CONTRIBUTIONS TO THE PROPERTIES OF TITANATES WITH ILMENITE STRUCTURE .1. ASPECTS OF A HIGH-TEMPERATURE PHASE-TRANSITION IN NITIO3

NiTiO3 (Ilmenite structure) shows an anomalous increase of the electrical conductivity in the temperature range between 1250-degrees-C and 1290-degrees-C. The temperature dependence of the Seebeck coefficient is also found to be anomalous in this region. DTA experiments are consistent with a phase transition; the transition enthalpy was estimated to DELTA-u(H)m = 17 +/- 3 kJmol-1. NiTiO3 powders and single crystals were quenched from 1350-degrees-C and 1200-degrees-C to room temperature; in principle no differences of the lattice parameters or atomic positions could be detected. The determination of the distribution of cations using x-ray powder methods failt because of strong texture effects. Structure refinements with single crystal methods suggest an ordered Ilmenite structure independent of the quenching temperature. The results are in agreement with a reversible order-disorder transition of higher order. It seems to be impossible to quench the high temperature phase by conventional methods.