High-Temperature Order−Disorder Transition in CoCr2Se4 and Trapping Co Disorder in Monoclinic CoCr2Se4: Structural Features of Cu1−x Co x Cr2Se4 Phases

X-ray diffraction studies on the Cu(1-x)Co(x)Cr(2)Se(4) system (x = 0, 0.2, 0.4, 0.6, 0.8, 1) proved the existence of the cubic spinel-type structure for x < or = 0.2 and the monoclinic Cr(3)S(4)-type one for x > or = 0.8. CoCr(2)Se(4) undergoes an order-disorder phase transition above 750 degrees C. The high-temperature CoCr(2)Se(4) polymorph crystallizes in the filled GdI(2)-type structure with the P3m1 space group. The trigonal-to-monoclinic symmetry breaking in CoCr(2)Se(4) is found to be first-order within Landau theory. Co distribution in the low-temperature monoclinic CoCr(2)Se(4) structure depends on cooling conditions of trigonal CoCr(2)Se(4): rapid quenching results in Co disorder in the monoclinic structure, while slow cooling yields a fully ordered monoclinic structure.