HIGH-TEMPERATURE PHASE-TRANSITION AT THE SYSTEM K2CRO4-K2SEO4

Compounds K2CrO4 and K2SeO4 are isomorphs. Solid solutions of general formula K-2(CrO4)(1-x)(SeO4)1(x), (0<x<1) were established. The orthor hombic symmetry of these phases at room temperature was confirmed. The transition of the orthorhombic-hexagonal phase for K2CrO4 and K2SeO4 was studied using X-ray powder diffraction. The mixed crystals showed that minimal substitution of chromate by selenate in K2CrO4 increases the temperature 4 interval in which the orthorhombic and hexagonal pha ses coexist. The phase diagram for this system presents the typical be haviour of systems which exhibit complete solid solubility. For the en d-member phases. K2CrO4 and K2SeO4, the transitions occur at fixed tem peratures. while for intermediate compositions two phases coexist over a range of temperatures. In the solid solutions two phases can coexis t because there is one degree of freedom. The phase transitions in end -members and solid solutions can best be explained by an order-disorde r mechanism.