Bulk <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>NdNi</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math> is thermodynamically unstable with respect to decomposition while hydrogenation reduces the instability and transforms it from metal to insulator

Abstract

The quest for a Ni-based oxide analog to cuprate ${\mathrm{Cu}}^{2+}({d}^{9})$ superconductors was long known to require a reduced form of ${\mathrm{Ni}}^{1+}({d}^{9})$ 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 $\frac{1}{2}[{\mathrm{Nd}}_{2}{\mathrm{O}}_{3}+\mathrm{NiO}+\mathrm{Ni}]$ 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 $\mathrm{NdNi}{\mathrm{O}}_{2}$ 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.

Topics & Concepts