The quest for a Ni-based oxide analog to cuprate ${\mathrm{Cu}}^{2+}\left(d^9\right)$ superconductors was long known to require a reduced form of ${\mathrm{Ni}}^{1+}\left(d^9\right)$ as in $A^{3+}{\mathrm{Ni}}^{1+}{\mathrm{O}}_2$, being an extremely oxygen-poor form of the usual $A^{3+}{\mathrm{Ni}}^{3+}{\mathrm{O}}_3$ compound. Through $\mathrm{Ca}{\mathrm{H}}_2$ chemical reduction of a parent $R^{3+}{\mathrm{Ni}}^{3+}{\mathrm{O}}_3$ perovskite form, superconductivity was recently achieved in Sr-doped $\mathrm{NdNi}{\mathrm{O}}_2$ on a $\mathrm{SrTi}{\mathrm{O}}_3$ substrate. Using density functional theory (DFT) calculations, we find that stoichiometric $\mathrm{NdNi}{\mathrm{O}}_2$ is significantly unstable with respect to decomposition into $\frac{1}{2}\left[{\mathrm{Nd}}_2{\mathrm{O}}_3+\mathrm{NiO}+\mathrm{Ni}\right]$ with exothermic decomposition energy of +176 meV/atom, a considerably higher instability than that for common ternary oxides. This poses the question of whether the stoichiometric $\mathrm{NdNi}{\mathrm{O}}_2$ nickelate compound used extensively to model the electronic band structure of the Ni-based oxide analog to cuprates, and found to be metallic, is the right model for this purpose. To examine this, we study via DFT the role of the common H impurity expected to be present in the process of chemical reduction needed to obtain $\mathrm{NdNi}{\mathrm{O}}_2$. We find that H can be incorporated exothermically, i.e., spontaneously in $\mathrm{NdNi}{\mathrm{O}}_2$, even from ${\mathrm{H}}_2$ gas. In the concentrated limit, such impurities can result in the formation of a hydride compound, $\mathrm{NdNi}{\mathrm{O}}_2\mathrm{H}$, which has significantly reduced instability relative to hydrogen-free $\mathrm{NdNi}{\mathrm{O}}_2$ (decomposition energy of +80 meV/atom instead of +176 meV/atom). Interestingly, the hydrogenated form has lattice constants similar to those of the pure form (leading to comparable x-ray diffraction patterns), but unlike the metallic character of $\mathrm{NdNi}{\mathrm{O}}_2$, the hydrogenated form is predicted to be a wide gap insulator, thus requiring doping to create a metallic or superconducting state, just like cuprates, but unlike unhydrogenated nickelates. While it is possible that hydrogen would be eventually desorbed, the calculation suggests that pristine $\mathrm{NdNi}{\mathrm{O}}_2$ is hydrogen stabilized. One must exercise caution with theories predicting new physics in pristine stoichiometric $\mathrm{NdNi}{\mathrm{O}}_2$ as it might be an unrealizable compound. Experimental examination of the composition of real $\mathrm{NdNi}{\mathrm{O}}_2$ superconductors and the effect of hydrogen on the superconductivity is called for.

• Received 5 July 2021
• Accepted 9 November 2021

DOI:https://doi.org/10.1103/PhysRevB.105.014106