nickelate

Rare-earth nickelates perovskites (RNiO₃, with R=rare earth) exhibit a plethora of electronic and magnetic phase transitions owing to the orbital structure of its trivalent Ni cations. Unlike other RNiO₃ compounds, LaNiO₃ is the only compound lacking a thermal metal-insulator transition. Nonetheless, oxygen-deficient LaNiO_3-δ exhibits interesting electronic/magnetic transitions with varying oxygen content δ owing to the reducibility of Ni³⁺. Specifically, the metal-semiconductor-insulator transition occurs concurrently with paramagnetic (PM)-ferromagnetic (FM)-antiferromagnetic (AFM) transition in bulk materials as δ varies [1]. Here, we explain the LaNiO_3-δ phase transitions and their dependencies upon (ordered) oxygen vacancy formation using first-principles calculations. We find oxygen vacancies form along the (110)_pc direction and transform NiO₆ octahedra to NiO₄ square planar units. The resultant NiO₄ unit is electronically and magnetically inactive, and this with the change in NiO₆ connectivity governs the phase transitions. We conclude with model interpretations of the transient states reported for LaNiO_2.5 and LaNiO₃, and their connection to experimental observations.

[1] R.D. Sanchez, M.T. Causa, A. Caneiro, A. B. Phys. Rev. B 1996, 54 (23), 574–578.