Thermochemistry of ( Ca _x Sr _{1‐ x} ) TiO ₃ , ( Ba _x Sr _{1‐ x} ) TiO ₃ , and ( Ba _x Ca _1‐x ) TiO ₃ Perovskite Solid Solutions

Abstract

High‐temperature solution calorimetry in molten 3Na 2 O • 4MoO 3 solvent at 975 K is used to measure the enthalpy of mixing of ( Ca x Sr 1‐ x ) TiO 3 and ( Ba x Sr 1‐ x ) TiO 3 solid solutions. Both solid solutions show positive enthalpy of mixing, however, the ( Ba x Sr 1‐ x ) TiO 3 shows less deviation from ideality, with interaction parameter λ = 8.5 ± 3.8 kJ/mol, than ( Ca x Sr 1‐ x ) TiO 3 with interaction parameter λ = 25.3 ± 2.0 kJ/mol. The smaller interaction parameter of the ( Ba x Sr 1‐ x ) TiO 3 system is attributed to the volume mismatch of the end‐members. The larger interaction parameter of ( Ca x Sr 1‐ x ) TiO 3 may be related to larger ionic radius difference of Ca and Sr and may also reflect the additional energy needed to overcome the lattice distortion of the orthorhombic CaTiO 3 to incorporate it into the cubic perovskite SrTiO 3 . ( Ba x Ca 1‐ x ) TiO 3 has a large positive heat of mixing and limited solid solubility as derived from its phase diagram. This study indicates the tendency of SrTiO 3 and BaTiO 3 to separate from a CaTiO 3 host matrix, which may be significant in perovskite‐bearing nuclear waste ceramics incorporating 90 Sr and 137 Ba .