Investigation of Hydrogen Incorporations in Bulk Infinite-Layer Nickelates

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 are 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.