2024/12/27 by D. M. Bhagat, Bhagat, Devang, Ranga Teja Pidathala +3
Materials Science · #Thermal Expansion and Ionic Conductivity #Ferroelectric and Piezoelectric Materials #Magnetic and transport properties of perovskites and related materials
paper · pdf · doi:10.48550/arxiv.2412.19700
Controlled introduction of oxygen vacancies offers an effective route to induce metal-to-insulator transition in strongly correlated rare-earth nickelates (RNiO3) at room temperature. However, the role played by the rare-earth cations on the structure, thermodynamic stability, and electronic properties of oxygen-deficient nickelates remains unclear. Here, we employ density functional theory calculations with Hubbard corrections (DFT + U) to investigate the whole family of RNiO2.5 (R = Pr-Er) compounds in two commonly observed oxygen-vacancy ordered configurations, namely brownmillerite, and square planar. We find that square planar polymorph is always more stable (∼0.4 eV/u.f) than the brownmillerite for all rare-earth cations, owing to the exceedingly low volumetric strains (< 1%). Formation energy of RNiO2.5 gradually increases with decreasing size of R owing to stronger Ni-O covalent interactions in pristine RNiO3 with small R3+ cations. This necessitates more oxygen-lean environments for synthesis of RNiO2.5 with smaller R3+ cations. Analysis of the density of states and band structures reveals that electronic structure of RNiO2.5 is governed by two factors: (a) localization of electron on NiO6 octahedra yielding a Mott insulating state with strong correlations as Ni eg is half filled, and (b) crystal field splitting in the NiO4 tetrahedra/square planar polyhedra. Brownmillerite RNiO2.5 is metallic, while square planar RNiO2.5 is an insulator with a predicted gap of ∼ 0.2-0.3 eV, depending on the R3+ cation. Crystal orbital Hamilton population (COHP) analysis indicates that the Ni-O bond belonging to square-planar NiO4 polyhedra exhibit much greater covalent character than those in NiO6 octahedra in square planar RNiO2.5.