High-pressure order-disorder transition in Mg$_2$SiO$_4$: Implications for super-Earth mineralogy

TL;DR

This study uses density functional theory to investigate high-pressure phase transitions in Mg$_2$SiO$_4$, revealing a stable disordered structure relevant for super-Earth interiors and its implications for mineral chemistry.

Contribution

It identifies a novel disordered I-42d structure in Mg$_2$SiO$_4$ at high pressures, expanding understanding of mineral phases in super-Earths.

Findings

Discovered a stable disordered I-42d phase at high pressure.

Explored the transition mechanism from ordered to disordered structures.

Analyzed the impact of disorder on electronic properties.

Abstract

(Mg, Fe)SiO$_3$ post-perovskite is the highest pressure silicate mineral phase in the Earth's interior. The extreme pressure and temperature conditions inside large extrasolar planets will likely lead to phase transitions beyond pPv. In this work we have explored the high-pressure phase relations in Mg$_2$SiO$_4$ using computations based on density functional theory. We find that a partially disordered I-42d type structure would be stable in the interiors of these super-Earth planets. The discovery of a structure where two very dissimilar cations, Mg$^{2+}$ and Si$^{4+}$ occupy the same crystallographic site opens up a domain of interesting crystal chemistry and provides a foundation for other silicates and oxides with mixed occupancy. We have explored the mechanism of the phase transition from the ordered ground state and the effect of the disordering on electronic properties of the silicate phase.