The effect of oxygen vacancies and aluminium substitution on the high-pressure properties of brownmillerite-structured Ca2Fe2−x Al x O5

Abstract

The structural evolution with pressure and the equations of state of three members of the brownmillerite solid solution, Ca2(Fe2−x Al x )O5, have been determined by single-crystal X-ray diffraction up to a maximum pressure of 9.73 GPa. The compositions of the samples were x = 0.00 and x = 0.37 (with Pnma symmetry) and x = 0.55 (with I2mb symmetry). No phase transitions were observed in the experiments. The equation of state parameters determined from the pressure-volume data are K 0T = 128.0 (7) GPa, K′0 = 5.8 (3) for the sample with x = 0.00, K 0T = 131 (2) GPa, K′0 = 5.5 (4) for x = 0.37, and K 0T = 137.5 (6) GPa, K′0 = 4 for x = 0.55. The bulk modulus therefore increases with Al content, being 11% higher in the x = 0.55 sample than in the Al-free sample. The unit-cell compression is anisotropic, with the c-axis being stiffer than a or b, and the anisotropy increases with increasing Al content of the structure. The structural response to pressure of all samples is similar. The (Al,Fe)O4 tetrahedra and the (Al,Fe)O6 octahedra undergo approximately isotropic compression. There is an increase in the twists of the chains of corner-sharing (Al,Fe)O4 tetrahedra, and an increase in the tilts of the (Al,Fe)O6 octahedra, because these framework polyhedra are stiffer than the Ca–O bonds to the extra-framework Ca site. The alignment of the two shortest Ca–O bonds sub-parallel to [001] accounts for the relative stiffness of the c-axis and thus the elastic anisotropy.