Influence of O2 molecular orientation on p-orbital ordering and exchange pathways in Cs4O6

The coupling of charge, lattice, orbital, and spin degrees of freedom was studied in a p-electron mixed-valence Cs₄O₆ compound by the electron paramagnetic resonance (EPR) and ¹³³Cs nuclear magnetic resonance (NMR). The dramatic differences in the evolution of the EPR and ¹³³Cs NMR spectra measured under different cooling protocols reveal two competing low-temperature phases: quenched high-temperature cubic and the low-temperature low-symmetry phases, respectively. They differ in the orientation of O₂^- anion axes, the ordering of π^* molecular orbitals, and the superexchange interactions through the Cs⁺ bridges. The transformation between the two phases involves large amplitude reorientations of O₂ groups and is extremely sluggish, thus explaining the coexistence of both phases and the deviations of the magnetic susceptibility from the simple Curie-Weiss dependence at low temperatures.