Structural and Valence Changes of Europium Hydride Induced by Application of High-Pressure ${\mathrm{H}}_2$

Europium hydride ${\mathrm{EuH}}_x$, when exposed to high-pressure ${\mathrm{H}}_2$, has been found to exhibit the following structural and valence changes: $Pnma(x=2,\mathrm{\text{divalent}})\to P{6}_3/mmc(x=2,7.2–8.7\mathrm{GPa})\to I4/m(x>2,8.7–9.7\mathrm{GPa})\to I4/mmm(x>2,9.7\mathrm{GPa}-,\mathrm{\text{trivalent}})$. With a trivalent character and a distorted cubic fcc structure, the $I4/mmm$ structure is the $\beta$ phase commonly observed for other rare-earth metal hydrides. Our study clearly demonstrates that ${\mathrm{EuH}}_x$ is no longer an irregular member of the rare-earth metal hydrides.

Figure 1

Integrated XRD profiles of (a) the ${\mathrm{EuH}}_x/{\mathrm{H}}_2$ and (b) the ${\mathrm{EuH}}_2/\mathrm{He}$. The inset graph in (a) indicates the XRD profile of ${\mathrm{EuH}}_x$ -III at 4.3 GPa when pressure is unloaded. The downward arrows in the inset graph show the satellite peaks of the ${\mathrm{EuH}}_x$-III phase. The “g” labels and upward arrows in (b) show the diffraction peaks of Re-metal gasket and EuO, respectively.

Figure 2

Structure models for (a) ${\mathrm{EuH}}_x$-III and (b) ${\mathrm{EuH}}_x$-IV, with corresponding Rietveld fits, the observed XRD profile and atomic positions. Large and small spheres indicate Eu-atoms and the possible H-atom positions, respectively. The final fit resulted in reliability factors of $R_{wp}=4.32\%$, $R_p=2.83\%$, and $R_{wp}(\mathrm{\text{without background}})=11.52\%$. The “r” in atomic position tables indicates that the $T$ sites, H3 and H4 in ${\mathrm{EuH}}_x$-III, ${\mathrm{H}}_2$ in ${\mathrm{EuH}}_x$-IV, are randomly occupied.

Figure 3

Pressure dependence of $V_{\mathrm{atom},\mathrm{Eu}}$. The solid lines represent the Birch-Murnaghn equation of state (BM-EOS). The fitting to BM-EOS gives the following results: $B_0=10.17(3)\mathrm{GPa}$ and $V_0=42.52(8){\AA }^3$ for ${\mathrm{EuH}}_x$ -I, $B_0=11.2(2)\mathrm{GPa}$ and $V_0=39.8(1){\AA }^3$ for ${\mathrm{EuH}}_x$ -II, $B_0=14.33(7)\mathrm{GPa}$ and $V_0=32.82(3){\AA }^3$ for ${\mathrm{EuH}}_x$ -IV. The volume data of pure bcc-Eu were taken from Ref. 17. The inset graph shows the pressure dependence of the $c/\mathrm{\text{textita}}$ ratio for ${\mathrm{EuH}}_x$-II, ${\mathrm{EuH}}_x$-III and ${\mathrm{EuH}}_x$-IV. ${\mathrm{EuH}}_x$-IV is stable up to the highest pressure of 50 GPa.

Figure 4

High-pressure Eu-Mössbauer spectra of ${\mathrm{EuH}}_x$-I at 2.3 GPa and ${\mathrm{EuH}}_x$-IV at 14.3 GPa. The solid lines are fit of the experimental data. The velocity scale was calibrated relative to the center of a single line of ${\mathrm{EuF}}_3$ under ambient conditions.