Evidence of hydrogen content and monovalent Ni oxidation state in non-superconducting bulk anchored infinite-layer nickelates

Abstract

Since superconductivity was first reported in nickelate thin films, many studies have been published about this family of materials, and different hypotheses have been proposed for explaining the mechanisms and structural dependence. Here, we report the synthesis of anchored infinite-layer LaNi0.9Al0.1O2.1 and its hole-doped derivatives by topotactic reduction from La1−xSrxNi0.9Al0.1O3 rhombohedral perovskites. LaNiO2 derivatives constitute a new family of high-temperature superconductors, with the same structure as high-Tc cuprates but based on nickel, only showing superconductivity in thin films for now. We describe a strategy to stabilize LaNiO2 derivatives in bulk: the presence of Al at the octahedral sites helps to stabilize/anchor the infinite-layer structure. The reasons for the bulk being non-superconductors are hotly debated in the literature. An important question is whether there is some hydrogen incorporated into the structure during the reduction process from LaNiO3 to LaNiO2, predicted theoretically but not reported experimentally. Our neutron powder diffraction data show that, indeed, hydrogen occupies the centers of the Ni–O squares, and spectroscopic evidence from EELS and XAS suggests that Ni is reduced to the Ni+ oxidation state, consistent with the crystallochemical data.