Synthesis of new nickel hydrides at high pressure

We predict two new nickel hydrides, $\mathrm{N}{\mathrm{i}}_{2}{\mathrm{H}}_{3}$ ($C2/m$) and $\mathrm{Ni}{\mathrm{H}}_{2}$ ($I4/mmm$), to be thermodynamically stable at 60 GPa by DFT calculations. The calculated structure of $\mathrm{Ni}{\mathrm{H}}_{2}$ is similar to the one observed earlier for $\mathrm{Fe}{\mathrm{H}}_{2}$. However, to the best of our knowledge, the monoclinic structure predicted for $\mathrm{N}{\mathrm{i}}_{2}{\mathrm{H}}_{3}$ was never reported in the other hydrides. We successfully synthesized the monoclinic $\mathrm{N}{\mathrm{i}}_{2}{\mathrm{H}}_{3}$ phase using a laser-heating technique in a diamond anvil cell at pressures around 60 GPa. The $\mathrm{N}{\mathrm{i}}_{2}{\mathrm{H}}_{3}$ phase can be retained down to 17 GPa, and DFT calculations predict it to be nonmagnetic and nonsuperconducting. However, we did not find in our experiments the theoretically predicted $\mathrm{Ni}{\mathrm{H}}_{2}$ phase. Higher pressures and/or different synthesis conditions might be needed to synthesize this and other polyhydrides of nickel. Our results show that the Ni-H system behaves differently in comparison to the Fe-H system: Ni extends its own chemical identity to pressures as high as 60--80 GPa. This finding may have significant implications for the high-pressure behavior of Fe-Ni alloys at the high-pressure conditions relevant for the Earth and planetary sciences.