Superconductivity of the hydrogen-rich metal hydride $\mathrm{L}{\mathrm{i}}_5\mathrm{Mo}{\mathrm{H}}_{11}$ under high pressure

Ternary metal hydrides are convenient and valuable systems for investigating the metallization and superconductivity of metal hydrides because they can be synthesized under mild conditions and recovered under ambient pressure. In this study, the conducting behavior and structural phase transition of a hydrogen-rich metal hydride, $\mathrm{L}{\mathrm{i}}_5\mathrm{Mo}{\mathrm{H}}_{11}$, were investigated at pressures up to 210 GPa in a diamond anvil cell. The results showed that $\mathrm{L}{\mathrm{i}}_5\mathrm{Mo}{\mathrm{H}}_{11}$ transforms from an insulator to a poor metal at around 100 GPa. Superconductivity was observed at 100 GPa and retained until 210 GPa, and its maximum onset transition temperature was 6.5 K at 160 GPa. High-pressure synchrotron x-ray diffraction experiments revealed that the ambient-pressure hexagonal crystal structure is retained until at least 130 GPa. Furthermore, apart from the influence of pressure on the conducting behavior of $\mathrm{L}{\mathrm{i}}_5\mathrm{Mo}{\mathrm{H}}_{11}$, the effect of annealing time on the conducting and superconducting behaviors at room temperature and high pressure were also observed. We hypothesized that this time-dependent behavior is due to the restoration of the $\mathrm{Mo}{\mathrm{H}}_9$ cage structure after distortion or rotation caused by pressurization. These findings provide insight on the conducting and superconducting behaviors of ternary metal hydrides that, until recently, have been mostly studied by theoretical methods.

Figure 1

(a) Pressure dependence of electrical resistance of $\mathrm{L}{\mathrm{i}}_5\mathrm{Mo}{\mathrm{H}}_{11}$ at room temperature. Temperature dependence of resistance at various pressures: (b) 100 GPa at various annealing times, (c) 130–170 GPa at 0–300 K (inset: 0–10 K), and (d) 140 GPa at various magnetic fields. The inset shows upper critical field $H_{c2}$ at various transition temperatures $T_c$, which are determined from the onset of resistance drop. The dashed line is the linear fitting with slope ${\left(d{H}_{c2}/dT\right)}_{T=T_c}$.

Figure 2

(a) Powder XRD pattern of $\mathrm{L}{\mathrm{i}}_5\mathrm{Mo}{\mathrm{H}}_{11}$ at ambient pressure and Rietveld refinement parameters $R_{wp}$ and $R_p$. (b) Powder XRD pattern at various pressures up to 130 GPa and room temperature. Open circles indicate the reflection from $\mathrm{L}{\mathrm{i}}_5\mathrm{Mo}{\mathrm{H}}_{11}$; closed circle, $\mathrm{L}{\mathrm{i}}_2\mathrm{O}$; and open triangle, tungsten (needle used to load sample into diamond anvil cell). (c) Pressure dependence of lattice parameters $a$ and $c$. (d) Unit cell volume V as a function of pressure. The solid curve is the best-fit curve to the third-order Birch-Murnaghan equation of state. Data fitting yields a bulk modulus $B_0$ of 51.6 (±2.2) GPa and its first pressure derivative $B_0^{\prime }$ of 4.65 (±0.17).

Figure 3

Phase diagram of $\mathrm{L}{\mathrm{i}}_5\mathrm{Mo}{\mathrm{H}}_{11}$ as a function of pressure. The black squares and blue triangles represent data from run 1 and run 2, respectively. The annealing time was 48 days for run 1 and 18 days for run 2. HP indicates high-pressure, and SC1 and SC2 indicate superconducting phases. The different values of $T_c$ at 160 and 165 GPa in SC2 indicate the coexistence of high-pressure superconducting phases.

Figure 4

(a) Resistance as a function of temperature at various magnetic fields and 160 GPa (inset: 150 GPa). (b) Transition temperature $T_c$ dependence of upper critical field $H_{c2}$ at 150 and 160 GPa. These three curves at 160 GPa correspond to three points of inflection.