Festkörperreaktionen in Chalkogenidsystemen. V. Verteilungsgleichgewichte zweiwertiger Übergangsmetalle in Chromthiospinell-Mischkristallen

Abstract

The solid-state equilibria of the chromium thiospinel solid solutions M x M′ 1− x Cr 2S 4 ( M, M′ = Mn, Co, Zn, Cd), with excess binary sulfides MS and M′S or M 1− x M′ x S mixed crystals, are investigated. At 600°C the following equilibrium compositions are found: Mn 0.38Co 0.62Cr 2S 4, Mn 0.36Zn 0.64Cr 2S 4, Mn 0.64Cd 0.36Cr 2S 4, Co 0.33Zn 0.67Cr 2S 4, Co 0.68Cd 0.32Cr 2S 4, and Zn 0.75Cd 0.25Cr 2S 4. The results show that metals with small crystal radii and high tetrahedral site preference energy are preferentially incorporated into the tetrahedral sites of chromium thiospinels. With increasing temperature the composition of the quaternary spinels approach M 0.5 M′ 0.5Cr 2S 4. From the temperature dependence of the equilibrium constants the reaction enthalpies could be determined. The binary sulfides MS and M′S are incompletely miscible excepting the system ZnS CdS . At 600°C the following miscibility gaps are found: Mn y Zn 1− y S: y = 0.43 – ≈ 1.0, Mn y Cd 1− y S: y = 0.50 – >0.9, Co y Mn 1− y S: y = <0.1 – ≈ 1.0, Co y Zn t− y S: y = 0.1 – ≈ 1.0, and Co y Cd 1− y S: y = 0.1 – ≈ 1.0. With increasing temperature the miscibility gaps, especially of the systems with CoS, get smaller. The spinel solid solutions and the ZnS CdS mixed crystals obey Vegard's rule.