Ab initio prediction of an order-disorder transition in Mg2GeO4: Implication for the nature of super-Earth's mantles

Here we present an ab initio prediction of an order-disorder transition (ODT) from a I (4) over bar 2d-type to Th3P4-type phase in the cation sublattices of Mg2GeO4, 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. Mg2GeO4 is a low-pressure analog of I (4) over bar 2d-type Mg2SiO4, a predicted major planet-forming phase of super-Earths' mantles. Therefore, a similar ODT is anticipated in I (4) over bar 2d-type Mg2SiO4 as well, which should impact the internal structure and dynamics of these planets. The prediction of this Th3P4-type phase in Mg2GeO4 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.