Perovskite Solid Solutions along the NaNbO₃−SrTiO₃ Join: Phase Transitions, Formation Enthalpies, and Implications for General Perovskite Energetics

Abstract

Perovskite solid solutions along the NaNbO3−SrTiO3 join have been synthesized using the sol−gel and solid-state sintering methods. XRD analysis indicates that as Sr+Ti content increases, the perovskite structure changes from the orthorhombic to tetragonal and to cubic. The enthalpies of formation from the constituent oxides (ΔH and from the elements (ΔH ) have been determined by drop solution calorimetry into molten 3Na2O·4MoO3 at 974 K. The formation enthalpy ΔH becomes less exothermic with increasing Sr+Ti content, suggesting a destabilization effect of the substitution, Na+ + Nb5+ → Sr2+ + Ti4+ on the perovskite structure with respect to the constituent oxides. The trend of decreasing thermodynamic stability with decreasing structural distortion (relative to the ideal cubic structure) is opposite to that seen in most ABO3 perovskites. We interpret this behavior in terms of the dominance of acid−base chemistry, expressed by the ionic potential ratio of B to A cation (z/r)B/(z/r)A, in determining phase stability. This approach can be applied to other perovskite systems. Moreover, the enthalpic variation with Sr+Ti content is nearly linear, and thus the enthalpies of the morphotropic transitions across the series are rather small.