Beryllium polyhydride ${\mathrm{Be}}_4{\mathrm{H}}_8{\left({\mathrm{H}}_2\right)}_2$ synthesized at high pressure and temperature

We report x-ray diffraction and Raman scattering measurements in combination with density functional theory calculations that reveal the formation of beryllium polyhydride ${\mathrm{Be}}_4{\mathrm{H}}_8{\left({\mathrm{H}}_2\right)}_2$ by the laser heating of a Be-${\mathrm{H}}_2$ mixture to above 1700 K at pressures between 5 and 8 GPa. ${\mathrm{Be}}_4{\mathrm{H}}_8{\left({\mathrm{H}}_2\right)}_2$ crystallizes in a $P{6}_3/mmc$ structure and consists of the corner-sharing ${\mathrm{BeH}}_4$ tetrahedrons and the ${\mathrm{H}}_2$ molecules that occupy an interstitial site. ${\mathrm{Be}}_4{\mathrm{H}}_8{\left({\mathrm{H}}_2\right)}_2$ is stable at least to 14 GPa upon compression and stable down to 4 GPa at room temperature. Our ab initio calculations suggest that ${\mathrm{Be}}_4{\mathrm{H}}_8{\left({\mathrm{H}}_2\right)}_2$ is a metastable phase of the Be-H system.

Figure 1

(a) XRD profiles of the Be-${\mathrm{H}}_2$ mixture before and after the laser heating with the Rietveld fitting result for the suggested structure model. (b) The $V_{\mathrm{f}.\mathrm{u}.}$ vs $P$ for h-${\mathrm{BeH}}_{\mathit{n}}$ (solid circle), $\alpha \text{−}{\mathrm{BeH}}_2$ (solid line), and $\alpha \text{−}{\mathrm{BeH}}_2+$(1/2)${\mathrm{H}}_2$ (dashed line). The $V_{\mathrm{f}.\mathrm{u}.}$ of $\alpha \text{−}{\mathrm{BeH}}_2+1/2({\mathrm{H}}_2$) is calculated using the data shown in Refs. [12, 24]. The lattice parameters of h-${\mathrm{BeH}}_{\mathit{n}}$ at 14 GPa are $a$ = 4.290(1) Å, $c$ = 6.993(2) Å, and $V=111.5\left(1\right){\AA }^3$. (c) Raman scattering spectra of the h-${\mathrm{BeH}}_{\mathit{n}}\text{−}{\mathrm{H}}_2$ mixture sample at 7.1 GPa, excess ${\mathrm{H}}_2$, the Be after being heated to 2000 K in Ar at 5 GPa, and the calculated spectrum for ${\mathrm{Be}}_4{\mathrm{H}}_8{\left({\mathrm{H}}_2\right)}_2$. The numbers indicate the correspondence between the observed and the predicted peaks. “dia.” is the signal from a diamond anvil.

Figure 2

${\mathrm{Be}}_4{\mathrm{H}}_8{\left({\mathrm{H}}_2\right)}_2$ (visualized using vesta [25]). Hydrogen atoms “H” are moving around the disordered positions. The ${\mathrm{H}}_2$ atoms are rotating in a dotted circle. Each tetrahedron is a corner-sharing ${\mathrm{BeH}}_4$ unit. See the Supplemental material for the crystal structure information file (CIF) of ${\mathrm{Be}}_4{\mathrm{H}}_8{\left({\mathrm{H}}_2\right)}_2$ [26].

Figure 3

(a) The Raman scattering spectra of h-${\mathrm{BeH}}_{\mathit{n}}$ as pressure decreases. The numbers indicate the peaks described in Fig. 1. The spectra at 1 bar were measured after the diamond anvil was removed. (b) The pressure dependence of the Raman shifts for peaks 1–16. The data obtained in three independent experiments are plotted together. Below 4 GPa (shaded area), h-${\mathrm{BeH}}_{\mathit{n}}$ is unstable at room temperature.