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 x Nb 1 - x O 3 - 0.5 x (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 NaNbO 3 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 Ti 4+ and Nb 5+ . Raman spectroscopy indicates that the O 2 - vacancies are locally associated with Ti 4+, resulting in [TiO 5 ] 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 3Na 2 O·4MoO 3 solvent at 974 K. As Ti 4+ substitutes for Nb 5+, 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 Ti 4+ and Nb 5+ .