Crystal chemistry and compressibility of Fe_0.5Mg_0.5Al_0.5Si_0.5O₃ and FeMg_0.5Si_0.5O₃ silicate perovskites at pressures up to 95 GPa.

Silicate perovskite, with the mineral name bridgmanite, is the most abundant mineral in the Earth's lower mantle. We investigated crystal structures and equations of state of two perovskite-type Fe³⁺-rich phases, FeMg_0.5Si_0.5O₃ and Fe_0.5Mg_0.5Al_0.5Si_0.5O₃, at high pressures, employing single-crystal X-ray diffraction and synchrotron Mössbauer spectroscopy. We solved their crystal structures at high pressures and found that the FeMg_0.5Si_0.5O₃ phase adopts a novel monoclinic double-perovskite structure with the space group of P21/n at pressures above 12 GPa, whereas the Fe_0.5Mg_0.5Al_0.5Si_0.5O₃ phase adopts an orthorhombic perovskite structure with the space group of Pnma at pressures above 8 GPa. The pressure induces an iron spin transition for Fe³⁺ in a (Fe_0.7,Mg_0.3)O₆ octahedral site of the FeMg_0.5Si_0.5O₃ phase at pressures higher than 40 GPa. No iron spin transition was observed for the Fe_0.5Mg_0.5Al_0.5Si_0.5O₃ phase as all Fe³⁺ ions are located in bicapped prism sites, which have larger volumes than an octahedral site of (Al_0.5,Si_0.5)O₆.