Showing papers in "Journal of Solid State Chemistry in 1979"

The crystal structure of Cs[VOF3] · 12H2O

Abstract: The crystal structure of Cs[VOF 3 ] · 1 2 H 2 O has been determined and refined on the basis of three-dimensional X-ray diffractometer data (MoKα radiation). The structure is monoclinic, a = 7.710(2), b = 19.474(7), c = 7.216(2)A, β = 116.75(1)°, V = 967.5A3, Z =8, space group Cc (No. 9). The final R and Rw were 0.0295 and 0.0300, respectively, for 1356 independent reflections and 117 variables. The structure contains two crystallographically different VOF5 octahedra linked so as to form complex chains. Two non-equivalent octahedra share one FF edge, forming V2O2F8 doublets. Two F atoms, connected to different V atoms within the doublet, form an edge in the adjacent equivalent V2O2F8 unit thus continuing the chain. The VO distances are 1.583(7) and 1.595(7) A. The VF distances are in the range 1.881-2.205 A, mean value: 1.989 A. The H2O group is a crystal water molecule.

Structural study of a new hexagonal form of tungsten trioxide

Abstract: A new form of tungsten trioxide WO3 has been obtained by dehydration of WO 3 · 1 3 H 2 O hydrate. The structural study was carried out from X-ray powder diffraction and selected area electron diffraction data. The crystallographic characteristics are: the hexagonal system; a = 7.298(2) A, c = 7.798(3) A; Z = 6. This hexagonal WO3 is built up of slightly distorted (WO6) octahedra sharing their corners arranged in six-membered rings in layers normal to the hexagonal axis; stacking of such layers leads to formation of large hexagonal tunnels. Some confirmations of this structure were made by high-resolution electron microscopy. Powder X-ray diffraction allowed us to determine an average structure. Absence of suitable single crystals has not permitted us to perform a complete structural determination. Although the existence of such a hexagonal structure for pure WO3 had been considered as likely, it had not been hitherto observed.

Synthèse et caractérisation d'une série de titanates pérowskites isotypes de [CaCu3](Mn4)O12

Abstract: A series of titanates which have perovskite-like arrangements and are isostructural with [CaCu3](Mn4)O12 have been synthesized. The total charge of the A sites can be modified (1) by substituting the Ca2+ cations with monovalent ones and the tetravalent manganese cations of the B sites by a mixture of (Ti4+ + M5+) in which M = Ta, Nb, Sb, or (2) by substituting the Ca2+ cations by a combination of cations plus vacancies. In this case, if the total charge of the A sites is 2, one obtains compounds such as [ Th 4+ 1 2 □ 1 2 Cu 3 ](Ti 4 )O 12 and [T 3+ 2 3 □ 1 3 Cu 3 ](Ti 4 O 12 (T = rare earth ) ; on the contrary, if the charge is less than 2, then one has to compensate it by changing that of the B sites. This leads to compounds such as [□Cu3](Ti2M2)O12 (M = Ta, Nb, Sb).

The dehydration of boehmite, γ-AlOOH, to γ-Al2O3

Abstract: Electron microscopic observations of microstructure, together with studies of the evolution of X-ray and electron diffraction patterns, have been used to provide mechanistic information on the dehydration of boehmite to γ-Al 2 O 3 . Firing boehmite in air at 400°C produces slow development of a fine, lamellar, porous microstructure, oriented parallel to (001) γ , the dimensions of which are consistent with the loss of one-quarter of the oxygen atoms of the boehmite lattice. The mechanism proposed for the dehydration is controlled by diffusion in a direction determined by the hydrogen bond chains in the boehmite structure and involves countermigration of the Al cations and protons with crystallographic formation of voids in a coherent cubic close-packed matrix. Final reorganization to give the spinel structure of γ-Al 2 O 3 is suggested to involve gradual filling of the tetrahedral cation sites.

Crystal structures and crystal chemistry of the RETiO3 perovskites: RE = La, Nd, Sm, Gd, Y

Abstract: The structures of the RE TiO 3 perovskites, RE = La, Nd, Sm, Gd, and Y, were solved using single-crystal, automated diffractometer techniques. All were found to belong to space group Pbnm and are, therefore, isostructural with the RE FeO 3 perovskites. The structure of LaTiO 3 was solved using a crystal exhibiting a complex twinning. The RE O and TiO coordination polyhedra were studied as a function of the RE ion and were compared with those for RE FeO 3 . The major difference in results between RE TiO 3 and RE FeO 3 is a more highly distorted TiO octahedron for RE = Gd and Y.

Energy transfer from UO22+ to Sm3+ in phosphate glass☆

Abstract: Energy transfer from UO22+ to Sm3+ is described. The transfer efficiencies are calculated from the decrease of donor luminescence and lifetimes and from the increase of the acceptor fluorescence. It is shown that the transfer is nonradiative. The energy transfer efficiencies are greater when the donor is excited at higher energy levels due to stronger overlap between electronic levels of donor UO22+ and acceptor Sm3+. From the comparison of energy transfer efficiencies from UO22+ to Sm3+ and Eu3+ it is deduced that the overlap between excitation levels of donor and acceptor is a sufficient condition for the transfer.

A thermodynamic study of sodium-intercalated TaS2 and TiS2

Abstract: The variation of the sodium chemical potential (μNa) with composition x in NaxTaS2 and NaxTiS2 has been measured electrochemically using propylene carbonate-based and β-alumina electrolytes. At 300°K the sodium chemical potential in NaxTaS2 varies from −63 to −25 kcal/mole as x increases from 0.003 to 0.92, respectively. Within experimental uncertainty, the compositional variation of μNa was linear. In NaxTiS2, the sodium chemical potential varies from −60 to −36 kcal/mole as x increases from 0.001 to 1.0, respectively. The compositional variation of μNa in NaxTiS2 exhibits two plateaus indicative of two-phase regions for 0.2 ≲ x ≲ 0.35 and 0.6 ≲ x ≲ 0.7. The standard free energies of intercalation for sodium-intercalated TaS2 and TiS2 are less negative than those reported for the respective lithium-intercalated compounds. The standard free energy of intercalation becomes more negative for both the sodium- and lithium-intercalated compounds as the dichalcogenide changes from TaS2 to TiS2.

The effect of oxygen vacancy on the magnetic properties in the system SrCoO3−δ (0 < δ < 0.5)

Abstract: The cubic perovskite SrCoO 3−δ (0.05 ⩽ δ ⩽ 0.26) was prepared under high oxygen pressures of 50–2600 bars, and its crystallographic and magnetic properties were examined. From the results, it was found that oxygen deficiency strongly influences the unit-cell parameters and the Curie temperatures. The unit-cell parameter a 0 = 3.836 A and the Curie temperature 222°K for SrCoO 3 were determined.

Studies of layered uranium(VI) compounds. I. High proton conductivity in polycrystalline hydrogen uranyl phosphate tetrahydrate

Abstract: We have found that hydrogen uranyl phosphate tetrahydrate HUO 2 PO 4 ·4H 2 O has a high proton conductivity. The ac conductivity was 0.4 ohm −1 m −1 at 290°K measured parallel to the faces of sintered disks of the compound. The activation energy was found to be 31 ± 3 kJ mole −1 . The values of conductivity were between 3 and 10 times lower when measured perpendicular to the disk faces due to preferred orientation of the plate-like crystals. Both the powder and sintered disks are stable in air and insoluble in phosphoric acid solution of pH 2.5. Experiments are described which enable possible grain boundary contributions to the conductivity to be determined in such hydrates. The extrinsic grain boundary contribution to the conductivity was found to be small from experiments in which the pH in a solution cell was varied. The abnormally high bulk H + conductivity thus inferred is attributed primarily to the high concentration of H + , which exists as H 3 O + in the interlamellar hydrogen-bonded network. A Grotthus-type mechanism of conduction is proposed which involves intermolecular transfer steps (hopping) and intramolecular transfer steps, in comparable numbers, the former facilitated by the high concentration of H 3 O + ions in the structure, and the latter most likely facilitated by the high concentration of H-bond vacancies.

Elastic and inelastic neutron studies of hydrogen molybdenum bronzes

Abstract: Hydrogen molybdenum bronzes HxMoO3 (∼0.3 < x < 2.0) have been investigated with elastic and inelastic neutron scattering. Neutron diffraction studies of orthorhombic D0.36MoO3 show that deuterium is incorporated as -OD without any major structural change to the MoO3 layer lattice. The inelastic neutron scattering spectra of HxMoO3 phases confirm that, for H0.34MoO3, H is present as -OH, but, for the monoclinic phases H0.93MoO3, H1.68MoO3 and H2.0MoO3, only peaks associated with -OH2 groups are found.

Fluorite-related phases Ln3MO7, Ln = rare earth, Y, or Sc, M = Nb, Sb, or Ta. I. Crystal chemistry

Abstract: Compounds Ln3MO7, where Ln = La, Nd, Gd, Ho, Er, Y, or Sc, and M = Nb, Ta, or Sb have been examined by powder X-ray diffraction, electron diffraction, and electron microscopy. For large Ln cations, an orthorhombic fluorite-related superstructure is formed, of probable space group Cmcm for Ln = La and C2221 for Ln = Nd, Gd, Ho, or Y, while for the smaller Ln cations, Er, and under some conditions, Ho and Y, the structure is defect fluorite containing microdomains of ordered, but undetermined, structure. The composition Sc3MO7 was not single phase under the conditions used. Compounds of the type Ln2ScMO7 have the pyrochlore structure.

The topology of three-dimensional 4-connected nets: Classification of zeolite framework types using coordination sequences

Abstract: Coordination sequences provide a practical numerical scale for expressing the degree of similarity of three-dimensional 4-connected nets found in zeolite structures when weighted mean values are used for topologically heterogeneous nets. Values of coordination sequences limited to 5 terms have been determined for nearly all of the known zeolite framework types. They are shown to be useful for classifying zeolite frameworks on purely topological grounds and also for finding the maximum possible space group symmetry in more difficult cases.

Fluorite-related phases Ln3MO7, Ln = rare earth, Y or Sc, M = Nb, Sb, or Ta: II. Structure determination

Abstract: The crystal structures of the orthorhombic fluorite-related compounds La3NbO7 (Cmcm), Y3TaO7, and Y2GdSbO7, (C2221) have been determined from X-ray powder diffraction intensities and single-crystal electron diffraction data. The structures are basically similar. One third of the Ln cations are 8-coordinated, and lie in [001] rows which alternate with parallel rows of corner-linked MO6 coordination octahedra within slabs parallel to (100). The remaining Ln cations lie in between these slabs in seven-fold coordination. This interslab cation arrangement distinguishes the structure from the closely related pyrochlore (A2B2O7) and weberite (Na2MgAlF7) structures. The compounds YGdScSbO7 and Nd2ScNbO7, also examined, have the pyrochlore structure, with Sc and Sb, or Sc and Nb distributed on the octahedral B sites.

The electrical, optical and photoconducting properties of Fe2−xCrxO3 (0 ⩽ x ⩽ 0.47)

Abstract: A study has been made of the electrical, optical and photoconducting properties of pure and reduced single crystals of composition Fe 2− x Cr x O 3 where 0 ⩽ x ⩽ 0.47. It has been found that pure α-Fe 2 O 3 is not a photoconductor. When defect-free crystals of α-Fe 2 O 3 are reduced a surface layer of Fe 3 O 4 is formed and the crystals exhibit photoconductivity. Removal of this layer resulted in the disappearance of photocurrents and an increase in the sample resistivity. A necessary condition for the observation of photocurrents in n -type Fe 2 O 3 is that some Fe 3 O 4 be present. In addition, it has been found that the substitution of chromium for iron in α-Fe 2 O 3 results in a monotonically decreasing optical band gap as the chromium concentration, x , increases.

Li3N: A promising Li ionic conductor

Abstract: Li 3 N is presented as one of the best solid lithium electrolytes available today. Its outstanding advantages are simple preparation of crystalline as well as sintered material, stability in moist atmosphere because of a protecting surface layer, stability with elemental lithium metal up to its melting point of 813°C, high Li-ion conductivity of the order of 10 −3 Ω −1 cm −1 at ambient temperature with negligible partial electronic conductivity, and a high decomposition voltage due to a strong kinetic hindrance of the decomposition reaction. Additional work has to be focused on optimization of the Li 3 N sinter preparation and on the kinetics of potential electrodes which can be used for a lithium-based battery with an Li 3 N electrolyte.

Intercalation chemistry and energy storage

Abstract: The reaction between lithium and titanium disulfide is used to show the relationship between intercalation chemistry and electrochemical energy storage. The maintenance of crystalline structure with only a 10% lattice expansion perpendicular to the sulfide sheets allows high rates of reaction with lithium and complete reversibility of the reaction. The behavior of TaS2 and VSe2 are compared with that of TiS2. Many oxides and other chalcogenides of the early group transition metals are also able to react with lithium to form ternary compounds, but none of these have to date shown the high reversibility of titanium disulfide.

Optical transitions of Sm3+ in oxide glasses

Abstract: Eigenvectors of the 4f5 electronic configuration of Sm3+ were calculated in intermediate coupling, and used to obtain the reduced matrix elements U(λ). Absorption spectra of Sm3+ were recorded in phosphate, borate, germanate and tellurite glasses. The Judd-Ofelt Ωλ's intensity parameters were then deduced from the measured oscillator strengths by least-squares fitting. Radiative transition probabilities and integrated cross-sections of stimulated emissions are obtained. Calculated branching ratios and decay lifetimes are compared with the experimental values.

Crystal structure of tetragonal boron related to α-AlB12

Abstract: Single crystals of the so-called β-tetragonal (or tetragonal II or III) boron modification have been obtained from boron deposits prepared by hydrogen reduction of BBr3 on tantalum filaments at 1200°C. Chemical analysis of the samples shows that this phase can be regarded as a true modification of pure elemental boron in contrast to α-tetragonal phases which require small amounts of foreign atoms to stabilize their boron framework. The lattice parameters (a = 10.14(1)A; c = 14.17(1)A) were obtained and refined from single crystal data. The unit cell contains four chemical units, B21 · 2B12 · B2.5 resulting in dc = 2.34 g cm−3 (dm = 2.36(2) g cm−3). The systematic extinctions are compatible with space group P41 or P43. The structure was determined from 1009 independent reflexions using a model derived from the recently solved structure of α - AlB12 (a = 10.161A; c = 14.283A; space group P41212 or P43212). The final R value (unweighted data) is 9.6%. Basically, the structure of this tetragonal form of boron consists of the same three-dimensional boron skeleton, built upon simple and twinned icosahedra, as that of α-AlB12. However, the defective twinned icosahedral B19 units in α-AlB12 are now completed (B21 units) in the related tetragonal boron. A number of interstitial sites, located at positions different from those occupied by aluminum in α-AlB12, are totally or partially filled by boron atoms and very probably increase the stability of the boron framework.

The effect of reduction and temperature on the electronic core levels of tungsten and molybdenum in WO3 and WxMo1−xO3—A photoelectron spectroscopic study

Abstract: X-Ray photoelectron spectroscopy (XPS) has been applied to electrochromic, reduced WO 3 and W x Mo 1− x O 3 crystals. In metal-reduced phases containing crystallographic shear planes the formation of Mo 5+ (preferentially) and W 5+ is observed in addition to that of the six-valent states. W 5+ and W 6+ are also dominant in H + -bombarded WO 3 indicating the formation of bronzes H x WO 3 . Significant differences are observed between single-crystal and “amorphous” oxides. The five-valent state is interpreted as being due to electron trapping and polaron formation. Under Ar + bombardment the crystallinity of the surface is destroyed and a continuous distribution of W 0 , W 4+ , W 5+ , and W 6+ is found similar to that observed for amorphous thin films. At low temperatures the (ϵ-δ) metal-insulator (M-I) transformation of H + :WO 3 is accompanied by a spontaneous change in the linewidth of W 5+ core levels but not of W 6+ states. This is in accordance with recent theoretical approaches to M-I transformations.

Phase diagrams of the SrF2(Y, Ln)F3 systems part I.—X-ray characteristics of phases

Abstract: The phase equilibria in the subsolidus part of 14 binary systems of SrF 2 (Y, Ln)F 3 type (Ln—all lanthanides except Pm and Eu) were studied at temperatures over 850°C in equilibrated and quenched specimens by the X-ray analysis method. The oxygen concentration in the specimens before and after the thermal treatment was checked. The crystallographic characteristics of phases formed in the systems i.e. nonstoichiometric phases Sr 1− x Ln x F 2+ x with fluorite type structure, phases with fluorite derived type structure, nonstochiometric phases Ln 1− y Sr y F 3− y with tysonite (LaF 3 ) type structure are given in this paper.

Substitution in barium-fluoride apatite: The crystal structures of Ba10(PO4)6F2, Ba6La2Na2(PO4)6F2 and Ba4Nd3Na3(PO4)6F2

Abstract: The crystal structures of the apatites Ba10(PO4)6F2(I), Ba6La2Na2(PO4)6F2(II) and Ba4Nd3Na3(PO4)6F2 (III) have been determined by single-crystal X-ray diffraction. All three compounds crystallize in a hexagonal apatite-like structure. The unit cells and space groups are: I, a = 10.153(2), c = 7.733(1)A, P6 3 m ; a = 9.9392(4), c = 7.4419(5) A, P 6 ; III, a = 9.786(2), c = 7.281(1)A, P 3 . The structures were refined by normal full-matrix crystallographic least squares techniques. The final values of the refinement indicators Rw and R are: I, Rw = 0.026, R = 0.027, 613 observed reflections; II, Rw = 0.081, R = 0.074, 579 observed reflections; III, Rw = 0.062, R = 0.044, 1262 observed reflections. In I, the Ba(1) atoms located in columns on threefold axes, are coordinated to nine oxygen atoms; the Ba(2) sites form triangles about the F site and are coordinated to six oxygen atoms and one fluoride ion. The fluoride ions are statistically displaced ∼0.25 A from the Ba(2) triangles. This displacement of the F ions is analogous to the displacement of OH ion in Ca10(PO4)6(OH)2. The structures of II and III contain disordered cations. In II there is disorder between La and Na in the column cation sites as well as triangle sites. In III, Nd and Na ions are ordered in the column sites, but there is disorder among Ba and the remaining Nd and Na ions in the triangle sites to give an average site population of 2 3 Ba , 1 6 Nd , 1 6 Na . The coordination of the rare earth ions and Na ions in the ordered column sites are nine and six oxygens, respectively, in accord with the greater charge of the rare earth ions as compared with Na. The F ions in both II and III suffer from considerable disorder in position, and their locations are not precisely known.

Electron microscopic study of dehydration transformations. I. Twin formation and mosaic structure in hematite derived from goethite

Abstract: A study by means of electron microscopy at high resolution has enabled the analysis of the microstructure of hematite as produced by dehydration from goethite. The apparently “polycrystalline” structure is found to consist of aggregates of well-oriented twin-related hematite crystals, separated by regularly spaced H(001) walls of voids resulting from the loss of water. The material is thus broken up into blocks with sizes smaller than 50 A. The crystallography of the twinned structure can be analyzed by electron diffraction data and dark-field imaging. These aggregates of hematite crystals provide an ideal mosaic structure. In particular the effect of this mosaic structure on the individual reflections of the diffraction patterns has been analyzed and discussed.

Electrical conductivity and defect chemistry of PbMoO4 and PbWO4

Abstract: The ionic and electronic conductivities of Czochralski-grown single crystals of PbMoO4 and PbWO4 are reported and discussed. Nominally pure crystals, as well as crystals with various aliovalent dopants, were used. From the electrical measurements it is concluded that oxygen vacancies are responsible for the ionic conductivities in both compounds. At high temperatures electronic conductivity predominates. Measurements of the oxygen partial pressure dependence of the electronic conductivity reveal that hole conduction prevails in PbWO4, while mixed conduction by holes and electrons is present in PbMoO4.

QMSCC calculations on thermal quenching of model phosphor systems

Abstract: Quantum mechanical single configurational coordinate (QMSCC) calculations have been performed on model phosphor systems. Variation of the QMSCC parameters provides information about the influence of these parameters on the temperature dependence of the luminescence efficiency. The results of the calculations are correlated with experimental results reported in the literature.

Transformation of three-connected silicon nets in CaSi2

Abstract: Up to 40 kbar and 1100°C, CaSi2 is dimorphic Trigonal/rhombohedral CaSi2I (CaSi2-type structure) with corrugated layers of three-connected Si atoms can be transformed by a high pressure-high temperature treatment into tetragonal CaSi2II (α-ThSi2-type structure) with a three-dimensional net of three-connected Si atoms The silicon net of CaSi2II is slightly distorted from the topologically simplest tetragonal three-dimensional three-connected net derived on a geometrical basis In order to correlate crystal chemical with thermochemical data the transformation between both polymorphs of CaSi2 has been studied at equilibrium and nonequilibrium conditions The pressure-temperature phase diagram of CaSi2 has been investigated by X-ray technique in quenched samples From the slope of the equilibrium line and the change in molar volume the approximate values of the entropy and energy of transformation CaSi2(I-II) have been determined ΔS = 32 eu, ΔU = 49 kcal/mole Under nonequilibrium conditions the transformation CaSi2(II-I) yielded ΔH = −42 kcal/mole at 500°C and ambient pressure in a DTA apparatus Complete transformation of metastable CaSi2II can be achieved within 5 min at a heating rate of 20°C/min Due to the relatively high speed of transformation simple structural relations between both polymorphs of CaSi2 are discussed

Thermochemistry of hydrogen molybdenum bronze phases HxMoO3

Abstract: The enthalpies of formation of five hydrogen molybdenum bronze phases, HxMoO3 (0 < x < 2), have been determined by solution calorimetry. Values obtained for formation from H2(g) and MoO3(s) at 298.15°K were (kJ mole−1): H0.28MoO3, −18.2 ± 0.5; H0.34MoO3, −22.5 ± 0.6; H0.93MoO3, −56.5 ± 0.8; H1.68MoO3, −83.9 ± 1.6; and H2.0MoO3, −80.4 ± 2.1. The thermodynamic stabilities of these phases are discussed in relation to their observed electrochemical behavior.

The properties of coordination sequences and conclusions regarding the lowest possible density of zeolites

Abstract: The topology of framework structures may be characterized by coordination sequences. It is shown that the information contained in these sequences is expressible in terms of two or three parameters. From diagrams involving such parameters it may be concluded that zeolites with densities considerably lower than those presently known would either contain three-membered rings or would have T -atoms connected to more than three four-membered rings. There are arguments against the realization of such properties, and zeolites of essentially lower densities will possibly not be found.

Preparation, structures, and properties of niobium chalcogenide halides, nbx2y2(x=s,se-y=cl,br,i)

Abstract: The preparation of the compounds NbX2Y2 (X = S, Se; Y = Cl, Br, I). including that of large single crystals, by chemical transport techniques is reported. Most of these compounds exist in a triclinic low-temperature form and a monoclinic high-temperature form. The crystal structures of triclinic low-temperature NbSe2Cl2 and monoclinic NbS2Cl2 are reported; both structures consist of cage-shaped units of Nb2X4 which are linked together by the Cl atoms to form sheets parallel to the a, b planes. Both the Nb and the X atoms are present in pairs which indicates that the compounds can be formulated as Nb4+2(X2)2−22Y−4, in agreement with XPS spectra. The presence of Nb4+ in pairs explains the observed diamagnetic and semiconducting properties of NbX2Y2. The preparation and some properties of MoS2Cl2 (orthorhombic) are also given.

Etude structurale de la forme ordonnée de LiAl5O8

Abstract: Resume La structure d'un monocristal de LiAl5O8 ordonne obtenu par fusion au four a image a ete determinee. Les intensites de 400 reflexions de diffraction X ont ete mesurees a l'aide d'un diffractometre automatique avec le rayonnement Kα du molybdene. La structure derive de celle du spinelle. Le groupe d'espace est P4332 (ou P4132), a = 7,908(2), A, Z = 4. La valeur de R est 0,022. La repartition obtenue pour les cations montre l'absence d'ions Li+ dans les sites tetraedriques et une mise en ordre 1–3 imparfaite des ions Li+ et Al3+ dans les sites octaedriques du spinelle.

Contribution à l'étude structurale de la wüstite solide de haute température

Abstract: Resume La structure de la wustite Fe1−zO est etudiee par diffraction de neutrons sur un echantillon polycristallin dans les conditions d'equilibre a haute temperature. Le facteur d'attenuation B est exprime par la somme de deux composantes BTh et BSt fonctions lineaires d'une seule variable, respectivement la temperature et le parametre z. Il est etabli une classification des amas ( m n ) formes de m lacunes en sites octaedriques et de n ions FeIII en sites interstitiels. Seize valeurs du rapport ϱ = (z + t) t = m n du taux de lacunes au taux d'ions FeIII interstitiels ont ete determinees experimentalement. Une valeur remarquablement constante inferieure a 3 est observee. Ce resultat caracteristique de l'ordre a courte distance ne peut etre interprete par la presence d'amas ( 4 1 ) isoles ou d'amas appartenant aux deux filiations definies par ϱ = (1 + 3n) n . Procedant du meme mode de formation, les amas ( 16 6 ) ou ( 40 14 ) pourraient convenir. Parmi les multiples amas obtenus a partir d'amas ( 4 1 ) lies par deux lacunes, les amas ( 8 3 ) ou ( 9 4 ) seraient les plus probables. Une analyse des bosses de diffusion exclut l'existence de grands domaines ayant la structure spinelle inverse. La distinction structurale entre les 3 varietes W1, W2 et W3 ne semble pas relever d'un changement de nature des amas.