Synthesis of ${\mathrm{Ni}}_2{\mathrm{H}}_3$ at high temperatures and pressures

In situ high-pressure high-temperature synchrotron x-ray diffraction studies of the nickel-hydrogen system reveals the synthesis of a nickel polyhydride, ${\mathrm{Ni}}_2{\mathrm{H}}_3$. We observe the formation of NiH at pressures above 1 GPa, which remains stable to 52 GPa at room temperature. Laser heating to above 1000 K at this pressure initiates a transition to a phase which we determine as ${\mathrm{Ni}}_2{\mathrm{H}}_3$, crystallizing in a body-centered monoclinic unit cell. The ${\mathrm{Ni}}_2{\mathrm{H}}_3$ phase was observed to convert back to NiH below 25 GPa, and upon further decompression to atmospheric conditions, NiH slowly releases hydrogen with time.

Figure 1

(a) High-pressure x-ray diffraction patterns $(\lambda =0.4131$ Å) showing the synthesis of ${\mathrm{Ni}}_2{\mathrm{H}}_3$ by laser heating above 50 GPa. Tick marks indicate the positions of Bragg reflections from the noted phases. (b) Atomic volume as a function of pressure for Ni (black), ${\gamma }_2$-NiH (blue), and ${\mathrm{Ni}}_2{\mathrm{H}}_3$ (green). Fitted equations of state are shown with dashed lines; the equation of state of Ni is shown with a solid line [13]. Closed (open) symbols correspond to data collected on (de)compression. Inset shows raw image plates before and after laser heating showing the clear formation of textured rings due to ${\mathrm{Ni}}_2{\mathrm{H}}_3$. (c) Representative Rietveld refinement of ${\mathrm{Ni}}_2{\mathrm{H}}_3$ data at 52 GPa with Ni atoms at $\mathrm{Ni}1\left[0.7101\left(6\right),0,0.0640\left(6\right)\right]$ and $\mathrm{Ni}2\left[0.0671\left(5\right),0,0.2439\left(6\right)\right],w{R}_p=1.94\%,w{R}_{\mathrm{all}}=14.19\%$.

Figure 2

(a) ${\mathrm{Ni}}_2{\mathrm{H}}_3$ unit-cell dimensions as a function of pressure. Inset: Crystal structure of ${\mathrm{Ni}}_2{\mathrm{H}}_3$ which crystallizes in space group $I2/m$ with two unique Ni atoms. (b) ${\mathrm{Ni}}_2{\mathrm{H}}_3$ unit-cell volume as a function of pressure. Inset: ${\mathrm{Ni}}_2{\mathrm{H}}_3$ structure showing four-coordinated H atoms in a distorted square planar arrangement. (c) Monoclinic angle, $\beta$, as a function of pressure. Data points in (a)–(c) are from experiments while lines correspond to values derived from DFT calculations at 0 K.

Figure 3

High-pressure x-ray diffraction patterns $(\lambda =0.4131$ Å) taken on decompression showing (left) the stepwise decomposition of ${\mathrm{Ni}}_2{\mathrm{H}}_3$ into ${\gamma }_2$-NiH and (right) the subsequent decomposition of ${\gamma }_2$-NiH into Ni and ${\mathrm{H}}_2$ at ambient pressure.