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 ΔfG(ox) (NiTiO3) has been measured over a wide temperature range (900 to 1700)K employing the solid-state electrochemical cell:PtNi+NiTiO3+TiO2 (Y2O3)ZrO2 Ni+NiOPt. 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, andE+/-0.35/(mV)=-16.21-0.03831(T/K),in the range T=(1565 to 1700)K. The change in slope at T=1565K is related to the transition from ordered ilmenite structure of NiTiO3 with space group R3-bar to a kind of disordered corundum structure with space group R3-bar 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