Thermodynamic evidence for order–disorder transition in NiTiO₃

Abstract

The standard Gibbs free energy of formation of NiTiO₃ from its component oxides NiO with rock salt structure and TiO2 with rutile structure Δ_fG °_(OX)(NiTiO₃) has been measured over a wide temperature range (900 to 1700) K employing the solid-state electrochemical cell: Pt|Ni+NiTiO₃+TiO₂||(Y₂O₃)ZrO₂||Ni+NiO|Pt. The measured EMF, which is directly related to the Gibbs free energy of formation of NiTiO₃ from component oxides, can be represented by two linear segments; E±0.38/(mV)=63.48-0.01264(T/K), in the range T = (900 to 1565) K, and E±0.35/(mV)=-16.21-0.0381(T/K), in the range T = (1565 to 1700) K. The change in slope at T = 1565 K is related to the transition from ordered ilmenite structure of NiTiO₃ with space group R3 to a kind of disordered corundum structure with space group R3c, involving essentially an order–disorder transition on the cationic sublattice. The new measurements help to resolve discrepancies in thermodynamic data on NiTiO₃ reported in the literature.