Quenching of magnetism in NaO2 due to electrostatic interaction induced partial orbital ordering.

Abstract

The transport properties of sodium superoxide (NaO2) are governed by the transfer of charge between O2- complexes. Although it goes through a plethora of structural phase transitions, its electronic and magnetic ground state remains shrouded in mystery. In this work, we perform first-principles density functional theory (DFT) calculations to understand the relationship between electronic structure and the reason for the non-observation of an antiferromagnetic (AFM) ground state in NaO2 vis-a-vis in KO2. In the cubic phase, uniform < Na-O-Na bond angles result in high symmetry and hence degeneracy in the O-2p orbitals. The freely rotating O2- molecules result in orbital degeneracy and hence paramagnetism at room temperature. Although the degeneracy between the bonding and anti-bonding orbitals of O2 dimers is lifted in the pyrite phase, the degeneracy between σ (σ*) and π (π*) states is still maintained and hence orbital degeneracy is partially lifted as the dimers are restricted to four directions now. The O-π* states are localized in such a manner that results in a substantial magnetic moment in the π*-orbital. The < O-Na-O bond angle (= 180°) in the c-axis facilitates a superexchange mechanism and thereby the system should be AFM in the pyrite phase. In the marcasite phase, the O-atoms are aligned parallel in alternative planes. The preservation of degeneracy among the two π* orbitals leading to only long-range orbital ordering negates any chance of quasi-one-dimensional AFM spin chains in NaO2. The difference in magnetic ground states of NaO2 and KO2 arises due to the difference in the electrostatic repulsion between electrons of Na+ and K+ ions with the O2- dimers.