Compressibility of brownmillerite (Ca2Fe2O5): effect of vacancies on the elastic properties of perovskites

The evolution with pressure of the unit-cell parameters brownmillerite (Ca2Fe2O5), a stoichiometric defect perovskite structure, has been determined to a maximum pressure of 9.46 GPa, by single-crystal X-ray diffraction measurements at room temperature. Brownmillerite does not exhibit any phase transitions in this pressure range. A fit of a third-order Birch–Murnaghan equation-of-state to the P–V data yields values of KT0=127.0(5) GPa and K0′=5.99(13). Analysis of the unit-cell parameter data shows that the structure compresses anisotropically. Compressional moduli for the axes are Ka0=141(1) GPa, Kb0=118(3) GPa and Kc0=122.2(2) GPa, with Ka0′=8.9(3), Kb0′=6.2(6) and Kc0′=4. The stiffest direction (i.e. along a) coincides with the direction of the FeO4 tetrahedral chains. Comparison of these data with the elasticity systematics of Ca-perovskites shows that the presence of oxygen vacancies in the brownmillerite structure softens the structure by ∼25% and that the ordering of vacancies in the perovskite structure increases the anisotropy of compression.