Thermochemistry of (Ca_xSr_1‐x)TiO₃, (Ba_xSr_1‐x)TiO₃, and (Ba_xCa_1‐x)TiO₃ Perovskite Solid Solutions

Abstract

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.

Thermodynamics of Nanoscale Calcium and Strontium Titanate Perovskites

“…The obtained enthalpy of formation from corresponding oxides, Δ H f,ox , is −131.54 ± 3.44 kJ/mol for SrTiO 3 and −83.76 ± 2.72 kJ/mol for CaTiO 3 . The enthalpy of formation from oxides (Δ H f,ox ) and enthalpy of drop solution (Δ H ds ) of SrTiO 3 and CaTiO 3 are consistent with previous reports …”

“…For this, the enthalpy of drop solution of SrO and CaO in 3Na 2 O·4MoO 3 at 975 K was taken from Refs. whereas the Δ H ds of SrTiO 3 and CaTiO 3 were determined in this study. The obtained enthalpy of formation from corresponding oxides, Δ H f,ox , is −131.54 ± 3.44 kJ/mol for SrTiO 3 and −83.76 ± 2.72 kJ/mol for CaTiO 3 .…”

Effect of ionic radii on the Curie temperature in Ba1-x-ySrxCayTiO3 compounds

“…This composition is close to the temperature dependent Ca solubility limit of 16% at 1300 °C19, 25% at 1430 °C20 and 1549 °C21. A good agreement between the values of the lattice parameters of Ba 1-x Sr x TiO 3 with those reported in the literature22 was observed. For the series A the increase of variance resulted in a slight decrease of the a lattice parameter and increase of the c lattice parameter whereas the c/a ratio showed an increase.…”

Relaxor Ferroelectric Behavior in Barium Strontium Titanate

“…Because nanoscale chemical inhomogeneity influences the extent of residual ferroelectricity, it will thus affect the loss in BST tunable dielectrics in the microwave range. Process variations due to incomplete mixing, differences in the decompositions of barium and strontium carbonates, or differences in the lattice matching between strontium titanate and barium titanate could lead to such local scale chemical variations …”