From Oxidized PrNi_0.9Al_0.1O₃ to Reduced PrNi_0.9Al_0.1O_2+δ Perovskite Nickelates: Stabilization of Infinite-Layer...

Recently, a new class of high-temperature superconductors, RNiO₂ (where R represents rare-earth elements) with infinite-layer (IL) structure, has been identified. They possess the same structural framework as the renowned high-T_c cuprates but with nickel replacing copper as the central element. In this study, we successfully synthesized infinite-layer samples of PrNi_0.9Al_0.1O_2+δ in the bulk polycrystalline form through topotactic reduction of the PrNi_0.9Al_0.1O₃ orthorhombic perovskite, via treatment with CaH₂. The incorporation of aluminum at the octahedral sites promotes the stabilization of bulk derivatives of the infinite-layer structure since unreduced [AlO₆] 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 RNiO₃ to RNiO₂─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 PrNi_0.9Al_0.1O_2.10H_0.16. 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 Ni³⁺ to the Ni⁺ oxidation state, aligning with the crystallochemical evidence.