Efforts to Synthesize Bulk Superconducting Infinite-Layer Nickelates: <i>R</i>₃Ni₃O₇ and Other Phases Prepared by Hydrogen Reduction of <i>R</i>NiO₃*

Recently, superconductivity in a nickelate was reported¹ in an apparently hole-doped RNiO₂ (“infinite-layer”) film (with R ≈ Nd_0.8Sr_0.2), after CaH₂ reduction. We have prepared both parent and electron- and hole-doped specimens of the bulk, nearly simple-perovskite RNiO₃ structures: R_1–xA_xNiO₃ [R = Nd, Pr, (La,Y); A = Sr, Ce, Th] using high oxygen pressure (200 bar) at high temperatures (T ≈1000°C). We reduced these phases in several ways in an effort to produce bulk infinite-layer materials. For reduction in 5% H₂ in Ar, and for T > 400°C, we find excess reduction to pure metal(s) or elemental oxides. But for temperatures in the range 300–375°C (reduction times of hours), we have synthesized a number of nickelate structures in bulk, dense form, suitable for transport and other measurements. One product is the “337” structure: R₃Ni₃O₇. X-ray diffraction, electrical resistivity, magnetic, and other data indicate that n-doping (Ce⁴⁺) induces structural and electronic changes in the 337 material, and thus may provide an alternative path to “Ni²⁺/Ni¹⁺” superconductivity. We also report other product phases and, thus, a rich reduced-nickelate phase diagram.
¹D. Li et al., Nature 572, 624 (2019).