Topotactic-hydrogen forms chains in $AB$O$_2$ nickelate superconductors

TL;DR

This study uses advanced computational methods to reveal that hydrogen chains form in nickelate superconductors, affecting their electronic and magnetic properties and complicating their synthesis and characterization.

Contribution

It demonstrates the tendency for hydrogen chain formation in LaNiO₂ superconductors and links this to charge order and magnetic phase coexistence, providing insights into synthesis challenges.

Findings

Hydrogen forms one-dimensional chains in LaNiO₂.

Hydrogen chains induce mixed Ni oxidation states and magnetic phases.

Flat infrared-active phonon modes can detect excess hydrogen.

Abstract

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 Nd(La)NiO$_3$ to Nd(La)NiO$_2$ using 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 LaNiO$_2$ superconductors using density-functional theory (DFT) supplemented by dynamical mean-field theory (DMFT). 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 are flat phonon modes, which are infrared active and quite insensitive to the exact arrangement of the H atoms.