Magnetic structure of perovskite-derived LaNiO_3-δ nickelate with ordered oxygen vacancies

Rare-earth nickelates perovskites (RNiO₃, with R=rare earth) are a family of compounds exhibiting temperature-dependent metal-insulator transitions with octahedral breathing distortion, while LaNiO₃ is an exception and lacks this distortion. Instead, oxygen-deficient LaNiO_3-δ exhibits electronic/magnetic transitions with varying oxygen content δ owing to the reducibility of Ni³⁺. Specifically, a metal-semiconductor-insulator transition occurs concurrently with a paramagnetic-ferromagnetic-antiferromagnetic transition in bulk materials as δ varies [1]. Here, we explain the electronic structure of LaNiO_2.5 and LaNiO_2.75 and their stable magnetic orders using first-principles calculations. We find square planar NiO₄ units formed by ordered oxygen vacancies are electronically and magnetically inactive, which along with changes in the NiO₆ connectivity, govern the phase transitions. Specifically, we discovered a columnar-type breathing distortion in LaNiO_2.75 with zigzag-type magnetic order. We conclude with model interpretations of the non-equilibrium states reported for LaNiO_2.5 and LaNiO₃, and their connection to experimental observations.

[1] Sanchez et al. Phys. Rev. B 54, 574–578 (1996)