Effect of FeAlO3 incorporation into MgSiO3 on the bulk modulus of perovskite

The effect of FeAlO3 incorporation into MgSiO3 on the bulk modulus of perovskite has been investigated in Mg0.85Fe0.15Al0.15Si0.85O3 on the basis of high-pressure and high-temperature in-situ   X-ray diffraction measurements in a laser-heated diamond anvil cell. The Birch–Murnaghan equation of state (K′0=4)(K′0=4) yields zero-pressure bulk moduli of 252(1) GPa and 259(1) GPa for Mg0.85Fe0.15Al0.15Si0.85O3 perovskite and MgSiO3 perovskite, respectively, using Tsuchiya's pressure scale. The effect of the FeAlO3 is a decrease in the bulk modulus. This effect is not explained by the compression mechanism of the oxygen vacancy or partial amorphisation. Rather, the decrease of the bulk modulus may be associated with high-pressure behavior of the perovskite structure that is distorted by the coupled substitution of Mg + Si = Fe + Al. Mg0.85Fe0.15Al0.15Si0.85O3 perovskite was also found to be stable up to 143 GPa and 2500 K based on Tsuchiya's pressure scale. The other effect of the addition of FeAlO3 is an expansion of the perovskite stability field towards higher pressures. There may be an empirical relation between the bulk modulus decrease and the post-perovskite transition pressure increase on incorporation of trivalent cations into perovskite by mechanisms other than the oxygen vacancy compression.