interlayer present

  1D) sequences of vacancy pairs are distributed at random along the b axis, the subcell loses its base-centered property in projection along the c * axis to become primitive. If these periodically distributed Me cations have octahedral coordination (Fig.10b), water molecules coordinating these cations (H 2 O Me ) form an empty octahedron with O layer from the adjacent layer, thus forming weak H-bonds. In contrast, if Me is tetrahedrally coordinated (Fig.10c), the short H 2 O Me -O layer distance leads to the more favorable formation of strong Hbonds between adjacent layers, as discussed byLanson et al. (2002a).

  keep the base-centered symmetry of the A = 5.25a, B = b and γ = 90°s upercell, sequences of vacancy pairs aligned along the a axis should be shifted by (nA + mb)/2 (n and m being integers) with respect to each other. As a result, any interlayer 1D sequences of vacancy pairs having identical periodicity but shifted with respect to each other by (A + b)/2 occur with equal probabilities and are interstratified at random along the b axis.

  distribution of newly formed vacant layer sites in the transformed NaBu structure, depend on the identity of Me. For example, PbBi and CdBi are characterized by only 1D super-cell periodicities, whereas Zn induces either 1D or 2D ordering in the distribution of vacant layer octahedra and Zn. It is thus likely that the distribution of vacant layer sites and Me, the location and coordination of Me, and the mutual arrangement of adjacent layers are interdependent, as will be shown.This mutual dependence may be illustrated at first for ZnBi micro-crystals of types I and II. In the 1M ZnBi structure, because of the +a/3 shift between adjacent layers, the three H 2 O Zn define an empty octahedron with the O layer of the next layer, thereby forming weak Hbonds. In spite of this unfavorable configuration, the ordered chalcophanite-like distribution of vacant octahedra capped on each side by Zn (Figs.8, 9) strongly constrains the mutual arrangement of adjacent layers. Indeed, this distribution can occur only when adjacent layers are shifted with respect to each other by +a/3, and is compatible neither with the orthogonal layer stacking nor with the -a/3 1M stacking because, for these two layer stacking modes, the octahedral coordination of Zn by H 2 O leads systematically to unrealistically short H 2 O Zn -H 2 O Zn distances (Fig.14).

FIGURE CAPTIONS Figure 1 .

 CAPTIONS1 FIGURE CAPTIONS

Figure 2 .

 2 Figure 2. MeBi type II. Observed (a) and schematic (b, c) SAED patterns of MeBi type II.

Figure 3 .

 3 Figure 3. MeBi type III. Observed (a, b) SAED patterns of MeBi type III. (a) Large arrows indicate the faint sub-cell reflections with h + k = 2n + 1. (b) Ticks indicate the positions of sub-and super-cell reflections (below and above reflections, respectively) to allow for an accurate determination of the super-cell periodicity (A * = a * /2.15).

Figure 4 .

 4 Figure 4. MeBi type IV. Observed (a, b) SAED patterns of MeBi type IV. (b) Ticks indicate the positions of sub-and super-cell reflections (below and above reflections, respectively) to allow for an accurate determination of the super-cell periodicity (A * = a * /5.25).

Figure 5 .

 5 Figure 5. MeBi type V. Observed SAED pattern.

Figure 6 .

 6 Figure 6. 2D distribution in projection on the ab-plane of lattice nodes corresponding to the super-cell reflection distribution in one of the twin networks present in SAED patterns of

Figure 7 .

 7 Figure 7. 3D distribution in projection on the ab-plane of Zn in ZnBi 69 in one of the twin networks present in SAED patterns of type I. (a), (b), and (c) refer respectively to the first, second and third interlayer regions of ZnBi type I. Symbols as in Figure 6, except for the shaded triangles, which correspond to the upper surface of the lower layer, and the lower surface of the upper layer, as illustrated on the right side of each Figure. The solid star indicates the origin in the first layer and is shown subsequently to indicate the total shift from that first layer. Layer 4 is equivalent to layer 1.

Figure 8 .

 8 Figure 8. 2D distribution in projection on the ab-plane of lattice nodes corresponding to the overall super-cell reflection distribution in the two twin networks of SAED patterns type of I. All symbols as in Figure 6, except where specified. The solid line outlines the righthanded supercell corresponding to the ordered vacancy distribution of the lower layer (large solid circles). The regular dashed line outlines the right-handed supercell corresponding to the ordered vacancy distribution of the upper layer (large open circles). The irregular dashed line outlines the left-handed supercell corresponding to the ordered vacancy distribution of the upper layer (large shaded circles). Vacant layer sites of the upper layer common to both right-and left-handed unit cells are shown as large half-and-

Figure 9 .

 9 Figure 9. 2D distribution in projection on the ab-plane of lattice nodes corresponding to the overall super-cell reflection distribution in the two twin networks of SAED patterns of

Figure 10 .

 10 Figure 10. Possible distribution of vacant sites and associated Me leading to SAED patterns

Figure 11 .

 11 Figure 11. Possible distribution of vacant sites and associated Me leading to SAED patterns of type III for the 1H phase. Adjacent layers are not translated along the a axis. (a) Projection on the ab-plane. All symbols as in Figure 8. Solid and open squares represent vacant layer sites from the lower and upper layers, respectively. (a) Projection along the b axis. All symbols as in Figure 10B. Me cations are located above/below empty tridentate cavities and are tetrahedrally coordinated.

Figure 12 .

 12 Figure 12. Possible distribution in projection on the ab-plane of vacant sites and associated

Figure 13 .

 13 Figure 13. Possible distribution in projection on the ab-plane of vacant sites and associated Me leading to SAED patterns of types I and II. All symbols as in Figure 8, except for the dark shaded octahedra which represent the octahedrally coordinated interlayer cations.4+ and 3+ indicate the initial distribution of Mn 4+ and Mn 3+ cations in rows parallel to the b axis. Irregular dashed lines outline the edges that Me octahedra would share with Mn 3+ in case of additional cation migration from the initial Mn 3+ -rich rows.

Figure 14 .

 14 Figure 14. Schematic 2D distribution in projection on the ab-plane of lattice nodes corresponding to the overall super-cell reflection distribution in the two twin networks of

Figure 15 .

 15 Figure 15. Schematic two-step reaction leading to a random distribution of Mn 2+ and Mn 3+

*

  Some SAED patterns of type IV were rotated with respect to each other by 120° around a common origin (not shown) ** Some of the SAED patterns of type V are rotated with respect to each other by 21.79° around the center of the electron diffraction pattern (not shown)Table 4. Summary of MeBi structural varieties determined from SAED. interlayer cations are located either above and below (Mn 3+ ) or above and below (Zn) vacant layer octahedra ( VI TC sites) -ZnBi 69, 122, 128 microcrystals of type II Twins of ZnBi structural fragments -Individual layers have a chalcophanite-like distribution of vacant layer octahedra with A H = b 7 = 7.53 Å -In adjacent layers, these distributions are not regularly shifted with respect to each other by 1/3 of the long diagonal of the hexagonal layer unit cell (A H = b 7 ) as in chalcophanite. -Octahedrally coordinated interlayer cations are located either above and below (Mn 3+ ) or above and below (Zn) vacant layer octahedra ( VI TC sites) -ZnBi 69 micro-crystals with a high IV Zn content -CdBi micro-crystals with a high content of 1M layer pairs 1M MeBi micro-crystals in which Me is only located above/below vacant layer octahedra (TC sites) -In projection along the c * axis, pairs of Me cations form ordered 1D sequences along the a axis with pseudo-periods equal to 2.15 a (type III) or 5.25 a (type IV) -Me may be tetrahedrally (Zn) or octahedrally (Mn 3+ , and Cd) coordinated -PbBi 58 1H MeBi micro-crystals in which Me is partly located above/below empty tridentate cavities ( VI TE sites) -In projection along the c * axis, pairs of Me cations form ordered 1D sequences along the a axis with pseudo-periods equal to 5.25 a (type IV). vacant layer sites and associated Me, or -a very low amount of Me, or -Me having a similar scattering power as that of Mn, and located either above or below vacant layer sites Drits et al. -

Table 2

 2

	shows that SAED patterns of CuBi and

  The measured 2.15a period can be obtained, within experimental precision, by assuming that the second and third pairs are separated by 2.67a or 1.67a with equal probability, as in this case the average

	Me periodicity is [2 + 2.33 + 2 + (2.67 + 1.67)/2]a/4 = 2.125a. Finally, if these one-
	dimensional

  First, chalcophanite-like ZnBi crystals may have a mixed-layered structure consisting of both right-and left-handed chalcophanite fragments. As shown in Figure8, if two adjacent layers have different orientations, only 11 of the 14 interlayer sites can be occupied

	simultaneously. As a result, a regular alternation of right-and left-handed chalcophanite
	fragments would lead to a Zn 2+ 11 (Mn 4+ 32.67 Mn 3+ 9.33 7 )O 82.67 (OH) 15.33 , or
	Zn 2+ 0.224 (Mn 4+ 0.667 Mn 3+ 0.190 0.143 )O 1.687 (OH) 0.313 cation composition with a 0.262 Zn/Mn
	ratio.

Table 1 .

 1 Main structural features of MeBi as determined from XRD and EXAFS results(Lanson et al. 2002a;[START_REF] Manceau | Structure of heavy metal sorbed birnessite. Part 3: Results from powder and polarized EXAFS spectroscopy[END_REF]. In TC sites Me is located above/below vacant layer octahedra and share three O layer with neighboring Mn layer

	Sample	Layer stacking	Well defined	Location and coordination of interlayer cations
			stacking faults
				Mn interlayer	Me	Add. Me
	ZnBi 69	1M (+a/ 3 )	1 H	-	-	-
	ZnBi 122	1M (+a/3)	1H -12%	VI TC sites	VI TC sites	IV TC sites
				above OR below	above AND below	above AND below
	ZnBi 128	1M (+a/3)	1H -15%	VI TC sites	VI TC sites	IV TC sites
				above OR below	above AND below	above AND below
	PbBi 6	1H	1M (-a/3) -2%	VI TC sites	VI TC sites	-
				above OR below	above OR below
	PbBi 58	1H	1M (-a/3) -3%	VI TC sites	VI TC sites	VI TE sites
				above OR below	above OR below	above OR below
	CdBi 106	1H	1M (+a/3) -8%	VI TC sites	VI TC sites	-
				above OR below	above AND below
	C u B i 1 5 6	-	-	-	-	-
	Note:

Table 2 .

 2 Occurrence of the different types of SAED patterns for the various samples.

	Sample	SAED Type I	SAED Type II	SAED Type III	SAED Type IV	SAED Type V
	ZnBi 69	++	+	±	++*	+
	ZnBi 122	-	+++	-	-	+**
	ZnBi 128	-	+++	-	-	+**
	P b B i 6	-	-	-	-	+ + +
	PbBi 58	-	-	++	-	++**
	CdBi 106	-	-	±	++	++
	C u B i 1 5 6	-	-	-	-	+ + + * *

Note: +++, ++, +, ±, -for prevalent, abundant, accessory, seldom observed, not observed.

ACKNOWLEDGMENTS

VAD, BL, and AIG are grateful to the Russian Science Foundation for financial support. BL and AM acknowledge financial support from INSU/Géomatériaux, CNRS/ACI "Eau et Environnement", and CNRS/PICS709 programs. Céline Bartoli is thanked for the preparation of the CdBi sample, and Anne-Claire Gaillot for the final prints of SAED micrographs. Steve Hillier kindly improved the English of an early version of this manuscript. Constructive remarks by David R. Veblen, A.E. David L. Bish and an anonymous reviewer helped to improved the original manuscript.

lead to a A = 3a super-cell periodicity along the a axis. This effect is minimized if disproportionation occurs after electron migration within the layers. Indeed, in the initial distribution of heterovalent Mn layer cations, any Mn 4+ row is adjacent to a Mn 3+ -rich row along the a axis (Figs. 13,15a). As a consequence, electrons can migrate from a Mn 3+ layer to any adjacent Mn 4+ layer to form Mn 3+ -Mn 3+ pairs oriented along the A or B axes (Fig. 15b).

Subsequent disproportionation enables the presence of Mn 2+ layer in any layer octahedron (Fig. 15c). As a consequence, such a random distribution of Mn 2+ and Mn 3+ over octahedral layer sites will lead to a random distribution of interlayer Zn-pairs and single Mn 3+ after their migration from the layers and the substitution of Zn for Mn 2+ . The above hypotheses account for the diffraction effects observed for ZnBi micro-crystals of types I and II, as well as for their layer and interlayer cation compositions.

SUMMARY

The main types of SAED patterns containing super-cell reflections were interpreted for synthetic MeBi. Analysis of the super-cell reflections observed for ZnBi micro-crystals of types I and II shows that layers of the two varieties exhibit an ordered distribution of vacant octahedra capped by Zn and Mn 3+ according to a hexagonal supercell with A = B = 7 b and γ = 120°. In ZnBi type-I micro-crystals right-and left-handed chalcophanite-like fragments occur with equal probability and are likely randomly interstratified. In ZnBi type-II microcrystals, vacant layer octahedra are not regularly shifted from one layer to the adjacent one as in chalcophanite, thus lowering the overall symmetry. For this latter variety, both the interstratification of right-and left-handed fragments and the peculiar distribution of vacant layer octahedra in adjacent layers explain why XRD patterns of ZnBi samples are devoid of super-cell reflections in contrast to that of chalcophanite.


Vers un droit commun des sûretés ?

Bourassin, Maître de conférences à l'Université Paris X-Nanterre L'ordonnance n° 2006-346 du 23 mars 2006 renforce-t-elle l'efficacité du droit des sûretés par l'instauration d'un droit commun ?

Au regard des impératifs d'accessibilité, de clarté et de cohérence du droit -composantes essentielles de la sécurité et de l'efficacité juridiques -, la réussite d'une réforme suppose que les règles spéciales viennent compléter ou affiner des règles générales. En effet, l'établissement de corps de règles communes, fondées sur les caractéristiques partagées par les divers mécanismes réglementés, présente de nombreux avantages : il favorise l'économie de textes et leur regroupement, permet l'absorption des créations de la pratique et, par conséquent, assure la prévisibilité et la stabilité du droit applicable, facilite l'interprétation des règles spéciales et évite des différences de régime injustifiées.

Le perfectionnement du droit des sûretés, grâce à l'émergence d'un tel droit commun, a pu paraître compromis par les termes de l'habilitation à réformer cette matière par ordonnance, donnée au gouvernement le 26 juillet 2005. L'ordonnance du 23 mars 2006 portant réforme du droit des sûretés redonne au contraire plein espoir. Certes, ce texte appelle certainement des critiques sur le fond, tant au regard du nécessaire renforcement de la sécurité des créanciers, que du non moins nécessaire respect des impératifs d'éthique et de justice contractuelle (sur ces conditions de l'efficacité du droit des sûretés quant à son contenu, qui ne seront pas envisagées ici, V. M. Bourassin, L'efficacité des garanties personnelles, LGDJ, 2006). L'ordonnance mérite en revanche un accueil tout à fait favorable en ce qu'elle ouvre véritablement la voie à une codification stratifiée -dispositions générales complétées par des dispositions spéciales -respectueuse de l'impératif de sécurité juridique.

A -La loi d'habilitation du 26 juillet 2005 : les espoirs déçus d'un droit commun

Au cours des débats préparatoires de la loi pour la confiance et la modernisation de l'économie, le gouvernement et les assemblées ont constamment exprimé leur volonté de rendre au droit des sûretés « sa pleine cohérence, sa lisibilité et sa simplicité de mise en oeuvre, gage de sécurité juridique » (V. www.senat.fr/dossierleg). L'habilitation finalement donnée ne laissait cependant guère espérer la réalisation de ces objectifs essentiels, tant l'émergence d'un droit commun paraissait entravée, aussi bien au niveau de l'ensemble des sûretés, qu'à celui des sûretés personnelles ou encore des sûretés réelles.

-L'idée abandonnée d'un droit commun des sûretés

Afin de remédier au manque de lisibilité et d'accessibilité du droit des sûretés unanimement dénoncé, le groupe de travail relatif à la réforme du droit des sûretés, présidé par le Professeur M. Grimaldi, avait suggéré, dans son rapport remis au garde des Sceaux le 31 mars 2005, de regrouper l'ensemble des sûretés dans un nouveau livre du code civil et de les soumettre à des principes généraux fondés sur leur qualité commune d'accessoires de la créance garantie.

Cette idée de créer un droit commun des sûretés n'a reçu aucun écho au cours de l'examen du projet de loi d'habilitation, le gouvernement ayant seulement exprimé le souhait de procéder à un regroupement de l'ensemble du droit des sûretés au sein d'un livre IV du code civil (l'actuel livre IV consacré à Mayotte devenant un livre V). Cette présentation resserrée ne pouvait, à elle seule, renforcer l'intelligibilité et la cohérence du droit des sûretés.

Le projet de loi d'habilitation du 14 avril 2005 visait, non seulement le cautionnement, mais également les garanties jusque-là innomées, à savoir les mécanismes du droit commun des obligations pouvant jouer un rôle de garantie personnelle (la délégation), ainsi que les garanties autonomes et lettres d'intention créées par la pratique. Il était permis d'espérer qu'à l'occasion de cette réforme globale, des règles impératives communes à l'ensemble des garanties personnelles seraient adoptées. Cet espoir fut malheureusement vite déçu.

En effet, le 12 mai 2005, la Commission des lois a proposé plusieurs amendements limitant le champ de l'habilitation. En matière de garanties personnelles, deux « coupes franches » ont ainsi été suggérées : d'une part, la suppression de toute référence au cautionnement ; d'autre part, l'abandon de la modification des mécanismes du droit commun des obligations utilisés comme garanties, faute pour le gouvernement d'en avoir précisé les finalités. Or, la Commission des lois a justement rappelé que, selon le Conseil constitutionnel, l'article 38 de la Constitution « doit être entendu comme faisant obligation au gouvernement d'indiquer avec précision au Parlement lors du dépôt d'un projet de loi d'habilitation [...] quelle est la finalité des mesures qu'il se propose de prendre » (décis. n° 76-72 du 12 janv. 1977).

Le Parlement ayant adopté ces deux amendements, l'habilitation à l'égard des sûretés personnelles se trouvait limitée à la reconnaissance d'une base légale pour la garantie autonome et la lettre d'intention (art. 24-4°). De l'aveu même d'un membre de la Commission des lois, cette forte limitation du champ de l'habilitation ne pouvait que « nuire à l'objectif premier de lisibilité et de mise en cohérence du droit des sûretés » (avis n° 437 du 29 juin 2005).

-L'absence annoncée d'un droit commun des sûretés réelles

En matière de sûretés réelles, l'habilitation donnée au gouvernement le 26 juillet 2005 ne présageait nullement l'édiction de règles communes et laissait au contraire craindre une accumulation incohérente de règles spéciales, ceci pour plusieurs raisons. D'abord, parce qu'au cours de l'examen du projet de loi d'habilitation, la discussion n'a jamais porté sur l'édiction de principes communs.

Ensuite, parce que, faute pour le gouvernement d'avoir défini avec suffisamment de précision les finalités poursuivies, des garanties réelles majeures ont été exclues de l'habilitation : les privilèges, les mécanismes du droit des obligations pouvant faire office de garanties réelles (cession de créance, vente à réméré) et encore les nantissements de meubles incorporels.

Enfin, la réforme s'annonçait très spéciale, parce qu'à l'égard des cinq garanties réelles finalement visées par l'habilitation, il fut moins question de rénovation globale que de mesures précises susceptibles de satisfaire les besoins économiques du moment (l'accession à la propriété, le soutien de la consommation et la relance de la croissance) : pour le gage, la suppression de son caractère réel, l'extension de son assiette aux choses futures ou ensemble de biens, comme les stocks d'entreprises, et l'autorisation du pacte commissoire (art. 24-1°) ; la consécration de la pratique de l'antichrèse-bail (art. 24-2°) ; à l'égard de l'hypothèque, l'amélioration de la mainlevée et l'introduction de deux « produits » d'origine anglo-saxonne, l'hypothèque rechargeable et le prêt viager hypothécaire (art. 24-2°) ; l'insertion à droit constant, dans le code civil, des dispositions relatives à la clause de réserve de propriété (art. 24-3°) ; enfin, la reconnaissance d'une base légale pour le droit de rétention (art. 24-4°).

B -L'ordonnance du 23 mars 2006 : la consécration d'un droit commun

La réforme opérée par l'ordonnance du 23 mars 2006 n'est pas l'occasion manquée de perfectionner le droit des sûretés que l'on pouvait redouter à la lecture de la loi d'habilitation. En effet, la matière fait l'objet d'une réorganisation globale, susceptible d'en améliorer les qualités formelles. Si l'on ne peut que se réjouir de cette avancée en termes de sécurité juridique et d'efficacité du droit, il importe néanmoins de se demander si le gouvernement, pour y parvenir, n'a pas outrepassé le domaine de l'habilitation et abusé, par là même, de la technique de la législation déléguée (V., sur l'ensemble de la réforme des sûretés, le dossier spécial : D. 2006, Chron. p. 1289).

-Une réorganisation globale du droit des sûretés

La nouvelle structure du droit des sûretés favorise la réalisation de l'objectif de valeur constitutionnelle d'accessibilité et d'intelligibilité de la loi.

a -L'accessibilité du droit des sûretés L'accessibilité matérielle, partant la connaissance et la prévisibilité du droit des sûretés, se trouvent renforcées par le regroupement dans un seul livre du code civil (les titres XIV, XVII et XVIII du livre III sont abrogés depuis le 24 mars dernier, date de publication et d'entrée en vigueur de l'ordonnance), non seulement des sûretés classiques, mais également de garanties jusqu'alors innomées -la garantie autonome (art. 2321) et la lettre d'intention (art. 2322) -ou uniquement soumises à des règles spéciales éparses -le droit de rétention (art. 2286) et la propriété retenue à titre de garantie (art. 2367 à 2372 et 2373).

Avec ce nouveau livre IV et la reconnaissance ou le rapatriement des garanties dites modernes dans le code civil, celui-ci redevient incontestablement le siège privilégié du droit des sûretés, sans empiéter pour autant, ni sur d'autres droits, ni sur les régimes spéciaux relevant d'autres codes. Deux points en attestent. D'une part, les nombreux renvois aux droits des procédures civiles d'exécution et des entreprises en difficulté. D'autre part, l'inscription, dans le code de la consommation, des règles propres aux sûretés couvrant un crédit à la consommation ou immobilier et l'insertion, dans le code de commerce, des règles intéressant les sûretés prises sur les biens des entreprises ou des personnes physiques dans l'exercice de leur activité professionnelle. Cette rigueur dans la place des textes concourt à une meilleure connaissance du droit des sûretés.

b -L'intelligibilité du droit des sûretés

L'intelligibilité du droit des sûretés paraît améliorée par trois biais : les classifications retenues, l'avènement de divers corps de règles communes et l'ampleur de la réforme.

L'intelligibilité du droit des sûretés est d'abord renforcée par les classifications qui en constituent désormais l'architecture. Le livre IV est logiquement divisé entre les sûretés personnelles (titre I) et les sûretés réelles (titre II). Le classement des sûretés réelles n'est plus fondé sur la technique d'affectation du ou des biens grevés (c'est-à-dire avec ou sans dessaisissement du débiteur), mais sur leur nature mobilière (sous-titre II) ou immobilière (sous-titre III). Cette classification est plus claire que la précédente, dont la rigueur avait été altérée par la création des diverses sûretés mobilières sans dépossession, et elle est tout aussi riche de conséquences, puisque les différences de régime entre les droits réels mobiliers et les droits réels immobiliers se prolongeront naturellement dans le régime des sûretés. Au sein des sûretés réelles mobilières, le sous-titre II du titre II distingue le gage de meubles corporels (chapitre II) et le nantissement de meubles incorporels (chapitre III).

Cette catégorisation nouvelle consacre, outre l'habitude prise en pratique de réserver le terme de nantissement aux biens incorporels, deux tendances modernes : d'une part, la montée en puissance de l'immatériel en droit des sûretés ; d'autre part, le fait que les biens incorporels sont appréhendés, non comme une espèce de biens meubles ou immeubles, mais comme un genre de biens à part entière. L'ordonnance a ainsi fait montre de réalisme en cessant de considérer le nantissement de meubles incorporels comme un gage spécial, dérogeant au droit commun du gage de meubles corporels. Il convient cependant de remarquer que la distinction entre le gage des meubles corporels et le nantissement des meubles incorporels n'est pas aussi nette que la nouvelle architecture du code pourrait le laisser penser, puisque l'article 2355 opère un renvoi qui laisse quelque peu dubitatif : le nantissement « qui porte sur d'autres meubles incorporels [que les créances] est soumis, à défaut de dispositions spéciales, aux règles prévues pour le gage de meubles corporels ». Pourquoi ne pas avoir fait plutôt référence aux règles du nantissement de créances si celui-ci est véritablement situé, aujourd'hui, sur le même plan que le gage de meubles corporels ? Par ailleurs, en cas d'incomplétude du régime du nantissement de créances, celui du gage de meubles corporels devra-t-il également s'appliquer ?

La compréhension du droit des sûretés est ensuite améliorée par l'avènement de divers corps de règles communes, complétés par des règles spéciales. En certains domaines, le droit commun n'est encore qu'en germe ou en filigrane ; dans d'autres, il a en revanche été expressément consacré.

Il est possible de déceler un droit commun en germe, chaque fois que des subdivisions s'ouvrent sur un article qui énumère les sûretés réglementées. Ainsi, les sûretés personnelles (art. 311-32, al. 3, c. consom. relatif au crédit à la consommation garanti par un gage de meuble corporel), soit de l'importance attachée à certains biens (en vertu des art. 2458 et 2459, la résidence principale du débiteur ne peut faire l'objet, en principe, que d'une vente forcée ; l'art. 527-2 c. com. répute non écrit le pacte commissoire convenu dans un gage de stocks), soit encore de l'ouverture d'une procédure de sauvegarde (art. L. 622-7 c. com.).

Une réglementation stratifiée (dispositions générales assorties de dispositions particulières) a, en outre, été expressément consacrée à quatre niveaux.

En premier lieu, à l'égard de l'ensemble des sûretés. Le livre IV s'ouvre, en effet, sur des dispositions générales concernant le droit de gage général, le droit de rétention et l'application des règles prévues en cas d'ouverture d'une procédure collective ou de surendettement. Ce droit commun des sûretés aurait pu être bien plus riche (les règles relatives à la propriété retenue à titre de garantie inscrites dans une section consacrée aux sûretés réelles mobilières auraient notamment pu y figurer, puisque cette forme de « sûreté-propriété » ne confère pas au créancier un droit de préférence, mais le soustrait purement et simplement au concours, comme le droit de rétention), mais son avènement est déjà tout à fait remarquable.

En deuxième lieu, le sous-titre Ier du titre II intitulé « Dispositions générales » consacre un droit commun des sûretés réelles. Il convient cependant de reconnaître que le contenu de cet ensemble n'est pas aussi général qu'annoncé, puisqu'il reprend en réalité les anciens articles 2094 à 2099 relatifs aux privilèges, qui ne sauraient servir de modèle en raison des spécificités qu'induit leur origine légale.

Une structure stratifiée a été adoptée, en troisième lieu, en matière de gage de meubles corporels. Après le « droit commun du gage » (art. 2333 à 2350), il est effectivement prévu un régime propre au gage portant sur un véhicule automobile (art. 2351 à 2353) et ont été également réservées les « règles particulières prévues en matière commerciale ou en faveur des établissements de prêt sur gage autorisés » (art. 2354).

En quatrième et dernier lieu, c'est la lisibilité du droit des hypothèques et des privilèges immobiliers qui se trouve améliorée par l'articulation systématique de « dispositions générales » et de « dispositions particulières », au sujet de leur constitution, de leur inscription, de leur effet et de leur purge.

L'intelligibilité du nouveau droit des sûretés est enfin favorisée par l'étendue de la réforme. Il ne s'agit pas d'une « réformette » venant greffer sur le droit en vigueur quelques dispositions nouvelles risquant de le dénaturer. Chacune des sûretés réelles visées par la loi d'habilitation a fait, au contraire, l'objet d'une refonte cohérente. S'agissant de l'antichrèse, l'ordonnance ne consacre pas uniquement la pratique consistant, pour le créancier, à donner à bail l'immeuble dont le débiteur s'est dépossédé à titre de garantie. Elle tire au contraire toutes les conséquences du nouveau classement des sûretés réelles fondé sur leur assiette, en renvoyant au régime de l'hypothèque conventionnelle chaque fois que la nature immobilière du bien grevé est en cause (art. 2388). Concernant l'hypothèque, si l'accent a été mis sur son rechargement et sur le prêt viager hypothécaire, la réforme ne se réduit pas à l'ajout de ces produits dans le système législatif en vigueur. Le régime de l'hypothèque conventionnelle est totalement réformé, de sorte que ces nouvelles utilisations, dont l'inanité sur la croissance économique et les dangers en terme de surendettement ont d'ores et déjà été critiqués, ne compromettent pas, en revanche, la cohérence du droit hypothécaire. Ainsi, l'hypothèque rechargeable (art. 2422) s'inscrit dans le prolongement de l'assouplissement du principe de spécialité quant à la créance garantie (art. 2421, al. 1er : « l'hypothèque peut être consentie pour sûreté d'une ou plusieurs créances, présentes ou futures »). Au-delà de ces innovations, il convient de souligner que la réforme globale de l'hypothèque clarifie nombre de règles existantes, en les rendant plus lisibles (par ex., l'art. 2419 prévoit que « l'hypothèque ne peut, en principe, être consentie que sur des immeubles présents » et l'art. 2420 regroupe les exceptions qui figuraient auparavant dans trois dispositions) ou en y intégrant les interprétations jurisprudentielles (V. l'art. 2114 relatif à l'hypothèque d'un immeuble indivis ou d'une quote-part).

A la lumière des impératifs d'accessibilité et d'intelligibilité du droit, il apparaît donc que la réforme opérée par l'ordonnance du 23 mars 2006, sans être parfaite (une réorganisation d'une telle ampleur aurait notamment gagné à être assortie de dispositions transitoires précises), est néanmoins source de progrès. Ce résultat a-t-il été atteint dans le respect de l'habilitation donnée en juillet 2005 ?

-Un vaste dessaisissement du Parlement

Les mesures énumérées par l'article 24-1° à 5° de la loi d'habilitation du 26 juillet 2005 pouvaient laisser craindre une réforme sans vue d'ensemble, essentiellement dictée par des objectifs économiques conjoncturels, qui aurait aggravé les défauts formels du droit des sûretés. Cet écueil a pu être évité au prix d'un vaste dessaisissement du Parlement.

Ce dessaisissement a été, pour l'essentiel, consenti par les assemblées, puisqu'elles ont autorisé, non seulement des innovations précises, mais aussi les mesures nécessaires pour « aménager et modifier toutes dispositions de nature législative permettant d'assurer la mise en et de tirer les conséquences des modifications apportées en application des 1° à 5° du présent article » (art. 24-6°). La généralité de ces termes offrait au gouvernement la possibilité de modifier l'architecture du droit des sûretés (au sein et en dehors du code civil) et de refondre entièrement le régime des sûretés réelles conventionnelles visées.

Il faut cependant reconnaître que certaines clarifications ou simplifications du droit positif résultent d'un dépassement du domaine de l'habilitation, qui pourrait justifier une annulation partielle de l'ordonnance par le Conseil d'Etat avant son éventuelle ratification expresse ou implicite (sur la nature réglementaire des ordonnances avant cette date, V. Cons. const., 29 févr. 1972, décis. n° 72-73 L ; sur la rareté de la jurisprudence du Conseil d'Etat relative à un tel dépassement de pouvoir, V. C. Boyer-Mérentier, Les ordonnances de l'article 38 de la Constitution du 4 octobre 1958, PUAM, 1996, préf. de L. Favoreu, p. 104 s.). Tout d'abord, l'ordonnance a établi un classement des privilèges mobiliers, alors que les privilèges avaient été expressément écartés au cours de l'examen du projet de loi d'habilitation. Il serait cependant surprenant que le Conseil d'Etat, s'il était saisi, déclarât l'ordonnance illégale sur ce point, dans la mesure où les dispositions nouvelles se contentent, en réalité, d'ordonner des solutions très anciennes, pour la plupart forgées par la Cour de cassation (il en va ainsi de la primauté des privilèges spéciaux sur les privilèges généraux, reconnue depuis 1849 sur le fondement de la règle generi per speciem derogatur, ou encore de l'ordre entre les privilèges spéciaux du bailleur d'immeuble, du conservateur et du vendeur de meuble).

Ensuite, l'ordonnance réforme le nantissement de meubles incorporels, alors que son exclusion a été votée. Il convient cependant de remarquer que la Commission des lois, à l'origine de l'amendement d'exclusion, s'était surtout montrée hostile à l'insertion dans le code civil du nantissement sur instruments financiers. Or, celui-ci n'est nullement visé par l'ordonnance, qui a seulement modifié les dispositions du code civil relatives au nantissement sur créance, dans le but mentionné à l'article 24-1° de la loi d'habilitation d'en simplifier les effets (l'opposabilité aux tiers et au débiteur de la créance nantie -art. 2361 et 2362 -est alignée sur le régime simplifié du nantissement Dailly).

Enfin, l'exigence d'insertion, à droit constant, dans le code civil, des dispositions relatives à la réserve de propriété n'a pas été strictement respectée, puisque l'ordonnance consacre la règle d'origine jurisprudentielle selon laquelle « la propriété ainsi réservée est l'accessoire de la créance dont elle garantit le paiement » (art. 2367, al. 2 ;Cass. com., 15 mars 1988, Bull. civ. IV, n° 106). Ce dépassement pourrait néanmoins être justifié par une interprétation large du terme « dispositions », et ce d'autant que cette solution prétorienne est admise par les droits étrangers et la plupart des auteurs. S'il est donc possible de relativiser les dépassements du champ de l'habilitation, on ne saurait en revanche occulter le dessaisissement non consenti du Parlement qu'ils entraînent. Mais peut-être ce nouveau type de détournement de la technique de la délégation législative (pour d'autres exemples de dérives, V. Y. Gaudemet, Sur l'abus ou sur quelques abus de la législation déléguée, Mélanges P. Pactet, Dalloz, 2003, p. 617 s.), était-il le prix à payer pour renforcer du droit des sûretés. M. B.

  2287-1), les sûretés mobilières (art. 2329) et les sûretés immobilières (art. 2373) n'ont certes pas fait l'objet de dispositions générales, mais la place d'un droit commun en ces matières est désormais toute trouvée. Un droit commun et des régimes dérogatoires existent en filigrane au sujet de la réalisation des sûretés réelles. Le droit commun réside dans l'option entre l'attribution judiciaire et l'exécution d'un pacte commissoire, option reconnue en matière de gage de meubles corporels civil (art. 2346 à 2348) ou commercial (art. L. 521-3 c. com.), de gage de véhicule automobile (art. 2353), de nantissement sur créance (art. 2365), d'antichrèse (art. 2388) et d'hypothèque conventionnelle (art. 2458 et 2459). Par exception, le pacte commissoire est nul en raison, soit de la faiblesse présumée du constituant (art. L.

  Le nouveau droit commun du gage couvre, lui aussi, tout le régime de cette sûreté. Le gouvernement ne s'est pas contenté, en effet, d'introduire les quelques « mesures phares » évoquées au cours de l'examen du projet de loi d'habilitation. Il a entièrement refondu le droit du gage en s'inspirant du Rapport Grimaldi et en consacrant des jurisprudences clarificatrices (ainsi, l'art. 2334, selon lequel « le créancier n'a d'action que sur le bien affecté en garantie » lorsque le gage est consenti par un tiers, reprend la solution de Cass. ch. mixte, 2 déc. 2005, D. 2006, AJ p. 61, obs. V. Avena-Robardet, Jur. p. 729, concl. J. Sainte-Rose, et p. 733, note L. Aynès ; AJ Famille 2006, p. 113, obs. P. Hilt ) et en veillant à harmoniser les règles nouvelles avec les solutions existantes. L'article 1415 ne pouvant plus fonder la protection du patrimoine familial en cas de constitution d'un gage sur un bien commun en garantie de la dette d'un tiers, cette protection procède désormais d'une extension de la cogestion. L'article 1422 a en effet été complété par un second alinéa ainsi rédigé : « ils [les époux] ne peuvent non plus, l'un sans l'autre, affecter l'un de ces biens à la garantie de la dette d'un tiers ».