A NEUTRON POWDER INVESTIGATION OF THE HIGH-TEMPERATURE PHASE-TRANSITION IN NITIO3

The structural changes at the phase transition in NiTiO3 have been followed by neutron powder diffraction up to 1650 K. At T-c approximate to 1560 K it transforms from an (ordered) ilmenite structure (R3 $($) over bar$$) to a disordered corundum like structure (R3c $($) over bar$$) with a random distribution of the cations. Detailed Rietveld refinements including anharmonic and split-atom models revealed that the transition process is not a simple direct interchange of two neighboring cations. Each cation species occupies its own specific position in its actual oxygen octahedron. Some interstitial cation densities are found, but only in a limited temperature region. This is assigned to temperature dependent mobilities of the cations. Taking into account further the temperature variation of the sizes of various interfacial windows and tetrahedral and octahedral cavities an elementary exchange process is deduced. The difficulties in defining atomic occupancies and positions (the order parameters of the transition), when (anharmonic) atomic density distributions are present, are outlined.