Jean-Claude Bureau

Etude comparative des hexahydrido-et des hexadeuteridoaluminates de lithium et de sodium. I - Spectres raman et infrarouge de Li3-et Na3AlH6, et Li3AlD6

Abstract Raman and Infrared spectra of Lithium and Sodium hexahydrio- and hexadeuteridoaluminates Li3AlH6, Na3AlH6 and Li3AlD6 have been investigated in order to study metal-hydrogen interactions. A vibrational analysis has been performed. Several force field models (Urey-Bradley, Orbital Valence force field, Shimanouchi, Murrell).have been used in order to compute the force constants of Al-H bonds. No significative difference has been observed between the results obtained from the various models, and the environnement of aluminum has been thus well determined : AlH63− ions form an almost perfect octahedron in Li3AlH6 and a distorted one in Na3AlH6.

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.

Etude comparative des hydro- et deuteroaluminates de lithium LiAlH4 et LiAlD4 II - Spectres de diffusion raman des composes solides

Abstract The Raman spectra of lithium tetrahydroaluminate (LiAlH 4 ) and tetradeuteroaluminate (LiAlD 4 ) in their room pressure phases have been investigated at room temperature. Internal vibrational modes as well as lattice modes have been observed. As the cristallographic structure of LiAlH 4 -α is not fully determined, a partial vibrational analysis has been performed. Using deuteration as a spectroscopic probe, the separation between librational and translational phonon modes has been established. Raman spectra of the new LiAlD 4 -α′ form have been analysed. They have confirmed the irreversible transformation of LiAlD 4 -α′ into the well known LiAlD 4 - α form, at ca.65°C.

Far-infrared and Raman spectra of mixed HClDCl crystals

Abstract The far-infrared and Raman spectra of mixed hydrogen and deuterium chloride crystals in their low temperature phase have been investigated in the lattice-mode region throughout the whole concentration range. The major part of librational modes shows a delocalized mode behavior, but one librational mode exhibits a splitting characteristic of a localized mode behavior. This is explained by the small dispersion of the corresponding phonon subband. Using the results of the mixed crystal study, the proposed C 2v 12 (C mc2 1 ) structure for the pure crystal has been reexamined. Good correspondences between HCl and DCl frequencies have been obtained and show, as expected, that librational modes are mixed with translational modes. Dynamic calculations made on pure HCl crystals in the orthorhombic phase are discussed. The interest of polarized light experiments is also presented, their results are used to assign frequency lines.

Etude par diffusion raman des fluorotellurates IV d'ammonium NH4TeF5 et (NH4)2TeF6 solides

Polycrystalline samples of ammonium fluorotellurates NH4TeF5 and (NH4)2TeF6 have been investigated using RAMAN spectroscopic techniques. The study has been limited by the absorption of the laser light that easily decomposes the samples. Compared to previously recorded spectra, the RAMAN spectra of NH4TeF5 show new features below 100 cm−1 and at about 3200 and 1100 cm−1. A vibrational analysis has been performed and observed frequencies have been assigned to internal modes of NH4+ and TeF5− ions and to external modes. The RAMAN spectra of (NH4)2TeF6 have been recorded in the range 20 → 1000 cm−1. RAMAN frequencies have been assigned assuming an Oh symmetry for the TeF62− ion. Force constant calculations have been performed using simple force field models.

Etude comparative des hexahydrido- et des hexadeuteridoaluminates de lithium et de sodium. IV — Ferroelectricite et proprietes thermiques de Li3AlH6 et Na3AlH6

Abstract Thermal and electrical properties of Na 3 AlH 6 and Li 3 AlH 6 have been investigated in order to study the behavior of [AlH 6 ] 3− ions in hexahydridoaluminates in the temperature range from room temperature to their decomposition temperature (293 → 530K). A thermal anomaly observed on the variation of heat capacity of Na 3 AlH 6 has been explained in terms of a ferro-paraelectric transition, determined by electrical measurements. The ferroelectric phase has been related to the distorted geometry of [AlH 6 ] 3− ions, confirming our previous vibrational studies. In the same temperature range, Li 3 AlH 6 showed no structural modification, which is in full agreement with the almost perfect octahedral geometry of its [AlH 6 ] 3− ions found using vibrational spectroscopic techniques.

Raman scattering study of the ferroelectric phase transition in tris-sarcosine calcium chloride (tscc)

Abstract We obtained Raman scattering spectra of normal and deuterated TSCC crystals and one TSCCI-xBx mixed crystal (cI-x = Cl, Bx = Br). The soft-mode behaviour is studied in the temperature range from 80 K to the Curie point. The L and T component of the soft mode are overdamped near 100 K while the oblique component can be observed all over the temperature range we studied. For both normal TSCC and mixed TSCCI-xBx crystals the critical exponent β = 1 3 is in agreement with dielectric measurements and theoretical calculations.

Sur l'existence d'un hexafluorotellurate (IV) de diammonium (NH4)2TeF6: mise au point

Abstract We present in this paper new developments which confirm that the product previously described as (NH 4 ) 2 TeF 6 actually contains tellurium (IV) atoms octahedrally coordinated by six fluorine atoms. It is shown that the product is a solid solution nNH 4 F, mTeF 4 with n/m close to 2 and containing small amounts of HF.

Etude comparative des hexahydrido- et des hexadeuteridoaluminates de lithium et de sodium.III-modelisation des ions [AlHnD6−n]3− dans Li3AlHnD6−n (n = 0,1,2,4,5,6)

Abstract Raman and Infrared spectra of mixed compounds Li 3 AlH n D 6−n (n=0,1,2,4,5,6) have been investigated in order to study the behavior of [AlH n D 6−n ] 3− ions and aluminum-hydrogen interactions. Since in a previous work no isotopic exchange was observed between H and D atoms, a vibrational analysis and force field calculations have been performed using theoretical models (Murrell and Shimanouchi). Experimental frequency assignments have been confirmed, the environnement of aluminum atoms has been fully explained in terms of strong covalent interactions, and a general model of formation (of AlH 6 3− from AlH 4 − ) has been proposed for M 3 AlH 6 compounds.