Thermodynamic evidence for order-disorder transition in NiTiO₃

The standard Gibbs free energy of formation of NiTiO₃ from its component oxides NiO with rock salt structure and TiO₂ with rutile structure Δ_f G_(ox)^{{ring operator}} (NiTiO₃) has been measured over a wide temperature range (900 to 1700) K employing the solid-state electrochemical cell:Pt | Ni + NiTiO₃ + TiO₂ {norm of matrix} (Y₂ O₃) ZrO₂ {norm of matrix} Ni + NiO | PtThe 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, andE ± 0.35 / (mV) = - 16.21 - 0.03831 (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 R over(3, ̄) to a kind of disordered corundum structure with space group R over(3, ̄) c, 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.