Jean-Pierre Bastide

Structural characterization of NaMgH2F and NaMgH3

The direct reaction of hydrogen on a mixture of Na + Mg or NaF + Mg allowed for synthesis of NaMgH3 and NaMgH2F, respectively. Both phases were indexed with an orthorhombic unit cell with dimensions a = 546.34, b = 770.30, c = 541.08 pm for NaMgH3 and a = 547.59, b = 769.68, c = 540.31 pm for NaMgH2F. The crystal structures were refined by X-ray Rietveld refinement. They derive from the perovskite structure type by a Pmm → Pnma distortion. In the hydridofluoride, H− and F− anions are equally distributed in the two anionic sites in a disordered way.

Reactions of lithium and sodium aluminium hydride with sodium or lithium hydride. Preparation of a new alumino-hydride of lithium and sodium LiNa2AlH6

Abstract A study of the reactions of MAlH4 with M′H (where M, M′ = Li, Na) has been carried out under various conditions of temperature and pressure. A new mixed hydride LiNa2AlH6 has been obtained. The X-ray structure has been determined on a Philips PW 1120 diffractometer and the product crystallises into a fcc structure with a cell having the dimension a = 7.405 A . Its diffraction pattern is very close to that of the high temperature modification of Na3AlH6 which possesses the ideal fcc cryolite structure. The thermal stability has been studied by DSC and the enthalpy of decomposition has been calculated and compared with the values of the other mixed hydrides of lithium and sodium

Polymorphisme de l'hexahydroaluminate trisodique Na3AlH6

Abstract Structural investigations on the cryolite-like compound Na 3 AlH 6 have been carried out using high-temperature diffractometry. α-Na 3 AlH 6 which crystallises in a monoclinic pseudo-cubic system under room-temperature conditions exhibits at 525 K a polymorphic transition into a f.c.c. form β-Na 3 AlH 6 . This result has been established by high-pressure-high-temperature studies. From structural point of view Na 3 AlH 6 -β is to be related to the high temperature modification of cryolite Na 3 AlF 6

Etude comparative des tetrahydro- et -deuteroaluminates de lithium LiAlH4, LiAlD4. I — Preparation, etude radiocristallographique et comportement thermique — Mise en evidence d'une variete metastable de LiAlD4

Abstract Toluene can precipitate lithium tetrahydroaluminate LiA1H 4 and trahydrodeuterate LiA1D 4 from concentrated solutions in diethylether at 20°C. This method gives small, solvent-free crystals with a purity better than 99.9%. X-ray and D.S.C. characterization show that LiA1H 4 is obtained in the usual “α” crystalline form. A new phase, LiA1D 4−α 1 can also be isolated, which transforms irreversibly upon heating to LiA1D 4−α . The latter phase has the same structure as LiA1H 4−α .

Etude comparative des tetrahydro- et -deuteroaluminates de lithium LiAlH4, LiAlD4. III - Caracterisation de la variete γ haute pression

Abstract The γ-crystalline form of LiAlH 4 (or LiAlD 4 ) has been obtained at high pressure using a belt type apparatus. The γ-phase is quenched and studied at ambient pressure. The crystalline structure has been redeterminated. On the basis of Raman spectra an octahedral environment of Al 3+ is proposed. The α → γ transition is reflected in a change of the CN of Al 3+ and Li + from 4 to 6, and an important decrease of the volume ( ⋍30 % ). Upon warming, the γ-phase transforms into the α-form with a large endothermal effect, as expected for a true hig pressure phase. From these results, a tentative (P,T) phase diagram for LiAlH 4 (or LiAlD 4 ) is proposed.

Etude spectrometrique infrarouge et raman du tetrahydridoaluminate de sodium NaAlH4 solide

Abstract Synthesis of sodium tetrahydridoaluminate has been optimized to allow the formation of a polycrystalline powder with crystallites of about 0.5 mm dia. This material showed very good optical properties for Raman scattering and lattice modes has been obtained by spectral analysis. This work which has been performed from 150 to 300 K shows that spectral parameters do not vary in this temperature range. In contrast, IR spectra show only very broad bands and an accurate analysis was not possible. Vibrational analysis of internal modes and lattice modes agrees very well with the crystalline structure and confirms the ionic character of the crystal.

Synthese et caracterisation du pentafluorotellurate (IV) de sodium NaTeF5

Abstract Sodium pentafluorotellurate (IV) NaTeF5 has been prepared by slow evaporation of a 1 1 mixture of sodium fluoride and tellurium dioxyde in 50 % aqueous hydrofluoric acid. NaTeF5 has an orthorhombic structure, parameters of which are a = 9.10 5 A b = 11.30 6 A c = 10.12 A

Enthalpies standard de formation des composés à structure perovskite KMgH3, KMgH2F, KMgHF2, KMgF3, et K2MgF4

Abstract The standard enthalpies of formation of KMgH 2 F and K 2 MgF 4 were determined by reaction with hydrochloric acid solutions (0.5 M HC1 and 3 M HC1 respectively). The standard enthalpies of formation of KMgF 3 , KMgHF 2 and KMgH 3 were estimated on the basis of the following measurements. Δ f H XXX (K 2 MgF 4 ,cr,298K) = −2317 ± 20kJmol −1 Δ f H XXX (KMgF 3 ,cr,298K) = −1727 ± 15kJmol −1 Δ f H XXX (KMgHF 2 ,cr,298K) = −1246.5 ± 5kJmol −1 Δ f H XXX (KMgH 2 F,cr,298K) = −762.6 ± 10kJmol −1 Δ f H XXX (KMgH 3 ,cr,298K) = −278.4 ± 5kJmol −1

Crystal structure of PbTeF6

Abstract PbTeF6 crystallizes with monoclinic symmetry (space group P2 1 c ) and the unit cell parameters a = 462.5(1) pm, b = 1281.2(2) pm, c = 851.9(1) pm, β = 102.36(1)°, Z = 4. Its crystal structure has been solved by a Rietveld analysis of its X-ray powder pattern and refined to the final agreement factors Rb = 0.059 and Rp = 0.073. The PbII and TeIV atoms are respectively seven- and five-fold coordinated and their lone pair E is stereochemically active. By sharing edges, the PbF7E polyhedra constitute infinite zig-zag chains parallel to Ox. These chains are connected to each other by sharing their free corners with the TeF5E polyhedra, one of the corners being non-bridging. Structural relationships with ReO3-type materials are described and analyzed.

Etude du comportement thermique des hexitols: Partie 2. Polymorphisme du dulcitol

Dulcitol, known in a single crystalline form (hereafter called I), in fact exhibits complex thermal behaviour. A second polymorph II appears when the glassy material is allowed to crystallize at room temperature. This form always coexists with I and cannot be well characterized. The mixture I + II transforms very slowly to pure I when annealed at room temperature for a week. When a melt of dulcitol is cooled very slowly vitrification does not occur and a new pure crystalline form III is obtained. Analysis of X-ray powder data leads to a possible monoclinic cell with a = 8.82 A, b = 11.41 A, c = 8.61 A, β = 110.55°, Dx = 1.49, Z = 4. These parameters are very close to those of I but the slightly smaller values for a and b could indicate a shrinking of some bonds in the molecular packing. Each of the two forms II (mixed with I) and pure III are thermodynamically unstable. The “normal” form I is always obtained by evaporation from methanol/water solutions.