AUTHOR=Koemets Iuliia , Wang Biao , Koemets Egor , Ishii Takayuki , Liu Zhaodong , McCammon Catherine , Chanyshev Artem , Katsura Tomo , Hanfland Michael , Chumakov Alexander , Dubrovinsky Leonid 

TITLE=Crystal chemistry and compressibility of Fe0.5Mg0.5Al0.5Si0.5O3 and FeMg0.5Si0.5O3 silicate perovskites at pressures up to 95 GPa

JOURNAL=Frontiers in Chemistry

VOLUME=11

YEAR=2023

URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2023.1258389

DOI=10.3389/fchem.2023.1258389

ISSN=2296-2646

ABSTRACT=<p>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 Fe<sup>3+</sup>-rich phases, FeMg<sub>0.5</sub>Si<sub>0.5</sub>O<sub>3</sub> and Fe<sub>0.5</sub>Mg<sub>0.5</sub>Al<sub>0.5</sub>Si<sub>0.5</sub>O<sub>3</sub>, 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<sub>0.5</sub>Si<sub>0.5</sub>O<sub>3</sub> phase adopts a novel monoclinic double-perovskite structure with the space group of <italic>P21/n</italic> at pressures above 12 GPa, whereas the Fe<sub>0.5</sub>Mg<sub>0.5</sub>Al<sub>0.5</sub>Si<sub>0.5</sub>O<sub>3</sub> phase adopts an orthorhombic perovskite structure with the space group of <italic>Pnma</italic> at pressures above 8 GPa. The pressure induces an iron spin transition for Fe<sup>3+</sup> in a (Fe<sub>0.7</sub>,Mg<sub>0.3</sub>)O<sub>6</sub> octahedral site of the FeMg<sub>0.5</sub>Si<sub>0.5</sub>O<sub>3</sub> phase at pressures higher than 40 GPa. No iron spin transition was observed for the Fe<sub>0.5</sub>Mg<sub>0.5</sub>Al<sub>0.5</sub>Si<sub>0.5</sub>O<sub>3</sub> phase as all Fe<sup>3+</sup> ions are located in bicapped prism sites, which have larger volumes than an octahedral site of (Al<sub>0.5</sub>,Si<sub>0.5</sub>)O<sub>6</sub>.</p>