M. Maurin

Etude comparée à diverses températures (173, 293 et 373°K) des structures de type quartz α des phases MIIIXVO4 (MIII = Al, Ga et XV = P, As)

Abstract The crystal structures of quartz α type M III X V O 4 ( M = Al, Ga; X = P, As) were refined at 173, 293, and 373°K in the P 3 1 21 space group. In this temperature range, the packing shows very high rigidity. Distortions of tetrahedral sites ( X O 4 and M O 4 ) have been evaluated and decrease when the temperature increases. The comparison of these packings shows an increasing value of their distortions with the size of the M III and X V elements.

Glass formation, structure and ionic conduction in the Na2SGeS2 system

Abstract Na2S forms stable glasses with GeS2 in a large range of composition (from 0–0.6 in molar ratio of Na2S). The Raman spectra of polycrystalline thiogermanates (Na4Ge4S10, Na2GeS3, Na6Ge2S7) have been recorded. A vibrational assignment in terms of terminal (GeS) and bridge (GeSGe) stretching has been made. The spectra of the glasses and the crystalline phases were also compared. The strong resemblance between these spectra shows that GeS4 tetrahedra are present in the glasses. In addition, it seems probable that the different types of chains found in the crystallized compounds are also present in the vitreous network. The electrical conductivity of these glasses was measured as a function of temperature (25–150°C) and composition by the complex impedance diagram method. At 25°C, the conductivity attained a maximum value of 5 × 10 −6 Ω −1 cm −1 for glass containing the highest mole-fraction of Na2S. The ionic transport number obtained by emf measurements was found to be 1. Substitution of sulphur by oxygen in oxide glasses is found to improve the ionic conductivity. This may be due to an increase in the number of charge carriers, weaker bonding between sodium and sulphur ions than between sodium and oxygen, and a greater mobility of the alkaline cations which would result from a greater polarizability of the sulphur.

Chemical bonding and electronic properties in antimony chalcogenides

The lone-pair antimony chalcogenides are investigated. A detailed qualitative classification of their properties is worked out. The authors show that the {sup 121}Sb Moessbauer isomer shift increases with the covalent character of bonds and with the distortion of the local antimony environment. This is quantitatively explained by a tight-binding band-structure calculation. The lone-pair stereochemical activity also weakens with the forbidden band gap while the conductivity and the metallic character of bonds increase.

Evaluation of high quality berlinite crystals grown in sulphuric acid medium

Abstract This investigation concerns berlinite crystal growth in sulphuric acid solvent through the reverse temperature gradient method and its consequences on crystalline quality and piezoelectric characteristics. Systematic study of growth parameters shows a high sensitivity of growth rates to crystal growth temperature, temperature gradient and acid concentration. The choice of good crystal growth conditions allows us to obtain crystals with a very good crystalline quality checked by X-ray topography. The -OH content, followed by infrared spectrometry, has been lowered to very small values and then electrical measurements of Y and Y-rotated resonators lead to high Q factors, first and second order temperature coefficients lower than those of quartz and a favourable coupling coefficients. For extensional bars, the existence of compensated cuts is also demonstrated.

The crystal structure of AgTIX phases (X = S, Se, Te)

Abstract The isomorphism of ternary compounds AgTIX (X = S, Se, Te) is pointed out. The compounds have an orthorhombic unit cell with four formulas and the space group is Pnam. The crystal structure of AgTlTe has been solved with a final R value of 0.098. Silver and tellurium atoms are covalently bonded in chains of AgTe4 tetrahedra delimiting channels where thallium atoms are located.

Structure et proprietes physiques des phases Cu2TlX2, Cu3 YTl2X4 (AVEC X = Se, Te et Y = Fe, Ga, Al)

Abstract Ternary (Cu 2 TlX 2 ) and quarternary (Cu 3 YTl 2 X 4 ) compounds (with X = Se, Te and Y = Fe, Ga, Al) have been prepared and characterized. All of them have the same lamellar structure where tetrahedras (CuYX 4 ) n and thallium atoms in coordination eight form successive layers. Some are metallic conductors (Cu 2 TlX 2 ) whereas others are semi-conductors (Cu 3 YTlX 4 ). The metallic behaviour could be due to thallium monovalent-thallium trivalent interactions within the layers.

Sur les chalcogeno-iodures d'antimoine SbXI(X =S, Se, Te): Structures et spectroscopie Mo¨ssbauer de121Sb

The crystal structure of SbXI(X =Se, Te) compounds has been determined by means of three-dimensional intensity data. The crystal structure of SbSeI, orthorhombic, space groupPnma witha = 8.698(2), b = 4.127(1), c = 10.412(2) A, was refined at several temperatures (180 K,R = 0.021; 293 K,R = 0.020; 320 K,R = 0.023) in correlation with the paraelectric structure or SbSI stable above 293 K. The crystal structure of SbTeI, triclinic, space groupP1¯, witha = 7.570(3), b = 7.159(3), c = 4.228(3) A, α = 107.22(5), β = 106.18(4), γ = 77.19(3)° has been determined by symbolic addition method and refined to a finalR value of 0.035. These structures are built up from infinite weakly linked ribbons (SbX2)n of trigonal SbX3 with SbX bonds of 2.605(1), 2.795(1)A(X =Se), and 2.829(1), 2.953(1), 2.955(1)A(X =Te). The nature of SbX and SbI bonds is discussed in terms of the S, Se, Te substitution. Antimony-121 Mo¨ssbauer spectra have been recorded at liquid helium temperature. The data are discussed with regard to the stereochimical activity of the antimony (III) lone pair of electrons. For SbTeI the Mo¨ssbauer parameters are interpreted in terms of direct population of conductance bands by nonbonding electron pairs.

Stereochimie du motif de coordination dans les complexes a ligands mixtes du type MII(bipy)(H2O)2AB4 (MII = NiII ou CuII; AB42− = SO42− ou BeF42−) bipy: 2,2′ bipyridine

Abstract The authors have studied three complexes of 2,2′ bipyridine: Ni(C10H8N2)(H2O)2SO4∗, Cu(C10H8N2)-(H2O) 2SO4∗∗, Cu(C10H8N2)(H2O)2BeF4. The crystal structure has been solved for two of them (∗ and ∗∗). Moreover their electronic, ESR and i.r. spectra are reported. By correlating these data the authors describe without ambiguity the crystal structure and the local molecular environment for copper (II) and nickel (II).

Etude structurale de Li2TeO4. Coordination du tellure VI et du lithium par les atomes d'oxygéne

A single crystal of Li2TeO4 has been prepared by hydrothermal synthesis and its structure has been determined from three-dimensional X-ray analysis. The crystals are tetragonal, space group P4122 with a = b = 6.045(3) A, c = 8.290(2) A, and Z = 4. The structure is a distorted inverse spinel with helicoidal chains of Te(VI) octahedra parallel to the [001] axis which can be formulated as [TeO4]2n−n.

Mise au point dans le ternaire TlBiTe. Existence de deux phases nonstoechiometriques de type TlBiTe2

Abstract The cross section BiTeTlTe of the phase diagram TlBiTe was constructed by X-ray, microstructure, and DTA methods. Two phases were emphasized: Tl1−xBi1+xTe2 with x = 0.06, and Tl1−yBi1+yTe2 with y = 0.2. The first one is a nonstoichiometric representation of what so many authors studied as TlBiTe2. The latter is a high-temperature phase which decomposes in eutectoid reaction (415°C): 1.18{ Tl 0.8 Bi 1.2 Te 2 } ⇄ Tl 0.94 Bi 1.06 Te 2 + 0.36 { BiTe } . The high-temperature form undergoes a phase transformation by quenching, giving a metastable low-temperature form. The discussion includes electrical resistivity and thermoelectric measurements of each of them.