Ambient pressure synthesis of unstable bulk phases of strongly correlated rare-earth nickelates

Abstract

Despite the outstanding electrical, electrochemical, and optical properties of rare-earth (R)-doped nickelates (RNiO₃), a bottleneck in its device applications is the need for high-pressure, typically in excess of 100 bars, to stabilize this phase during synthesis. To date, no known near-ambient pressure synthesis process exists for the synthesis of bulk RNiO₃ with ionic radii of R lower than that of Nd (such as Sm, Eu, and Gd) in the lanthanide series due to the increasing thermodynamic instability of Ni³⁺ cations at ambient pressures. In the present study, we report a set of conditions for the successful synthesis of bulk SmNiO₃ and NdNiO₃ through a sol–gel synthesis procedure followed by annealing at ambient pressure to stabilize Ni³⁺. Rietveld refinement analysis shows the composition of crystalline SmNiO₃ and NdNiO₃ phases to be as high as 50 wt% and 96 wt%, respectively. Consequently, sharp, well-defined insulator–metal transitions (IMTs) involving resistance changes of 2–3 orders of magnitude could be achieved, which is comparable to that seen in high-pressure synthesized samples reported in the literature.