A New Series of Oxygen-Deficient Perovskites in the NaTixNb1-xO3-0.5x System: Synthesis, Crystal Chemistry, and Energetics

Abstract

A series of perovskites with the compositions NaTixNb1-xO3-0.5x (0 ≤ x ≤ 0.2) has been synthesized for the first time using the sol−gel method. Combined Rietveld analysis of powder neutron and X-ray diffraction data reveals that these phases adopt the structure of the end-member perovskite NaNbO3 with the space group Pbma. However, the substituted structure is deficient in oxygen over its O1 and O3 sites, which compensates the charge imbalance between the substituting Ti4+ and Nb5+. Raman spectroscopy indicates that the O2- vacancies are locally associated with Ti4+, resulting in [TiO5] coordination. There is no evidence for long-range order or superstructure. With increasing Ti content, the orthorhombic structure becomes more like the cubic, as reflected by the smaller differences among its cell parameters (ap, bp, and cp) and the cell angle βp approaching 90° (in terms of the pseudocubic subcell). Enthalpies of formation from the 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, suggesting a destabilizing effect of the substitution. We attribute this behavior to a number of structural factors, including the occurrence of O2- vacancies and of Ti−O−Nb linkages as well as the size mismatch between Ti4+ and Nb5+.