Elastic properties of calcium oxide perovskites

The equations of state and axial moduli of the CaBO₃ perovskites (B=Zr,Sn,Ti,Ge) and CaFeO_2.5 with the brownmillerite structure have been determined using high-pressure, single-crystal X-ray diffraction. The bulk modulus-specific volume relationship for the Ca-perovskites is nonlinear, with CaSnO₃ and CaZrO₃ displaying anomalous stiffening (higher bulk moduli) than previously reported and predicted [1,2]. The axial moduli of the a- and c-axes decrease steadily by ∼30% from the least-distorted of the Pbnm perovskites, CaGeO₃, to the most distorted, CaZrO₃, while the b-axis shows little change. The net result is a threefold increase in the anisotropy of the axial moduli of CaSnO₃ and CaZrO₃ (∼21%) relative to CaGeO₃ and CaTiO₃ (∼ 4-8%). The bulk modulus of CaFeO_2.5 falls significantly below the trend for the stoichiometric perovskites. The introduction of 1/6 vacancies, on the oxygen positions softens the perovskite structure by 25%.