Bulk NdNiO2 is thermodynamically unstable with respect to decomposition while hydrogenation reduces the instability and transforms it from metal to insulator

The quest for a Ni-based oxide analog to cuprate Cu2+ (d(9)) superconductors was long known to require a reduced form of Ni1+ (d(9)) as in A(3+)Ni(1+)O(2), being an extremely oxygen-poor form of the usual A(3+)Ni(3+)O(3) compound. Through CaH2 chemical reduction of a parent R3+Ni3+O3 perovskite form, superconductivity was recently achieved in Sr-doped NdNiO2 on a SrTiO3 substrate. Using density functional theory (DFT) calculations, we find that stoichiometric NdNiO2 is significantly unstable with respect to decomposition into 1/2[Nd2O3 + NiO + Ni] 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 NdNiO2 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 NdNiO2. We find that H can be incorporated exothermically, i.e., spontaneously in NdNiO2, even from H-2 gas. In the concentrated limit, such impurities can result in the formation of a hydride compound, NdNiO2H, which has significantly reduced instability relative to hydrogen-free NdNiO2 (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 NdNiO2, 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 NdNiO2 is hydrogen stabilized. One must exercise caution with theories predicting new physics in pristine stoichiometric NdNiO2 as it might be an unrealizable compound. Experimental examination of the composition of real NdNiO2 superconductors and the effect of hydrogen on the superconductivity is called for.

Automated summary

The instability of stoichiometric NdNiO2 and the effect of hydrogen impurities on its properties are studied. Hydrogenation is found to reduce the instability of NdNiO2, and the hydrogenated form is predicted to be a wide gap insulator.