Metal-insulator transition in yttrium- and neodymium-doped lanthanum nickelates

Abstract

Discovery of high-temperature superconductivity (SC) in (La,Ba)₂CuO₄ opened many lines of investigation into similar perovskite-related compounds, such as the square-planar “infinite-layer” superconductor, Sr₁₋ₓRₓCuO₂ (R = rare-earth element). Until recently, SC had not yet been found in any nickelates, including the isostructural infinite-layer RNiO₂. Recently, chemical reduction of RNiO₃/STO thin films (R =Nd₀.₈Sr₀.₂) using CaH₂ into the RNiO₂ infinite-layer phase was found to have a zero-resistivity measurement below T [subscript C]= 9 − 15 K³. This study re-invigorated the SC community while leaving many open questions, mainly regarding the effect on superconductivity of oxygen defects (vacancies and interstitials, e.g., apical) and of strain (due to cation substitutions and substrate-film mismatch). The study also re-prompted the search for SC in the bulk material. The parent RNiO₃ bulk compounds are known to have a rich structural phase diagram and a metal-insulator transition T [subscript M−I] that varies based on the rare-earth element. This work investigates steric and doping effects in bulk nickelates La₁₋ₓRₓNiO₃ (R = Y, Nd) and both develops and tests the prediction of a universal T → 0 Metal-Insulator (M-I) transition for all (La, R)NiO₃ nearly-perovskite materials. Polycrystalline bulk samples were prepared using a sol-gel precursor that was then subject to high oxygen pressure (150-200 bar), high temperature (950°C − 1050°C) conditions, or high quasi-hydrostatic pressures using a hot piston-cylinder apparatus (2.0 − 2.5 GPa) with an oxidizer at high temperatures (900°C − 1000°C) for La₁₋ₓYₓNiO₃ (x > 0.2). These compounds can be reduced several ways to produce the infinite-layer phase, as well as some other RₙNiₙOᵧ phases (e.g., “337”). Similarly, thin films of Nd₁₋ₓAₓNiO₃ (A = La, Sr) deposited using Pulsed Laser Deposition can be reduced to compare the effects in the bulk to that in thin films. The synthesized samples are studied using x-ray diffraction, electrical resistivity, and other techniques to correlate structural, transport, and magnetic properties.