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 K-T0= 127.0(5) GPa and K-0' = 5.99(13). Analysis of the unit-cell parameter data shows that the structure compresses anisotropically. Compressional moduli for the axes are K-a0 = 141(1) GPa, Kb0 = 118(3) GPa and K-c0 = 122.2(2) GPa with K-a0' = 8.9(3), K-b0' = 6.2(6) and K-c0' = 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 similar to25% and that the ordering of vacancies in the perovskite structure increases the anisotropy of compression. (C) 2002 Elsevier Science B.V. All rights reserved.