Ultra-high pressure disordered eight-coordinated phase of Mg${}_2$GeO${}_4$: Analogue for super-Earth mantles

Abstract

Mg2GeO4 is an analogue for the ultra-high pressure behavior of Mg2SiO4, so we have investigated magnesium germanate to 275 GPa and over 2000 K using a laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction and density functional theory (DFT) computations. The experimental results are consistent with a novel phase with disordered Mg and Ge, in which germanium adopts eight-fold coordination with oxygen: the cubic Th3P4- type structure. Simulations using the special quasirandom structure (SQS) method suggest partial order in the tetragonal I-42d structure, indistinguishable from I-43d Th3P4 in our experiments. These structures have not been reported before in any oxide. If applicable to silicates, the formation of this highly coordinated and intrinsically disordered phase would have important implications for the interior mineralogy of large, rocky extrasolar planets.

13 pages, 5 figures and 18 pages of Supplementary Material

References in corpus (5)

• Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
• Canonical sampling through velocity-rescaling
• Phase transitions in MgSiO3 post-perovskite in super-Earth mantles
• Ab initio linear response and frozen phonons for the relaxor PMN (PbMg1/3Nb2/3O3)
• Ab initio prediction of an order-disorder transition in Mg${}_2$GeO${}_4$: implication for the nature of super-Earth's mantles

Cited by in corpus (4)

• High-pressure order-disorder transition in Mg${}_2$SiO${}_4$: Implications for super-Earth mineralogy
• Exo-Geoscience Perspectives Beyond Habitability
• Structure and Melting of Fe, MgO, SiO2, and MgSiO3 in Planets: Database, Inversion, and Phase Diagram
• A Validated Low-to-Intermediate Mass Planetary Interior Structure Model and New Mass-Radius Relations