Composition Dependence of the Structure and Electronic Properties of Liquid Ga-Se Alloys Studied by Ab Initio Molecular Dynamics Simulation

Ab initio molecular dynamics simulation is used to study the structure and electronic properties of the liquid Ga-Se system at the three compositions Ga$_2$Se, GaSe and Ga$_2$Se$_3$, and of the GaSe and Ga$_2$Se$_3$ crystals. The calculated equilibrium structure of GaSe crystal agrees well with available experimental data. The neutron-weighted liquid structure factors calculated from the simulations are in reasonable agreement with recent neutron diffraction measurements. Simulation results for the partial radial distribution functions show that the liquid structure is closely related to that of the crystals. A close similarity between solid and liquid is also found for the electronic density of states and charge density. The calculated electronic conductivity decreases strongly with increasing Se content, in accord with experimental measurements.Comment: REVTeX, 8 pages and 12 uuencoded PostScript figures, submitted to Phys. Rev. B. corresponding author: [email protected]

Angle-resolved photoemission study and first principles calculation of the electronic structure of GaTe

The electronic band structure of GaTe has been calculated by numerical atomic orbitals density-functional theory, in the local density approximation. In addition, the valence-band dispersion along various directions of the GaTe Brillouin zone has been determined experimentally by angle-resolved photoelectron spectroscopy. Along these directions, the calculated valence-band structure is in good concordance with the valence-band dispersion obtained by these measurements. It has been established that GaTe is a direct-gap semiconductor with the band gap located at the Z point, that is, at Brillouin zone border in the direction perpendicular to the layers. The valence-band maximum shows a marked \textit{p}-like behavior, with a pronounced anion contribution. The conduction band minimum arises from states with a comparable \textit{s}- \textit{p}-cation and \textit{p}-anion orbital contribution. Spin-orbit interaction appears to specially alter dispersion and binding energy of states of the topmost valence bands lying at $\Gamma$. By spin-orbit, it is favored hybridization of the topmost \textit{p}$_z$-valence band with deeper and flatter \textit{p$_x$}-\textit{p$_y$} bands and the valence-band minimum at $\Gamma$ is raised towards the Fermi level since it appears to be determined by the shifted up \textit{p$_x$}-\textit{p$_y$} bands.Comment: 7 text pages, 6 eps figures, submitted to PR

Pulsed laser deposition of Ga-La-S chalcogenide glass thin film optical waveguides

Thin film optical waveguides of the chalcogenide glass Ga-La-S have been deposited on substrates of CaF2 and microscope glass by the technique of pulsed laser deposition. The chalcogenide properties of photobleaching, photodoping, and photoinduced refractive index changes have been observed and preliminary experiments carried out. The refractive index and thickness of the layer were verified using a waveguide "dark mode" analysis technique

NON-STŒCHIOMÉTRIE ET MACLES DANS LES POLYSÉLÉNIURES DE LANTHANE

Trois polyséléniures de lanthane A, B, C ont été trouvés au sein de différentes préparations de LaSe2-x (0 ⩽ x ⩽ 0,12). Tous trois sont des surstructures de type-anti Fe2As. On ne peut obtenir de monocristal d'aucun de ces composés : ils sont tous maclés. Nous avons complétement défini les structures de A et B avec des facteurs de confiance cristallographique respectivement égaux , à 0,07 et 0,06. Le composé B répond à la formule non stœchiométrique LaSe1,88, les lacunes y sont totalement désordonnées. Le composé A répond à la formule stœchiométrique LaSe2 Les lois de macle de A et B sont différentes. La macle de B est vraisemblablement polysynthétique.Three lanthanum polyselenides A, B, C have been found in various preparations of LaSe2-x (0 ⩽ x ⩽ 0.12). They are superstructures of anti-Fe2As type. We have not been able to find a single crystal because all that compounds are twinned. The A and B structures have been completely determined, the final R values are respectively 0.07 and 0.06. The B compound is non stoichiometric, its formula is LaSe1.88, and the vacancies are entirely disordered. The A compound is stoichiometric with the formula LaSe2. The A and B twin-laws are different. Probably B consists of a polysynthetic twin

EVIDENCE FOR SHORT Cu(I)-Cu(I) DISTANCES IN COPPER-EXCESS SPINELS FROM EXAFS

L'étude EXAFS des spinelles Cu2x Cr2x Sn2-2x S4 a été réalisée dans le but de déterminer les caractéristiques structurales du cuivre monovalent spécialement pour les formulations surstoechiométriques en cuivre (2x >1). Nous avons montré que le cuivre se partage entre les sites tétraédriques 8a habituels et les sites octaédriques normalement vides où il occupe une position excentrée proche d'une face triangulaire. De plus, des distances courtes Cu-Cu (2,35 Å) sont mises en évidence, en accord avec l'arrangement relatif des sites.The Cu2x Cr2x Sn2-2x S4 spinel compounds have been investigated using EXAFS, especially as far as the structural features of monovalent copper excess are concerned, i.e. for 2x > 1. It has been shown that part of Cu is tetrahedrally surrounded (8a-sites) whereas the remaining part is triangularly coordinated in the vicinity of a face of the usually empty octahedral sites. In addition, the evidence for short Cu-Cu distances (2.35 Å) is given, in agreement with the octahedral- and tetrahedral- site arrangement

Composés du Pt(II) à visée antitumorale, dérivés de l’ellipticine et de ses analogues

L’environnement du platine dans des composés du Pt(II) à visée antitumorale dérivés de l’ellipticine et de ses analogues, est étudié par spectroscopie d’absorption X. Il est montré que, suivant les conditions de synthèse, on peut obtenir soit des sels de l’anion [PtCl4]2-, soit des composés de coordination dans lesquels le platine est entouré par deux atomes de chlore