NEUTRON-SCATTERING INVESTIGATION OF THE HIGH-TEMPERATURE PHASE-TRANSITION IN NITIO3

Nickel titanate, NiTiO3, undergoes a second order phase transition at T(c) = 1570(10) K from the ilmenite structure with space group R3BAR to the corundum structure with space group R3cBAR. It is predominantly due to a Ni/Ti order-disorder transition. The temperature dependence of certain superstructure reflexions, investigated by powder neutron diffraction suggests, however, a coupling to other displacive degrees of freedom of the order parameter. The behaviour of the essential part of the order parameter below the critical region may be described by the law of mass action applied to the exchange of Ni and Ti on nonequivalent sites.