High Hydrostatic Pressure (0.5–2.5 GPa) Synthesis and Properties of Bulk “<i>nnx</i>” Rare-Earth Nickel Oxides (<i>R_n</i>Ni<i>_n</i>O<i>_x</i>)*

We have undertaken rare-earth and alkaline-earth substitutions in known nickel oxide phases, and a search for new such phases, using a hot press. We prepared starting materials both by ambient solid-state reaction (650°C–975°C) and by using high-oxygen-pressure (150–200 bar), high-temperature (T ≈ 1000°C) syntheses, producing either nominal-composition mixed-phase RNiO_x materials, or the nearly-simple-perovskite RNiO₃ phases, respectively. We then subjected such parent materials [R = Pr, Nd, (La,Y)], as well as hole- and electron-doped varieties, to high quasi-hydrostatic pressures using a hot piston-cylinder apparatus (5–25 kbar, i.e., 0.5–2.5 GPa) at high temperatures (950°C–1050°C). The technique¹ can include oxidizers (KClO₄), or can be naturally reducing, the latter augmentable by hydrogen. We report product phases; initial syntheses have provided specimens with decreases in undesired starting phases. Our study of other nickelate phases, e.g., R₃Ni₃O₇ and infinite-layer RNiO₂, with this technique is ongoing, exploring both steric effects and hole- (Sr²⁺) and electron-doping (Ce⁴⁺) in targeted R_nNi_nO_x structures.
¹G. Démazeau et al., J. Solid State Chem. 3, 582 (1971).