Phase diagram of the system K2CrO4-Rb2CrO4

Compounds K2CrO4 and Rb2CrO4 are isomorphs. Solid solutions of general form ula K2-x RbXCrO4, (0<x<1) were established. The orthorhombic symmetry of th ese phases at room temperature was confirmed. The transition of the orthorh ombic-hexagonal phase for K2CrO4 and RxCrO4 was studied using X-ray powder diffraction. The mixed crystals showed that minimal substitution of potassi um by rubidium in K2CrO4 increases the temperature interval in which the or thorhombic and hexagonal phases coexist. The phase diagram for this system presents the typical behaviour of systems that exhibit complete solid solub ility. For the end-member phases, K2CrO4 and Rb2CrO4, the transitions occur at fixed temperatures, while for intermediate compositions two phases coex ist over a range of temperatures. In the solid solutions two phases can coe xist because there is one degree of freedom. The phase transitions in end-m embers and solid solutions can best be explained by an order-disorder mecha nism.