| Abstract
| - A series of perovskites with the compositions NaTixNb1-xO3-0.5x (0 ≤ x ≤ 0.2) has beensynthesized for the first time using the sol−gel method. Combined Rietveld analysis of powderneutron 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 structureis deficient in oxygen over its O1 and O3 sites, which compensates the charge imbalancebetween the substituting Ti4+ and Nb5+. Raman spectroscopy indicates that the O2- vacanciesare locally associated with Ti4+, resulting in [TiO5] coordination. There is no evidence forlong-range order or superstructure. With increasing Ti content, the orthorhombic structurebecomes 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 bydrop solution calorimetry into molten 3Na2O·4MoO3 solvent at 974 K. As Ti4+ substitutesfor Nb5+, the formation enthalpies become less exothermic, suggesting a destabilizing effectof the substitution. We attribute this behavior to a number of structural factors, includingthe occurrence of O2- vacancies and of Ti−O−Nb linkages as well as the size mismatchbetween Ti4+ and Nb5+.
- A new series of oxygen-deficient perovskites, NaTixNb1-xO3-0.5x (0 < x ≤ 0.2), has been synthesized using the sol−gel method. These phases adopt the structure of the end-member NaNbO3 (space group Pbma) with their oxygen vacancies occurring over half the available sites (O1 and O3) and being locally associated with Ti4+. With increasing Ti content, the orthorhombic structure becomes more like the cubic, and its enthalpy of formation less exothermic, suggesting a destabilizing effect of the Ti4+ + 0.5VO•• → Nb5+ substitution.
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