Intermediate ground states of hydrogen in LaNiO2 : A first-principles study

Infinite-layer nickelate superconductors ${R\mathrm{Ni}\mathrm{O}}_{2}$ $(R=\mathrm{La}, \mathrm{Nd}, \mathrm{Pr})$ are synthesized via chemical reduction using the ${\mathrm{CaH}}_{2}$ agent, and the unintentional introduction of H dopant during the reduction process may affect ${R\mathrm{Ni}\mathrm{O}}_{2}$ quality and ultimately determine their electronic properties. However, the ground-state configurations and electronic structures of ${R\mathrm{Ni}\mathrm{O}}_{2}$ under varying H concentrations (\ensuremath{\delta}) remain unclear. In this work, we employ the cluster-expansion method and first-principles calculations to examine the intermediate ground-state structures, electronic structures, and magnetic properties of ${\mathrm{LaNiO}}_{2}{\mathrm{H}}_{\ensuremath{\delta}}$ $(0<\ensuremath{\delta}<1)$. Interestingly, our results show that H always occupies apical oxygen vacancies, forming ordered H-Ni-H chains along the out-of-plane direction but with different \ensuremath{\delta}-dependent patterns along the in-plane direction. Notably, the interstitial $s$-like orbital in the ${\mathrm{LaNiO}}_{2}$ host is annihilated by H and the filling of the Ni $3d$ bands, especially the $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and $3{d}_{{z}^{2}}$ orbitals, is greatly altered. With H doping, both enhanced magnetic stability of G-type antiferromagnetic states and simultaneous metal-to-insulator transition occur. Our study demonstrates the remarkable doping impact of H in ${\mathrm{LaNiO}}_{2}$, allowing us to gain deeper insight into the structure and superconductivity of infinite-layer nickelates.