Christian Cros

Clathrate Structure of Silicon Na8Si46 and NaxSi136 (x < 11).

The crystal structure of two new cubic phases in the silicon-sodium system have been solved from their x-ray diffraction patterns. Both structures are of the clathrate type found for gas hydrates, consisting of tetrahedral networks which are combinations of pentagonal dodecahedra with 14-face polyhedra in one case and with 16-face polyhedra in the other case. There is strict correspondence between the silicon positions and the oxygen positions of the hydrate structures. For one compound, Na8Si46, the centers of all polyhedra are occupied by sodium atoms. For the other compound, there occurs only partial occupancy of the polyhedral cages.

Sur une nouvelle famille de clathrates minéraux isotypes des hydrates de gaz et de liquides. Interprétation des résultats obtenus

Abstract The thermal degradation of the alkali silicides and germanides of formula MeSi and MeGe (Me = Na, K, Rb, Cs) leads to intermediate phases having the clathrate structure of the gas and liquid hydrates. The alkali atoms occupy holes in a distorted silicon or germanium lattice; a radius intermediate between the ionic and metallic radii accounts for the experimental results, involving a certain localization of the peripheral s electron. This conclusion is confirmed by the magnetic and electric properties.

Structural characterisations of the NaxSi136 and Na8Si46 silicon clathrates using the Rietveld method

The crystal structure of the non-stoichiometric Na x Si 136 silicon clathrate has been refined using the Rietveld method, in order to determine accurately the distribution of the sodium atoms within the two available sites. In agreement with the previous data, it was found that for x≤8, the alkali atoms occupy exclusively, and not only preferentially the eight larger Si 28 sites. For 8<x<24, the filling of the sixteen smaller Si 20 cages occurs gradually with increasing x, and a slight increase of the unit cell parameter is then observed. The crystal structure of the stoichiometric Na 8 Si 46 clathrate, which is present as impurity in the studied samples, has also been refined.

Contribution a l'étude des propriétés magnétiques des phases AVX3 (A = Rb, Cs; X = Cl, Br, I)

The magnetic properties of RbVCl3, CsVCl3, CsVBr3, and CsVI3 have been investigated in the temperature range 4.2–800°K. In agreement with the 2H structure of these phases, the obtained results have been interpreted on the basis of a monodimensional Heisenberg model. The values of the exchange integrals have been determined by means of Smith and Friedberg [Phys. Rev. 176, 660 (1968)] calculations. They involve strong antiferromagnetic interactions, which decrease from the chloride to the iodide. The nature of the coupling between vanadium atoms is discussed.

Ionic Conductivity and Phase Transition of the Bromide Spinels, Li2 − 2x M 1 + x Br4 ( M = Mg , Mn )

A new bromide spinel has been found in the system. The orthorhombic cell dimensions are , , and at room temperature. The electrical conductivities of the new bromide spinel and the related spinel have been measured. and have been found to have high ionic conductivities of and at 400°C, respectively. High temperature x‐ray diffraction analysis indicates a phase transition from a distorted structure to a cubic structure around 300°C for and around 270°C for. Electrical conductivity measurements suggest that the transitions correspond to those from the low to high ionic conductivity phase. A solid solution is observed on substitution of lithium ions for Mg2+ or Mn2+ ions. The substitution leads to higher ionic conductivity only for the cubic spinel. No significant composition dependence of the conductivity or the activation energy for conduction has been observed in the distorted structures.

A brief overview on low sodium content silicides: are they mainly clathrates, fullerenes, intercalation compounds or Zintl phases?

Low sodium content silicides discovered by some of us in the sixties are compared to some analogous families of materials. As endofullerenes constituted by the smallest Si 20 ,S i 24 and Si28 clusters, they are polymerized in 3D-structures of face sharing polyhedra, giving rise to clathrate-like structures (similar to those of the gas and liquid hydrates). Several physical properties (conductivity, 23 Na and 29 Si NMR, EPR) show that the electronic transfer from sodium to the silicon host is rather low and appears only gradually with rising Na-content as a consequence of Na 3s and Si 3s-3p orbital mixing which gives rise to an antibonding band. Silicon clathrates differ from the homologous germanium and tin compounds characterized by a large number of defects in the host structure which leads to stronger ionization of the alkali atoms and to the formation of some Ge − anions within Zintl–Klemm-type phases.  2002 Editions scientifiques et medicales

Sur le systeme LiClVCl2

Abstract The phase diagram of the system LiClVCl2 has been investigated by X-ray diffraction on slowly cooled and quenched samples, as well as by D.T.A. measurements. In the solid state three homogeneity ranges have been observed, all with a c.f.c. anionic packing. The first one corresponds to the presence of V2+ ions and vacancies in a Li-deficient LiCl type structure, this appearing to be disordered at high temperature and more or less ordered at lower temperature. The second range is a deficient spinel-type solid solution, in which there occurs another kind of ordering between cation and vacancies in both the octahedral and tetrahedral sites. The last range corresponds to an insertion of Li+ ions in a CdCl2 type lattice, with a lamellar arrangement of divalent cations in the octahedral sites. The significant degree of disorder, which characterizes all these solid solutions at high temperature may be expected to lead to high mobility of the Li+ ions.

Les bronzes fluores de vanadium

Abstract The investigation of the ternary systems AF-VF2-VF3 (A = Li, Na, K, Rb, Cs) for molar ratios MF VF 2 = 1 leads to a new series of compounds, generally non-stoichiometric, of formula AxVF3, where vanadium has simultaneously the oxidation states +II and +III. The structural type depends upon the composition and the size of the alkali metal. The systems AxVF3, AxCrF3 and AxFeF3, which are all related to the tungsten bronzes AxWO3, are compared.

Sur la phase CsVCl3: Etude cristallographique et magnetique

The crystal structure of CsVCl3 has been determined by means of a Patterson synthesis and refined by full-matrix least-squares to a residual R = 0.06. The space group is P63mmc with a = 7.228 (3) A and c = 6.030 (3) A. The structure of CsVCl3 belongs to the CsNiCl3 type. The magnetic properties have been interpreted on the basis of a S = 32 model for Heisenberg one-dimensional interactions. The strong antifer-romagnetic intrachain V-V interactions (Jk = −115 K) seem to result from direct t2g - t2g coupling and from 90° correlation superexchange coupling using the 3s chlorine orbital.

High pressure synthesis of an iodine doped silicon clathrate compound

A silicon clathrate compound doped with an electronegative element, I8Si46 − xIx (x = 1.5 ± 0.5), has been prepared for the first time using high pressure and high temperature conditions.