Figure 1

(Color online) (a) (Top panel) X-ray diffraction pattern from run 1. Note that the initial pressure was $16\mathrm{GPa}$ in which the sample was already a mixture of the low pressure $Pnma$ phase and high pressure $P{6}_3∕mmc$ phase. The sample pressure was then decreased to $5\mathrm{GPa}$ and subsequently increased to finally reach $30\mathrm{GPa}$. Background has been subtracted from all patterns and offsets have been introduced for clarity. (b) (Bottom panel) X-ray diffraction pattern (symbols) recorded at $22.9\mathrm{GPa}$ in run 2 and Rietveld fit (solid line) considering a mixture of three phases, namely, high-density $\mathrm{Ca}{\mathrm{H}}_2$ $(P{6}_3∕mmc)$, hexagonal stainless steel gasket T301 $(P{6}_3∕mmc)$, and cubic stainless steel gasket T301 $\left(Im\text{−}3m\right)$. For the $\mathrm{Ca}{\mathrm{H}}_2$ phase, Ca atomic positions as fixed by symmetry and full site occupancies are assumed. Preferred orientation for the high-pressure hydride and the stainless steel gasket, both hexagonal phases, has been taken into account. The $R$ factors for the Rietveld fit (FullProf) are $R_{\exp }=7.8\%$, $R_{\mathrm{wp}}=10.3\%$, and $R_{\mathrm{wp}}∕R_{\exp }=1.3$. The bottom trace shows the difference between the Rietveld fit and the experimental data. The inset illustrates the low- and high-density structure models for $\mathrm{Ca}{\mathrm{H}}_2$.Reuse & Permissions

Figure 2

(Color online) Room temperature pressure dependence of lattice parameters and relative unit cell volume of the orthorhombic $Pnma$ and hexagonal $P{6}_3∕mmc$ (in the orthorhombic setting) phases of $\mathrm{Ca}{\mathrm{H}}_2$. Open and solid symbols distinguish results obtained in run 1 and run 2, respectively. Data points below $16\mathrm{GPa}$ were recorded with decreasing pressures in run 1. Solid lines represent theoretical calculations.Reuse & Permissions

Figure 3

(Color online) Raman data for run 1. (a) (Top panel) Raman spectra as a function of pressure, following the sequence of pressure changes from bottom to top. The asterisk indicates the Raman line due to the high-pressure phase in the case of phase mixtures. (b) (Bottom panel) Pressure dependence of the Raman shifts for decreasing pressures (open symbols) and increasing pressures (solid symbols).Reuse & Permissions

Figure 4

(Color online) (a) (Top panel) Unit cell volume calculated as a function of pressure. The data points (open symbols) for the $P{6}_3∕mmc$ are those for the equivalent orthorhombic setting. The inset illustrates the change of total energy for both phases. (b) (Bottom panel) Atomic coordinates calculated as a function of pressure.Reuse & Permissions