Das system chrom-selen

Abstract

The system chromium-selenium was investigated by differential thermal analysis and X-ray powder methods. The samples were annealed at 575, 775 and 1375 K. The system is dominated at higher temperatures by a broad region of solid solutions of Cr 1− x Se (40–50 mol.% chromium) which crystallize in the space group P3̄m1. Some experiments indicate that the solid solutions may transform at high temperatures to a structure with the space group P6 3/ mmc. On cooling the solid solutions separate into four different compounds: α-Cr 2Se 3 ( R 3 ̄ ), Cr 0,68Se ( P 3 ̄ 1c ), Cr 3Se 4 ( C2/ m) between 41.7–44.5 mol.% chromium and Cr 7Se 8 ( C2/ m). In the primary cristallization field of α-Cr 2Se 3 a fifth compound was observed: Cr 2−δSe 3 ( C2/ m) is metastable and can be obtained only during cooling experiments in the presence of an excess of selenium. The prototype of all the structures of these compounds is the NiAs-type structure, from which their structures can be derived. The chromium-rich part of the solid solutions, usually called Cr 1−ϵSe, decomposes at lower temperatures, as found in protracted annealing experiments.