Pressure-Induced Hydrogen-Dominant Metallic State in Aluminum Hydride

Two structural transitions in covalent aluminum hydride ${\mathrm{AlH}}_3$ were characterized at high pressure. A metallic phase stable above 100 GPa is found to have a remarkably simple cubic structure with shortest first-neighbor H-H distances ever measured except in ${\mathrm{H}}_2$ molecule. Although the high-pressure phase is predicted to be superconductive, this was not observed experimentally down to 4 K over the pressure range 120–164 GPa. The results indicate that the superconducting behavior may be more complex than anticipated.

Figure 1

X-ray diffraction patterns of phases I, II, and III of ${\mathrm{AlH}}_3$. Incident wavelength is 0.412 Å. Solid lines are calculated diffraction patterns. Asterisks mark diffraction peaks from the pressure-transmitting medium. The hexagonal ($a=4.44\AA$, $c=11.8\AA$) unit cell of phase I at ambient contains 6 formula units, Al in 6b $(0,0,0)$ positions and H in 18e $(0.63,0,\frac{1}{4})$ positions, in agreement with Refs. 14, 15. Phase II at 72 GPa was described in a monoclinic (space group $P2$) unit cell with lattice parameters $a=3.21\AA$, $b=6.18\AA$, $c=2.95\AA$, $\widehat{a}=119.7°$ containing 3 f.u. [solid (red) line] or in a trigonal ($a=3.83\AA$, $b=5.12\AA$, $c=6.72\AA$, $\alpha =112.6°$, $\beta =111.9°$, $\gamma =56.6°$) unit cell (space group $P1$) containing 6 f.u. suggested by theory (blue line). Phase III has a cubic structure with Al atoms in positions $(0,0,0)$ and $(\frac{1}{2},\frac{1}{2},\frac{1}{2})$, $a=3.08\AA$ at 110 GPa. Bragg $R$ factor for phase III $R_B=7\%$.

Figure 2

Volume per formula unit in ${\mathrm{AlH}}_3$ as a function of pressure. Black lines are fits of the experimental data by the Murnaghan-Birch equation. The volume of phase II was calculated in the monoclinic model. The fitting parameters are the following: $V_0=33.5(1)\AA$, $B_0=42.3(5)\mathrm{GPa}$, $B_1=3.5(1)$ for phase I; $V_0=31.7(5)\AA$, $B_0=44.9(10)\mathrm{GPa}$, $B^{\prime }=2.7(2)$ for phase II; $V_0=27.0(5)\AA$, $B_0=50.4(10)\mathrm{GPa}$, $B^{\prime }=3.5(2)$ for phase III. Red lines are results from ab initio calculations. In insets, left: $c/a$ ratio in phase I versus pressure; right: partial hydrogen volume per H atom in ${\mathrm{AlH}}_3$ (circles) compared with volume per H atom (blue curve) and per ${\mathrm{H}}_2$ molecule (red curve) expected in molecular hydrogen. Double line is a guide for the eye.

Figure 3

Electrical resistance in ${\mathrm{AlH}}_3$ at the pressures of 120 and 164 GPa as a function of temperature. Residual resistance at the lowest temperature in part originates from the electrical contacts due to the quasi-four-probe technique. In insets, top: image of the sample and the electrical contacts before and after the insulator-metal transition; bottom: electrical resistance measured at room temperature and different pressures.

Figure 4

(a) Left: the $Im3m$ (up) and $Pm\mathrm{\text{−}}3n$ (down) structures suggested for phase III. Aluminum atoms are shown in red and hydrogen atoms are shown in blue. Each cell contains 2 f.u., aluminum atoms occupy positions $(0,0,0)$ and $(\frac{1}{2},\frac{1}{2},\frac{1}{2})$. In the $Pm\mathrm{\text{−}}3n$ structure, hydrogen atoms occupy positions $(\frac{1}{2},\frac{1}{4},0)$, $(\frac{1}{2},\frac{3}{4},0)$, $(\frac{1}{4},0,\frac{1}{2})$, $(\frac{3}{4},0,\frac{1}{2})$, $(0,\frac{1}{2},\frac{1}{4})$, $(0,\frac{1}{2},\frac{3}{4})$. Right: calculated enthalpy in the $R\mathrm{\text{−}}3c$, $Im3m$, and $Pm\mathrm{\text{−}}3n$ structures. (b) Calculated electronic band structure (left) and projected density of states (right) in the $Pm\mathrm{\text{−}}3n$ phase. (c) Plots of the nesting function at 110 GPa and 165 GPa (left) showing significant nesting in the $R–M$ direction at 110 GPa but diminished at 165 GPa. (Right) Possible nesting vectors connecting the 2 pieces of Fermi surfaces at 110 GPa.