Structural phase transition inCaH2at high pressures

The structural and vibrational properties of $\mathrm{Ca}{\mathrm{H}}_{2}$ have been examined up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ at room temperature. Under ambient conditions, $\mathrm{Ca}{\mathrm{H}}_{2}$ has a $Pnma$ (cotunnite-type) structure. A structural phase transformation was observed around $15\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and completed at $20\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The high pressure structure is identified as hexagonal $P{6}_{3}∕mmc$. First-principles calculations reproduced the first-order nature of the transition. Since $P{6}_{3}∕mmc$ is a supergroup of $Pnma$ the structural change can be traced back to gradual displacements of the hydrogen atoms from the $4c$ positions in the cotunnite structure to the special $2a$ and $2d$ positions in the hexagonal structure. The observed phase transition pressure is much lower than that predicted for $\mathrm{Mg}{\mathrm{H}}_{2}$.