A new series of oxygen-deficient perovskites in the NaTi_xNb_1-xO_3-0.5x system: Synthesis, crystal chemistry, and energetics

A series of perovskites 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. 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 O^2- 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 (a_p, b_p, and c_p) and the cell angle β_p 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 O^2- vacancies and of Ti-O-Nb linkages as well as the size mismatch between Ti⁴⁺ and Nb⁵⁺.