Thermochemistry of substituted perovskites in the NaTi_xNb_1-xO_3-0.5x System

A suite of perovskite phases with the compositions NaTi_xNb_1-xO_3-0.5x, 0 ≤ x ≤ 0.2, has been synthesized for the first time using the sol-gel method. Rietveld analysis of powder XRD data reveals that with increasing Ti content, the orthorhombic perovskite structure becomes more cubic-like, as evidenced by the smaller differences among its three cell parameters. Enthalpies of formation of the synthesized phases from the constituent oxides and from the elements have been determined by drop solution calorimetry into molten 3Na₂O·4MoO₃ solvent at 974 K. As Ti⁴⁺ substitutes for Nb⁵⁺, the formation enthalpies become less exothermic in a nearly linear manner. This behavior suggests that the Ti→Nb substitution destabilizes the perovskite structure, presumably because of the concomitant occurrence of O²⁺ vacancies, which compensate the charge imbalance between Ti⁴⁺ and Nb⁵⁺ in the structure.