Ternary nickel hydrides: A platform for unconventional superconductivity and quantum magnetism

Hydrides are famous for the possibility of reaching high-Tc superconductivity under high pressure within a conventional mechanism. Here we propose ternary nickel hydrides MNiH2 (M=Li, Na) as materials that mirror key aspects of unconventional superconducting cuprates and nickelates while presenting unique characteristics. Compared to Ni oxides, Ni-H bands are wider due to shorter bond lengths and show a smaller charge-transfer energy. This leads to a larger scale of magnetic interactions than for LaNiO2, which previous works in cuprates suggest should lead to a larger Tc. The presence of an interstitial anionic electron orbital, which hybridizes with the dz2 band, induces self-doping in the Ni dx2-y2 band, enabling metallicity and superconductivity in stoichiometric forms. A t-J model computation yields dominant dx2-y2 superconducting symmetry for all doping levels. This, combined with the improved thermodynamic stability over known Ni1+ oxides, positions MNiH2 as compelling materials for unconventional high-Tc superconductivity under standard conditions.