Critical Role of Hydrogen

The newly discovered nickelate superconductors so far only exist in epitaxial thin films synthesized via topotactic reaction with metal hydrides¹. This method changes the nickelates from the perovskite to the infinite-layer structure by deintercalation of apical oxygens^1-3. Such chemical reaction may introduce hydrogen (H) influencing the physical properties of the end materials^4-9. Unfortunately, H is insensitive to most of the characterization techniques and is difficult to detect due to its light weight. Here, in the optimally doped Nd_0.8Sr_0.2NiO₂H epitaxial film, secondary-ion mass spectroscopy reveals abundant H existing in the form of Nd_0.8Sr_0.2NiO₂H_<em>x</em> (<em>x</em> = 0.2 ~ 0.5). Remarkably, zero resistivity is found within a very narrow H doping window of 0.22 ≤ x ≤ 0.28 showing unequivocally the critical role of H in superconductivity. Resonant inelastic X-ray scattering demonstrates the existence of itinerant interstitial <em>s</em> (IIS) orbital originating from apical oxygen deintercalation. Density function theory calculations show that electronegative H^– occupies the apical oxygen sites annihilating IIS orbitals, reducing the IIS – Ni 3<em>d</em> orbital hybridization. This leads the electronic structure of H-doped Nd_0.8Sr_0.2NiO₂H_<em>x</em> to be more two-dimensional-like, which might be relevant for the observed superconductivity. We highlight that H is an important ingredient for the superconductivity in epitaxial infinite-layer nickelates.