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

Octacopper germanium hexaselenide, Cu8GeSe6, M(r) = 1054.7, hexagonal P6(3)cm for the low-temperature form with a = 12.648 (5) and c = 11.76 (4) angstrom, V = 1629 (6) angstrom 3, Z = 6, F(000) = 2808; and P6(3)mc for the high-temperature form with a = 7.307 (3), c = 11.75 (1) angstrom, V = 543 (1) angstrom 3, Z = 2, F(000) = 936. D(m) = 6.60 (8), D(x) = 6.60 Mg m-3, lambda(Mo K-alpha) = 0.71069 angstrom, mu = 40.0 mm-1, 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 are in a tetrahedral coordination while the Cu atoms are in triangular or in tetrahedral coordinations of Se atoms. At 328 K Cu8GeSe6 exhibits a reversible phase transition, the passage of the high-temperature form to the low-temperature one resulting from ordering of the Cu atoms.