High temperature neutron diffraction studies of phase transformations in NiCr2S4

High temperature structural transformations in NiCr2S4 have been investigat ed by neutron powder diffraction in situ. At 835(5)degrees C, NiCr2S4 under goes an order-disorder transition from the monoclinic Cr3S4 structure to a hexagonal cation-deficient NiAs structure. Above the transition temperature , cation vacancies, which at room temperature are ordered in every second c ation layer, become statistically distributed between all cation layers. Pr ior to this structural transition, intralayer disordering of vacancies occu rs within the half-occupied layer. This is not sufficiently complete to cau se the change to trigonal symmetry which has been reported for binary sulfi des.