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Through CaH2 chemical reduction of a parent R3+Ni3+O3 perovskite form,superconductivity was recently achieved in Sr-doped NdNiO2 on SrTiO3 substrate.Using density functional theory (DFT) calculations, we find that stoichiometricNdNiO2 is significantly unstable with respect to decomposition into 1/2[Nd2O3 +NiO + Ni] with exothermic decomposition energy of +176 meV/atom, a considerablyhigher instability than that for common ternary oxides. This poses the questionif the stoichiometric NdNiO2 nickelate compound used extensively to model theelectronic band structure of Ni-based oxide analog to cuprates and found to bemetallic is the right model for this purpose. To examine this, we study via DFTthe role of the common H impurity expected to be present in the process ofchemical reduction needed to obtain NdNiO2. We find that H can be incorporatedexothermically, i.e., spontaneously in NdNiO2, even from H2 gas. In theconcentrated limit, such impurities can result in the formation of a hydridecompound NdNiO2H, which has significantly reduced instability relative tohydrogen-free NdNiO2. Interestingly, the hydrogenated form has a similarlattice constant as the pure form (leading to comparable XRD patterns), butunlike the metallic character of NdNiO2, the hydrogenated form is predicted tobe a wide gap insulator thus, requiring doping to create a metallic orsuperconducting state, just like cuprates, but unlike unhydrogenatednickelates. While it is possible that hydrogen would be eventually desorbed,the calculation suggests that pristine NdNiO2 is hydrogen-stabilized. One mustexercise caution with theories predicting new physics in pristinestoichiometric NdNiO2 as it might be an unrealizable compound. Experimentalexamination of the composition of real NdNiO2 superconductors and the effect ofhydrogen on the superconductivity is called for.

physical review. b./physical review. b

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