Magnetic structure of oxygen-deficient perovskite nickelates with ordered vacancies

The oxygen vacancy concentration in LaNiO3-delta nickelate perovskites affects magnetic interactions and longrange magnetic order through changes in local electronic configurations, crystal field splitting energies, and polyhedral arrangements. Here we use density functional theory calculations to examine the magnetic structure of LaNiO2.5 and LaNiO2.75 with structural ordered oxygen vacancies (OOV). These OOV phases exhibit columnar arrangements of NiO4 square planar units, which adopt low-spin Ni2+ (d(8)) configurations with nominally zero magnetic moment (S = 0), interconnected by NiO6 octahedral units. The magnetic structure of the OOV phases are governed by the flexible charge state of the NiO6 octahedral units, whose density and connectivity depends on the oxygen vacancy concentration. LaNiO2.5 is stable in an insulating A-type antiferromagnetic (AFM) phase derived from octahedral units comprising Ni2+ in AFM chains. LaNiO2.75 is a narrow-gap insulator with zigzagtype AFM order originating from weakly localized electrons in columnar breathing distortions to the NiO6 units. Our results suggest that nanoscale OOV phases within single-phase LaNiO3-delta crystals can account for its reported complex magnetic ground-state structure.