Pressure-induced structural transitions in MgH${_2}$

TL;DR

This study uses density-functional calculations to explore pressure-induced structural phase transitions in MgH2 up to 20 GPa, identifying multiple phase changes and their transition pressures.

Contribution

The paper provides a detailed theoretical prediction of phase transition pressures in MgH2, including new insights into coexistence regions and structural data validation against experiments.

Findings

MgH2 transforms from α to γ phase at 0.39 GPa

γ to β phase transition occurs at 3.84 GPa

Additional transitions to δ and ε phases identified at higher pressures

Abstract

The stability of MgH$_2$ has been studied up to 20~GPa using density-functional total-energy calculations. At ambient pressure $\alpha$-MgH${_2}$ takes a TiO$_2$-rutile-type structure. $\alpha$-MgH$_2$ is predicted to transform into $\gamma$-MgH${_2}$ at 0.39~GPa. The calculated structural data for $\alpha$- and $\gamma$-MgH${_2}$ are in very good agreement with experimental values. At equilibrium the energy difference between these modifications is very small, and as a result both phases coexist in a certain volume and pressure field. Above 3.84~GPa $\gamma$-MgH${_2}$ transforms into $\beta$-MgH${_2}$; consistent with experimental findings. Two further transformations have been identified at still higher pressure: i) $\beta$- to $\delta$-MgH${_2}$ at 6.73 GPa and (ii) $\delta$- to $\epsilon$-MgH${_2}$ at 10.26~GPa.