CONTRIBUTIONS TO THE PROPERTIES OF TITANATES WITH ILMENITE STRUCTURE .2. STUDY ON THE THERMODYNAMICS AND THE ELECTRICAL-CONDUCTIVITY OF NITIO3 AND OTHER PHASES WITH ILMENITE STRUCTURE

NiTiO3 shows a phase transition at high temperatures (T(c) = 1290-degrees-C). The standard enthalpy and entropy of the reaction NiO + TiO2 = NiTiO3, was estimated for temperatures above and below the transition temperature using emf-measurements based on the following solid state galvanic cell: Ni,TiO2,NiTiO3 \ZrO2(+CaO)\Ni,NiO. The transition enthalpy was found to be 18 +/- 2 kJmol-1, the transition entropy is 12 +/- 1 JK-1mol-1. This is in good agreement with the calculated entropy change for an order-disorder transition (11.5 JK-1mol-1). The influence of other cations like Mg2+ and CO2+ on the transition temperature was investigated by measurements of the electrical conductivity as a function of comPosition. Ni1-xMgxTiO3 shows a strong shift of the transition to higher temperatures if a small part of the Ni2+ was replaced by Mg2+. A linear correlation between the temperature shift and the amount of Co2+ was found for Ni1-xCoxTiO3. Thermoanalytical investigations reveal an endothermic peak during the heating period some degrees below the melting point of CoTiO3. The substitution of Ge4+ for Ti4+ is without any influence on the transition temperature. By doping the NiTiO3 with Ga2O3, the anomalous increase of the electrical conductivity with temperature is shifted to lower temperatures.