<i>Ab initio</i> prediction of an order-disorder transition in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Mg</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi>Ge</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>: Implication for the nature of super-Earth's mantles
Koichiro Umemoto, Renata M. Wentzcovitch
Abstract
Here we present an ab initio prediction of an order-disorder transition (ODT) from a $I\overline{4}2d$-type to ${\mathrm{Th}}_{3}{\mathrm{P}}_{4}$-type phase in the cation sublattices of ${\mathrm{Mg}}_{2}\mathrm{Ge}{\mathrm{O}}_{4}$, a post-post-perovskite phase. This uncommon type of prediction is achieved by carrying out a high-throughput sampling of atomic configurations in a 56-atom supercell followed by a Boltzmann ensemble statistics calculation. ${\mathrm{Mg}}_{2}\mathrm{Ge}{\mathrm{O}}_{4}$ is a low-pressure analog of $I\overline{4}2d$-type ${\mathrm{Mg}}_{2}\mathrm{Si}{\mathrm{O}}_{4}$, a predicted major planet-forming phase of super-Earths' mantles. Therefore, a similar ODT is anticipated in $I\overline{4}2d$-type ${\mathrm{Mg}}_{2}\mathrm{Si}{\mathrm{O}}_{4}$ as well, which should impact the internal structure and dynamics of these planets. The prediction of this ${\mathrm{Th}}_{3}{\mathrm{P}}_{4}$-type phase in ${\mathrm{Mg}}_{2}\mathrm{Ge}{\mathrm{O}}_{4}$ further enhances the relationship between the crystal structures of Earth/planet-forming silicates and oxides at extreme pressures and those of rare-earth sesquisulfides at low pressures.