High-Temperature Order−Disorder Transition in CoCr2Se4 and Trapping Co Disorder in Monoclinic CoCr2Se4: Structural Features of Cu1−xCoxCr2Se4 Phases
datasetposted on 15.06.2009, 00:00 by Volodymyr Svitlyk, Yurij Mozharivskyj
X-ray diffraction studies on the Cu1−xCoxCr2Se4 system (x = 0, 0.2, 0.4, 0.6, 0.8, 1) proved the existence of the cubic spinel-type structure for x ≤ 0.2 and the monoclinic Cr3S4-type one for x ≥ 0.8. CoCr2Se4 undergoes an order−disorder phase transition above 750 °C. The high-temperature CoCr2Se4 polymorph crystallizes in the filled GdI2-type structure with the P3̅m1 space group. The trigonal-to-monoclinic symmetry breaking in CoCr2Se4 is found to be first-order within Landau theory. Co distribution in the low-temperature monoclinic CoCr2Se4 structure depends on cooling conditions of trigonal CoCr2Se4: rapid quenching results in Co disorder in the monoclinic structure, while slow cooling yields a fully ordered monoclinic structure.