M. Guittard

Raman and infrared spectroscopic studies of GeGaAg sulphide glasses

Gallium sulphide forms stable glasses with GeS2 over a wide range of composition, and in the same system complex materials are obtained by dissolving silver sulphide. In such glasses Ga2S3 acts as a co-former with GeS2 whereas Ag2S plays the role of modifier. Raman scattering and infrared absorption measurements have been carried out on the Ge2S4Ga2S3Ag2S glass system. The structural arrangement of these glasses is studied as a function of the composition in the ternary glass-forming region. The vibrational assignment is made mainly in terms of bridging and non-bridging sulphur in the vitreous network. The tendency of gallium atoms to form GaS4 tetrahedra forces germanium atoms to induce the formation of S3GeGeS3 structural groups due to the shortage of sulphur atoms. When Ag2S is associated with Ge2S4Ga2S3 glasses, its defavors the edge-shared GeS4 tetrahedra and contributes to the cohesion of corner-shared tetrahedra. When Ag2S content increases, we observe a band located at low frequency that we have attributed to charge-coupled silver cation motion. A slight distortion of the tetrahedral groups is also seen from the vibrational features.

Verres formes par les sulfures L2S3 des terres rares avec le sulfure de gallium Ga2S3

Abstract Formation of glasses in the systems L 2 S 3 Ga 2 S 3 (L = Y and La to Er, and La to Er, except Eu). Conditions of preparation. Extent of the glassy regions as a fonction of the quenching temperatures and of the rare earth element. Crystallisation by heating: vitreous transition temperature Tg, and crystallisation temperature Tc. Mechanism of crystallisation. Preliminary study of the absorption spectra.

Transitions de la forme de haute température α de In2Se3, de part et d'autre de la température ambiante

Abstract The high-temperature form of In 2 Se 3 , α, encompasses several hexagonal or rhombohedral polytypes, having the same fundamental basis, hexagonal a 1 = 4.02A and c 1 = 9.56A, and having c parameters which are integral multiples of c 1 . The α form is stable above 550°C. Its transformation into γ-In 2 Se 3 is relatively slow, and only possible just below the transition temperature—between about 400 and 550°C. At ordinary temperature, it exists in a metastable state, but on heating a transformation occurs at 200°C involving the formation of a new form, β-In 2 Se 3 , which seems also to be metastable. This again exists in several polytypic forms depending on the type of parent α form. The α(2) polytype has a metal/nonmetal transition at 200 K.

Preparation et structure de Ga2S3α type wurtzite lacunaire

Abstract Preparations of powdered and crystalline αGa 2 S 3 are described. Structure of αGa 2 S 3 is established from single crystal determination (R = 0.058). These is a superstructure of wurtzite type, with ordered vacancies on gallium positions. A polymorphism of Ga 2 S 3 is described.

Oxysulfides and oxyselenides in sheets, formed by a rare earth element and a second metal

Abstract The preparation and structural features of compounds of the general formula (R O) n (M x X y ) , with R =rare earth, M =IB, IIIA, IVA, or VA elements, and X =S or Se, are described. Four families of oxysulfides can be prepared by the reaction of a rare earth oxysulfide and a post-transition metal sulfide. These include (R O) 4 Ga 2 S 5 for R =Ce(tetragonal P4/mmm ) and R =Pr to Sm(orthorhombic Pbca of the type (NdO) 4 Ga 2 S 5 ); (R O) 4 Ca 1.33 S 4 for R =La and Ce(tetragonal P4/mmm ); (R O) 4 Ge 1.5 S 5 for R =La to Nd(orthorhombic Pbca type (NdO) 4 Ga 2 S 5 ); (R O) 4 Sn 2 S 6 for R =La to Nd(orthorhombic pbnm type (LaO) 4 Sn 2 S 6 ). The reaction of lanthanum oxyselenide and the metal selenide gives (LaO)GaSe 2 , (LaO)InSe 2 , (LaO) 4 Ge 1.5 Se 5 , (LaO) 4 Sn 2 Se 6 , and (LaO)SbSe 2 . Crystal data are given for these systems. Although these compounds crystallize in several different space groups, they all have sheet structures formed by alternation of an oxide sheet ( R O) and a sulfide or selenide sheet ( M x X y ). They belong to 7 new structural types. The ( R O) sheets have the same tetragonal or pseudotetragonal array of R 4 O tetrahedra, but the ( M x X y ) sheets have various structures, with 2, 3, or 4 layers of X atoms. The structural characters of these compounds are discussed.

Systeme gallium-tellure: Diagrammes de phases, étude structurale de GaTe, Ga2Te5 et de Ga6SnTe10

Abstract Three phases are present in the GaTe system: Ga 2 Te 5 , Ga 2 Te 3 , and GaTe. Ga 2 Te 5 is stable in a narrow temperature domain, between about 400 and 495°C (its peritectic decomposition). The phase diagram is established from DTA and X-ray diffraction studies. The crystal structures of GaTe and Ga 2 Te 5 are described. The first one contains GaGa pairs and the second one a square plane coordination of Te, in which the atoms are bonded by covalency. The formation of compounds involving monovalent Ga is discussed, in connection with the existence of SnGa 6 Te 10 , in which Sn can be substituted by other monovalent or divalent cations.

Sur une nouvelle famille de combinaisons soufrees, de type “melilite”

Abstract Two kinds of compounds belong to this family: the compounds L 10/3 Ga 6 S 14 (L = La and Ce) and the compounds MLGa 3 S 7 and MLAl 3 S 7 (M = Ca, Sr or Eu″; L = La and in some cases Ce, Pr, Nd). All these compounds have the same tetragonal structure of the melilite type: Al or Ga cations have a tetrahedral environment of sulphur, M and L cations occupy the same 8 coordinated crystal sites. The absence of a continuous solid solution between La 10/3 Ga 6 S 14 and CaLaGa 3 S 7 could be explained by the presence of a superstructure in the last compound.

Mise en évidence d'une solution solide de type spinelle dans le diagramme de phase du systeme InS

Abstract Evidence is presented for normal-spinel domain of homogeneity between the compositions InS 1.50−e and InS 1.35 . Structural study of a single crystal of composition InS 1.44 indicates indium vacancies on the tetrahedral sites. The compound In 2 S 3 , regardless of conditions of formation, is a tetragonal superstructure of the spinel lattice ( a 0 ) with a = a 0 2 1 2 and c = 3 a 0 . The spinel-type domain shows peritectic decomposition at 850°C for the composition InS 1.40 .

Systèmes L2S3-Ga2S3 (L = La, Ce, Dy, Er et Y) diagrammes de phases

Abstract The intermediate phases are: L 6 Ga 10 3 S 14 hexagonal, Ce 6 Al 10 3 S 14 -type, for L: La to Tb, and Y, which have an order disorder transition; LGaS3 of unknown structure for L = La and Ce; L3GaS6, orthorhombic Er3GaS6-type, for L = Dy to Er and Y. Description of the 5 phase diagrams. In each case the formation of glasses is closely related to the phase diagram.

Dimorphisme et structure du disulfure de lanthane LaS2

Abstract The stoichiometric lanthanum disulfide LaS 2 presents a reversible phase transition at about 750°C. The α low-temperature form is monoclinic with the LaSe 2 type. All the crystals are twinned with the same twin law (100). The cell parameters are a = 8.18, b = 8.13, c = 4.03A, γ = 90°, space group P2 1 a . The β high-temperature form has the orthorhombic structure previously described with the parameters a = 8.13, b = 16.34, c = 4.14 A; space group Pnma . The two structures are compared.