Aus der DTA und Röntgenbeugungsuntersuchungen wird das Phasendiagramm La2Se3‐Cu2Se konstruiert.

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Abstract. CuLaS2, monoclinic, P2Jb, a = 6.646 (2), b = 6.938 (2), c = 7.325 (2)A, ~= 98.73 (2)°, Z = 4, d m = 5.29 (8), d x = 5.3 Mg m -3. The structure has been solved by direct methods and refined by leastsquares methods to a final R value of 0.063 for 999 reflexions. The structure is built up of sheets parallel to the b and c axes. One sheet is made of (LaaS) tetrahedra, and has the composition (LaS); the second sheet is formed by (CuS4) tetrahedra arranged in pairs, which alternate with pairs of empty tetrahedra.

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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.

Description and Structural Study of a New Family of Sheet Compounds of the General Composition (UO)2RS3 (R = Gd to Lu and Y) The reaction of UOS was tested on various sulfides. It was only positive in the case of the R2S3 sulfides at very high temperature (1500—1900°C) with formation of the title compounds. The crystal structure was solved in the case of (UO)2ErS3: tetragonal cell, space group 14/m mm, a = 3.785, c = 20.83 Å, Z = 2. A sheet structure is formed by alternating (UO) and (ErS3) sheets. The apparent oxydation state of U is 3.5, and there is only one serie of positions for this element. But it is presently impossible to choose between mixed valencies or intermediate valency to represent the electronic state of U.