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arXiv · 2012.08056

Ab initio prediction of an order-disorder transition in Mg$_2$GeO$_4$: implication for the nature of super-Earth's mantles

Koichiro Umemoto·Renata M. Wentzcovitch
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Abstract

Here we present an ab initio prediction of an order-disorder transition (ODT) from $I\bar{4}2d$-type to a Th$_3$P$_4$-type phase in the cation sublattices of Mg$_2$GeO$_4$, a post-post-perovskite (post-PPv) 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. Mg$_2$GeO$_4$ is a low-pressure analog of $I\bar{4}2d$-type Mg$_2$SiO$_4$, a predicted major planet-forming phase of super-Earths' mantles. Therefore, a similar ODT is anticipated in $I\bar{4}2d$-type Mg$_2$SiO$_4$ as well, which should impact the internal structure and dynamics of these planets. The prediction of this Th$_3$P$_4$-type phase in Mg$_2$GeO$_4$ enhances further 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.

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Koichiro Umemoto, Renata M. Wentzcovitch. 2020-12-15. Ab initio prediction of an order-disorder transition in Mg$_2$GeO$_4$: implication for the nature of super-Earth's mantles. https://doi.org/10.1103/physrevmaterials.5.093604

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