Interplay between Spin , Charge , and Orbital Order with Molecular Orientation in Anionic p-Electron Systems

The interplay between spin, orbital, charge, and lattice degrees of freedom has been explored widely in strongly correlated transition metal (TM) compounds with partially filled d shells. However, similar phenomena are also observed in open shell p electron systems. For instance, there is some analogy between the properties of TM compounds with Jahn-Teller t2geg (Cu) or t2geg (Mn) electron configurations and alkali metal superoxides, AO2, with paramagnetic O2 ions that have a degenerate (π*) electron configuration. Here, the π* orbitals are antibonding molecular orbitals formed from O 2pπ atomic orbitals. A prototype system is CsO2, where, similar to the Cu compound KCuF3, a quasi-one-dimensional magnetic ordering was observed, which is driven by orbital ordering [1]. The orbital ordering involves a reorientation of the molecular anions, which is an important degree of freedom in molecular anionic p electron systems. The main focus of our work is on alkali sesquioxides, A4O6 ( A = Rb, Cs), which are anionic mixed valence compounds with two paramagnetic O2 anions and one diamagnetic O2 anion per formula unit and thus also feature the charge degree of freedom [2]. In addition, new insights into the low-dimensional physics of CsO2 were obtained recently.