High-pressure transitions and thermochemistry of MGeO3 (M=Mg, Zn and Sr) and Sr-silicates:: systematics in enthalpies of formation of A2+B4+O3 perovskites

Phase transitions in MgGeO3 and ZnGeO3 were examined up to 26 GPa and 2,073 K to determine ilmenite - perovskite transition boundaries. In both systems, the perovskite phases were converted to lithium niobate structure on release of pressure. The ilmenite perovskite boundaries have negative slopes and are expressed as P(GPa) = 38.4 - 0.0082T(K) and P(GPa) = 27.4 - 0.0032T( K), respectively, for MgGeO3 and ZnGeO3. Enthalpies of SrGeO3 polymorphs were measured by high-temperature calorimetry. The enthalpies of SrGeO3 pseudowollasonite - walstromite and walstromite - perovskite transitions at 298 K were determined to be 6.0 +/- 8.6 and 48.9 +/- 5.8 kJ/mol, respectively. The calculated transition boundaries of SrGeO3, using the measured enthalpy data, were consistent with the boundaries determined by previous high-pressure experiments. Enthalpy of formation (Delta H-f degrees) of SrGeO3 perovskite from the constituent oxides at 298 K was determined to be - 73.6 +/- 5.6 kJ/mol by calorimetric measurements. Thermodynamic analysis of the ilmenite perovskite transition boundaries in MgGeO3 and ZnGeO3 and the boundary of formation of SrSiO3 perovskite provided transition enthalpies that were used to estimate enthalpies of formation of the perovskites. The Delta H(f)degrees of MgGeO3, ZnGeO3 and SrSiO3 perovskites from constituent oxides were 10.2 +/- 4.5, 33.8 +/- 7.2 and - 3.0 +/- 2.2 kJ/mol, respectively. The present data on enthalpies of formation of the above high-pressure perovskites were combined with published data for A(2+) B4+ O-3 perovskites stable at both atmospheric and high pressures to explore the relationship between Delta H(f)degrees and ionic radii of eightfold coordinated A(2+) (R-A) and sixfold coordinated B4+ (R-B) cations. The results show that enthalpy of formation of A(2+) B4+ O-3 perovskite increases with decreasing R-A and R-B. The relationship between the enthalpy of formation and tolerance factor (t = R-A + R-o)/root 2(R-B + R-o), R-o: O2- radius) is not straightforward; however, a linear relationship was found between the enthalpy of formation and the sum of squares of deviations of A(2+) and B4+ radii from ideal sizes in the perovskite structure. A diagram showing enthalpy of formation of perovskite as a function of A(2+) and B4+ radii indicates a systematic change with equi-enthalpy curves. These relationships of Delta H(f)degrees with R-A and R-B can be used to estimate enthalpies of formation of perovskites, which have not yet been synthesized.