Stabilization of Infinite-Layer NdNi1–xAlxO2 Nickelates Containing Monovalent Ni as Bulk Polycrystalline Materials

The series of RNiO₂ (where R is a rare-earth element) oxides with infinite-layer structure constitutes a novel family of high-temperature superconductors, with the same structural framework as the well-known high-T_c cuprates but with Ni instead of Cu for the covalent matrix. Despite these similarities, superconductivity has only been described in nickelates in thin films, but its origin remains controversial, being associated either with a reduction of Ni or with the stress effect in the monolayer. In the present work, we have successfully synthesized infinite-layer bulk samples with nominal formulas NdNiO₂ and NdNi_0.9Al_0.1O_2+δ, by topotactic reduction from the corresponding “oxidized” perovskites with NdNi_1–xAl_xO₃ (x = 0, 0.1) stoichiometry. We show that the substitution of 10% Al at the octahedral Ni positions contributes to the stabilization of the infinite-layer structure. Indeed, the topotactic removal of axial oxygen atoms on the [NiO₆] octahedra leads to a chemical reduction of Ni³⁺ to Ni⁺ and thus, in the absence of unreducible Al atoms, to structural instabilities. Synchrotron X-ray diffraction (SXRD) data, collected after treatment at increasing temperatures and therefore increasingly reducing conditions, permitted us to unveil the structural evolution upon oxygen removal. Additional neutron diffraction measurements allowed the axial oxygen content to be assessed and revealed an almost monovalent oxidation state for Ni in the Al-containing sample. In addition, the neutron data evidenced the absence of occluded, long-range ordered hydrogen in the crystal structure, consistent with observations on LaNiO₂ materials. Spectroscopic results from X-ray absorption spectroscopy (XAS) show that Ni ions are indeed reduced to the Ni⁺ oxidation state, in agreement with the crystallochemical data of NdNiO₂. The local atomic structure around the Ni absorber was evaluated using the EXAFS technique and showed that Al doping enhances the rigidity of the Ni–O bonds and medium-range interactions within the Ni–Nd sublattice. Magnetic measurements could not provide evidence of superconductivity, as susceptibility is masked by the presence of tiny amounts of Ni metal.