Variétés de haute et basse température du séléniure double de cuivre et de germanium

Abstract

1799 un poly6dre KO7. Ainsi les atomes d'oxyg6ne sont tricoordinds sauf 0(4) qui est t~tracoordin& Les poly6dres ThO8 partagent quatre de leur ar~tes avec des poly+dres KO7; ces derniers partagent entre eux deux de leurs ar6tes et une seule avec un poly6dre THOR.Les cations se regroupent dans des plans moyens (001) distants de c/4, avec un 6cart maximum zlz = 0,119 A. Une projection de la structure selon c et comprenant les atomes tels que 0,01 < z < 0,24 ( Fig. 1) fait apparakre un site anionique vide et les relations g6om6triques entre les mailles quadratiques de K4Th(MoO4)4 et de CaWO4 (Kay, Fraser & Almodovar, 1964): a = 2as + bs, b = -as + 2bs, c = %. La maille du molybdate double a donc une surface de base 6gale fi cinq lois celle de la scheelite CaWO4. La Fig. 2 Abstract. Octacopper germanium hexaselenide, CuBGeSe6, M, = 1054.7, hexagonal P63cm for the low-temperature form with a = 12.648 (5) and c--11.76 (4) A, V = 1629 (6) A 3, Z = 6, F(000) = 2808; and P63mc for the high-temperature form with a--7.307(3), c=11.75(1)]k, V=543(1)• 3, Z=2, F(000) = 936. Dm = 6"60 (8), Dx = 6"60 Mg m-3, MMo Ka) = 0"71069/~, /.~ = 40"0 mm-i, T = 293 K for the low-temperature form with final R = 0.059 for 787 observed reflections and T= 350 K for the high-temperature form with final R = 0.061 for 268 observed reflections. In both structures the Ge atoms 0108-2701/91/091799-05503.00 are in a tetrahedral coordination while the Cu atoms are in triangular or in tetrahedral coordinations of Se atoms. At 328 K CusGeSe6 exhibits a reversible phase transition, the passage of the high-temperature form to the low-temperature one resulting from ordering of the Cu atoms.

Argyrodite‐Type Cu8GeSe6–xTex (0 ≤ x ≤ 2): Temperature‐Dependent Crystal Structure and Thermoelectric Properties

“…At room temperature, Cu 8 GeSe 6 crystallizes in the hexagonal crystal system with a = 12.648(5) Å and c = 11.76(4) Å (Cu 8 GeSe 6 structure type) , . There are five Cu atom positions, which are fully occupied.…”

“…Samples of Cu 8 GeSe 6 and Cu 8 GeSe 4 Te 2 can be prepared as single phases according to SEM‐EDX and Rietveld refinements on PXRD data (Figure and Tables S2–S6, Supporting Information). The model for Cu 8 GeSe 6 was taken from literature, the model for Cu 8 GeSe 4 Te 2 from the single crystal refinement at 50 °C with Te only on 4 d . [GeSe 4 ] 4– appears as an invariant entity, no substitution with Te occurs in the coordination sphere of Ge, probably due to the strong covalent character of the Ge–Se bond in the complex anion.…”

Crystal growth of argyrodite-type phases Cu8–xGeS6−xIx and Cu8–xGeSe6−xIx (0⩽x⩽0.8)

“…For the Cu 8 GeS 6 samples, a structure model with orthorhombic symmetry (space group Pmn2 1 ) [4] or with cubic symmetry (space group F43m) [14] were used in the refinement. The structure model with hexagonal symmetry (space group P6 3 mc [15] or P6 3 mc [9], respectively) was used in the refinement of the Cu 8 GeSe 6 samples. Refinement parameters for both structures are unit cell parameters and atomic positions.…”

“…[8][9][10], Cu 8 GeSe 6 crystallizes exclusively in hexagonal space groups: the low temperature phase in the hexagonal space group P6 3 cm, the high temperature one in the space group P6 3 mc [10]. The solid-solid phase transition occurs at 55 1C [8].…”

“…[3] the structure of the studied argyrodites at T ¼ 298 K can be described within the cubic space group F43m, which is in a noticeable contradiction to Refs. [8][9][10] for Cu 8Àx GeSe 6Àx I x .…”