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

TL;DR

This study investigates Mg2GeO4 under extreme pressures and temperatures, revealing a novel disordered phase with eight-fold coordinated germanium, which could inform models of super-Earth mantles.

Contribution

It reports the discovery of a previously unobserved high-pressure phase of Mg2GeO4 with disordered Mg and Ge, and provides insights into its structure through combined experimental and computational methods.

Findings

Identification of a new cubic Th3P4-type structure at high pressure

Evidence of partial order in the disordered phase from simulations

Implications for mineralogy of super-Earth interiors

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.