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

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.

References in corpus (2)

• Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
• Phase transitions in MgSiO3 post-perovskite in super-Earth mantles

Cited by in corpus (3)

• Ultra-high pressure disordered eight-coordinated phase of Mg${}_2$GeO${}_4$: Analogue for super-Earth mantles
• High-pressure order-disorder transition in Mg${}_2$SiO${}_4$: Implications for super-Earth mineralogy
• Ab initio investigation of H-bond disordering in $\delta$-AlOOH