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

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.

4 pages, 4 figures

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