High‐temperature solution calorimetry in molten 3Na2O•4MoO3 solvent at 975 K is used to measure the enthalpy of mixing of (CaxSr1‐x)TiO3
and (BaxSr1‐x)TiO3 solid solutions. Both solid solutions show positive enthalpy of mixing, however, the (BaxSr1‐x)TiO3
shows less deviation from ideality, with interaction parameter λ = 8.5 ± 3.8 kJ/mol, than (CaxSr1‐x)TiO3 with interaction
parameter λ = 25.3 ± 2.0 kJ/mol. The smaller interaction parameter of the (BaxSr1‐x)TiO3 system is attributed to
the volume mismatch of the end‐members. The larger interaction parameter of (CaxSr1‐x)TiO3 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 CaTiO3 to incorporate it into the cubic perovskite SrTiO3.
(BaxCa1‐x)TiO3 has a large positive heat of mixing and limited solid solubility as derived from its phase diagram. This study indicates the tendency of
SrTiO3 and BaTiO3 to separate from a CaTiO3 host matrix, which may be significant in perovskite‐bearing nuclear waste ceramics
incorporating 90Sr and 137Ba.