Structural stability and pressure-induced phase transitions inMgH2

The structural stability of ${\mathrm{MgH}}_{2}$ has been studied up to $16\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ using a high-pressure synchrotron x-ray diffraction technique. Several pressure-induced phase transitions have been identified in this pressure range. Owing to the close structural similarity between the $\ensuremath{\alpha}$ and $\ensuremath{\gamma}$ modifications the high-pressure $\ensuremath{\gamma}$ form can be stabilized as a metastable phase after pressure release. The experimentally observed structural transition sequence and the volume changes at the transition points as well as bulk modulii are found to be in good agreement with theoretically calculated data. The bonding nature of ${\mathrm{MgH}}_{2}$ is analyzed with the help of charge-density, charge-transfer, electron-localization-function, and Mulliken-population analyses which clearly show that all polymorphs of ${\mathrm{MgH}}_{2}$ are to be classified as ionic materials with Mg and H in nearly 2+ and $1\ensuremath{-}$ states, respectively.