| Abstract
| - As the Sr+Ti content increases, the structure of Na1-xSrxNb1-xTixO3 perovskite changes from the orthorhombic to tetragonal and to cubic, while its enthalpy of formation ΔHbecomes less exothermic. This energetic trend is opposite to that seen in most ABO3 perovskites and can be interpreted in terms of the dominance of acid−base chemistry in phase stability.
- Perovskite solid solutions along the NaNbO3−SrTiO3 join have been synthesized using the sol−geland solid-state sintering methods. XRD analysis indicates that as Sr+Ti content increases, the perovskitestructure changes from the orthorhombic to tetragonal and to cubic. The enthalpies of formation fromthe constituent oxides (ΔHand from the elements (ΔH) have been determined by drop solutioncalorimetry into molten 3Na2O·4MoO3 at 974 K. The formation enthalpy ΔHbecomes lessexothermic 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 ofdecreasing thermodynamic stability with decreasing structural distortion (relative to the ideal cubicstructure) is opposite to that seen in most ABO3 perovskites. We interpret this behavior in terms of thedominance 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, theenthalpic variation with Sr+Ti content is nearly linear, and thus the enthalpies of the morphotropictransitions across the series are rather small.
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