Pressure-Induced Structural Transitions inMgH2

The stability of ${\mathrm{M}\mathrm{g}\mathrm{H}}_2$ has been studied up to 20 GPa using density-functional total-energy calculations. At ambient pressure $\alpha \mathrm{\text{−}}{\mathrm{M}\mathrm{g}\mathrm{H}}_2$ takes a ${\mathrm{T}\mathrm{i}\mathrm{O}}_2$-rutile-type structure. $\alpha \mathrm{\text{−}}{\mathrm{M}\mathrm{g}\mathrm{H}}_2$ is predicted to transform into $\gamma \mathrm{\text{−}}{\mathrm{M}\mathrm{g}\mathrm{H}}_2$ at 0.39 GPa. The calculated structural data for $\alpha$- and $\gamma \mathrm{\text{−}}{\mathrm{M}\mathrm{g}\mathrm{H}}_2$ 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 $\gamma \mathrm{\text{−}}{\mathrm{M}\mathrm{g}\mathrm{H}}_2$ transforms into $\beta \mathrm{\text{−}}{\mathrm{M}\mathrm{g}\mathrm{H}}_2$, consistent with experimental findings. Two further transformations have been identified at still higher pressure: (i) $\beta$- to $\delta \mathrm{\text{−}}{\mathrm{M}\mathrm{g}\mathrm{H}}_2$ at 6.73 GPa and (ii) $\delta$- to $ϵ\mathrm{\text{−}}{\mathrm{M}\mathrm{g}\mathrm{H}}_2$ at 10.26 GPa.