From Oxidized PrNiAlO to Reduced PrNiAlO Perovskite Nickelates: Stabilization of Infinite-Layer Specimens with Monovalent Ni in the Bulk Polycrystalline Form.

Recently, a new class of high-temperature superconductors, NiO (where represents rare-earth elements) with infinite-layer (IL) structure, has been identified. They possess the same structural framework as the renowned high- cuprates but with nickel replacing copper as the central element. In this study, we successfully synthesized infinite-layer samples of PrNiAlO in the bulk polycrystalline form through topotactic reduction of the PrNiAlO orthorhombic perovskite, via treatment with CaH. The incorporation of aluminum at the octahedral sites promotes the stabilization of bulk derivatives of the infinite-layer structure since unreduced [AlO] octahedra keep the layers together and prevent their decomposition. The lack of superconductivity in bulk samples has been a subject of intense debate in recent literature. One major theoretical question concerns whether hydrogen becomes incorporated into the structure during the reduction from RNiO to RNiO─as suggested by theory. Here, we present neutron powder diffraction data demonstrating that hydride ions indeed reside within the IL lattice in samples of stoichiometry PrNiAlOH. Additional crystallographic analyses were carried out using temperature-dependent synchrotron X-ray diffraction on both reduced and oxidized phases. Furthermore, spectroscopic analysis via XAS and magnetometry confirms the reduction of Ni to the Ni oxidation state, aligning with the crystallochemical evidence.