Structural stability and pressure-induced phase transitions in ${\mathrm{MgH}}_2$

The structural stability of ${\mathrm{MgH}}_2$ has been studied up to $16\mathrm{GPa}$ 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 $\alpha$ and $\gamma$ modifications the high-pressure $\gamma$ 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 $1-$ states, respectively.

Figure 1

Observed (solid line) and simulated (points marked by +) XRD pattern for $\alpha \text{−}{\mathrm{MgH}}_2$. Positional parameters obtained by theoretical calculation are used for the simulated XRD patterns together with experimental unit-cell dimensions derived separated by Le Bail fitting of the observed data.

Figure 2

Observed (solid line) and simulated (points marked by +) XRD patterns of $\alpha \text{−}{\mathrm{MgH}}_2$ at $2.5\mathrm{GPa}$, $\gamma \text{−}{\mathrm{MgH}}_2$ at ambient pressure (after pressure release), $\beta \text{−}{\mathrm{MgH}}_2$ at $8.56\mathrm{GPa}$, and ${\delta }^{\prime }\text{−}{\mathrm{MgH}}_2$ at $15.36\mathrm{GPa}$. Simulated patterns are obtained with positional parameters from theoretical calculations and experimental unit-cell dimensions from separate Le Bail fittings of the observed data.

Figure 3

Crystal structures of the (a) $\alpha$, (b) $\beta$, (c) $\gamma$, and (d) ${\delta }^{\prime }$ modifications of ${\mathrm{MgH}}_2$.

Figure 4

(Color online) Calculated cell volume vs total energy difference (with respect to $\alpha \text{−}{\mathrm{MgH}}_2$) relations for ${\mathrm{MgH}}_2$ in actual and possible structural arrangements (see legends on the illustration). Arrow indicates transition points, but the actual transition pressures are calculated from the Gibbs free-energy curves.

Figure 5

(a) Calculated and (b) observed pressure vs volume relation for ${\mathrm{MgH}}_2$. Pressure stability regions for the different modifications (see Table and text) are indicated.

Figure 6

(a) Experimentally observed [at room temperature (Ref. 32)] and (b) calculated (at $0\mathrm{K}$) charge-density distribution in the (001) plane of $\alpha \text{−}{\mathrm{MgH}}_2$. The contour lines are drawn from 0.0 to 1.5 at $0.15e{\mathrm{\AA }}^{-3}$ intervals.

Figure 7

(Color online) (a) Calculated charge-transfer (see text) and (b) electron-localization-function maps for $\alpha \text{−}{\mathrm{MgH}}_2$ in the (001) plane.

Figure 8

Calculated crystal-orbital-Hamiltonian-population (COHP) profiles for the Mg-to-H bonds in different ${\mathrm{MgH}}_2$ polymorphs. Integrated COHP value for $\alpha$-, $\beta$-, $\gamma$-, and ${\delta }^{\prime }\text{−}{\mathrm{MgH}}_2$ is $-1.61$, $-1.65$, $-1.70$, and $-1.62\mathrm{eV}$, respectively.