High temperature neutron diffraction studies of phase transformations in NiCr₂S₄

High temperature structural transformations in NiCr₂S₄ have been investigated by neutron powder diffraction in situ. At 835(5) °C, NiCr₂S₄ undergoes 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 cation layer, become statistically distributed between all cation layers. Prior to this structural transition, intralayer disordering of vacancies occurs within the half-occupied layer. This is not sufficiently complete to cause the change to trigonal symmetry which has been reported for binary sulfides.