Cs 2Cu 5Se 4, ein neues metallreiches kupferselenid mit einer fragmentgerüststruktur des kupfers

Abstract

The reaction of caesium carbonate with copper and selenium at temperatures between 850 and 1100 °C yields the ternary compounds Cs 3Cu 8Se 6 and Cs 2Cu 5Se 4, the latter unknown until now. Single-crystal investigations on Cs 2Cu 5Se 4 revealed the rhombic space group Cmcm; the cell constants are a = 399.2(3)pm, b = 1941.7(7)pm and c = 1321.9(4)pm. The atomicarrangement is related to that of Cs 3Cu 8Se 6, which deviates little in stoichiometry from Cs 2Cu 5Se 4. In a sequence of close-packed layers of selenium and caesium atoms the copper atoms occupy such holes which are built up only by selenium atoms. The distances between the copper atoms are only a little larger than those in the structure of the element and their geometrical arrangement shows a slightly distorted fragment framework of the f.c.c. lattice. Fragment framework structures are also observed in other metal-rich copper or silver chalcogenides.