Synthesis ofNi2H3at 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.