[S-IT16] Structure and Dynamics of Earth and Planetary Mantles

[SIT16-10] 第一原理計算によるMg₂GeO₄の秩序無秩序相転移予言

*梅本 幸一郎^1,2、Wentzcovitch Renata³ (1.東京工業大学地球生命研究所、2.理化学研究所、3.コロンビア大学)

キーワード：高温高圧相転移、地球型系外惑星、第一原理計算

Here we present an first-principles prediction of a temperature-induced order-disorder transition (ODT) from I-42d-type to Th₃P₄-type phase in I-42d-type Mg₂GeO₄. 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 [1] of ultrahigh pressure I-42d-type Mg₂SiO₄ [2]. The latter was predicted to occur as a product of dissociation/recombination transitions in MgSiO₃ post-perovskite at multi-Mbar pressures (post-PPV transitions) [3]. In the Mg-Ge-O system, this post-PPV phase occurs beyond ~150 GPa [1]. The new ODT predicted here alters the sequence of post-PPV phases in Mg₂GeO₄ at high temperatures (>~2,000 K) and likely also in Mg₂SiO₄. I-42d-type Mg₂SiO₄ is predicted to occur in the deep interiors of super-Earths [2-5]. Therefore, this newly found ODT should be relevant for modeling the internal dynamics and structure of super-Earth-type planets. The prediction of this Th₃P₄-type phase in Mg₂GeO₄ 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:
[1] K. Umemoto and R. M. Wentzcovitch, Phys. Rev. Materials 3, 123601 (2019).
[2] S. Q. Wu et al., J. Phys.: Condens. Matter, 26, 035402 (2014).
[3] K. Umemoto et al., Earth Planet. Sci. Lett. 478, 40-45 (2017).
[4] H. Niu et al., Scientific Reports 5, 18347 (2015).
[5] A. van den Berg, et al. Icarus 317, 412-426 (2019).