The effect of octahedral distortions on the electronic properties and magnetic interactions in O3 NaTMO2 compounds (TM = Ti–Ni & Zr–Pd)

Abstract

The interplay between the coordination environment and magnetic properties in O3 layered sodium transition metal oxides (NaTMO₂) is a fascinating and complex problem. Through detailed and comprehensive density functional investigations on O3 NaTMO₂ compounds, we demonstrate that the TM ions in O3 NaMnO₂, NaFeO₂ and NaCoO₂ adopt a high spin state. Structurally, NaMnO₂ and NaPdO₂ undergo Jahn–Teller distortions while NaNbO₂ undergoes puckering distortion. Furthermore, in addition to Jahn–Teller distortion, NaPdO₂ exhibits charge disproportionation as it contains Pd²⁺, Pd³⁺ and Pd⁴⁺ species. These distortions stabilize the inter-plane ferromagnetism. Additionally, the inter-plane ferromagnetic coupling is stabilized by kinetic p–d exchange mechanism in undistorted NaCoO₂, NaNiO₂ and NaTcO₂. The intra-plane coupling in this family of compounds on the other hand was found to be generally weak. Only NaMnO₂, NaNiO₂ and NaTcO₂ are predicted to show bulk ferromagnetism with estimated Curie temperatures below ∼50 K.