Investigation of hydrogen incorporations in bulk infinite-layer nickelates

TL;DR

This study investigates hydrogen incorporation during the topotactic reduction of LaNiO3 to LaNiO2, revealing hydrogen mainly resides at grain boundaries or secondary phases without affecting the primary crystal structure.

Contribution

It provides new insights into hydrogen behavior during the topotactic transformation of nickelates, showing hydrogen does not incorporate into the main lattice but remains at boundaries or secondary phases.

Findings

Hydrogen content increases over time during reduction.

Neutron diffraction shows no hydrogen in the LaNiO2 lattice.

Hydrogen is confined to grain boundaries or secondary phases.

Abstract

Infinite-layer (IL) nickelates are an emerging class of superconductors, where the Ni$^{1+}$ valence state in a square planar NiO$_2$ coordination can only be reached via topotactic reduction of the perovskite phase. However, this topotactic soft chemistry with hydrogenous reagents is still at a stage of rapid development, and there is a number of open issues, especially considering the possibility of hydrogen incorporation. Here we study the time dependence of the topotactic transformation of LaNiO$_3$ to LaNiO$_2$ for powder samples with x-ray diffraction and gas extraction techniques. While the hydrogen content of the powder increases with time, neutron diffraction shows no negative scattering of hydrogen in the LaNiO$_2$ crystal lattice. The extra hydrogen appears to be confined to grain boundaries or secondary-phase precipitates. The average crystal structure, and possibly also the physical properties, of the primary LaNiO$_2$ phase are therefore not noticeably affected by hydrogen residues created by the topotactic transformation.