Despite enormous experimental and theoretical efforts, obtaining generally accepted conclusions regarding the intrinsic magnetic and electronic properties of superconducting nickelates remains exceptionally challenging. Experiments show a significant degree of uncertainty, indicating hidden factors in the synthesized films, which call for further investigations. One of those hidden factors is the possibility of intercalating hydrogen during the chemical reduction process from $\mathrm{Nd}\left(\mathrm{La}\right){\mathrm{NiO}}_3$ to $\mathrm{Nd}\left(\mathrm{La}\right){\mathrm{NiO}}_2$ using ${\mathrm{CaH}}_2$. While hydrogen has been detected in experimental samples, not much is known about its distribution through the crystal and its influence on the electronic environment. Here, we show the tendency toward the formation of one-dimensional hydrogen chains in infinite-layers ${\mathrm{LaNiO}}_2$ superconductors using density-functional theory supplemented by dynamical mean-field theory. The formation of such hydrogen chains induces a coexistence of different oxidation states of Ni and competing magnetic phases, and possibly explains the recently observed charge order states in nickelate superconductors. Furthermore, it contributes to the difficulty of synthesizing homogeneous nickelates and determining their ground states. The smoking gun to detect excess hydrogen in nickelates is the flat phonon modes, which are infrared active and quite insensitive to the exact arrangement of the H atoms.

• Received 23 August 2022
• Revised 22 February 2023
• Accepted 22 March 2023

DOI:https://doi.org/10.1103/PhysRevB.107.165116