Correlation-driven metal-insulator transition in unconventional magnetic metal superoxides

Abstract

Using first-principles electronic structure calculations, we have extensively studied the electronic and magnetic properties of alkali sodium superoxide (NaO2) in comparison with that of potassium superoxide (KO2) both at high and low temperatures. These properties of these superoxides are governed by the unpaired electron donated by the alkali atoms Na and K to the O atoms forming dimers. This unpaired electron is the source of orbital fluctuations in the O-π* manifold for both cases. In order to reduce this orbital fluctuation, both go through several structural phase transitions. In these plethora of structures, the O2- dimers undergo rotation, leading to a complex linking of its orbital degrees of freedom with its spin degrees of freedom. Hence the magnetic properties are found to be controlled by this unpaired electron vary as the orientations of these O2 - dimers change. Due to the change in the orientations of O2- dimers, the alkali ion cages around the O2 - dimers change from square in the pyrite phase to rhombus and rectangle for the orthorhombic phase for NaO2 and square in the tetragonal phase to parallelogram in the monoclinic phase for KO2 on the plane cutting through the dimers. The band structures of NaO2 in the low-temperature orthorhombic phase and KO2 in the monoclinic phase show that the lifting of degeneracy in the O-π* manifold is due to the redefined electrostatic interaction between the K/Na cages and the O2 - dimers. This, inaddition to electron correlation among the localized O-π* electrons, establishes complete orbital ordering (OO) in turn drives metal-insulator transition (MIT) in both the systems. Furthermore, K doping for Na in NaO2 also results in correlation-induced MIT, predicted to take place at a temperature higher than that in NaO2. This opens up the possibility of MIT in Rb/Cs-doped NaO2 at even higher temperatures.