Spin-dimer ground state driven by consecutive charge and orbital ordering transitions in the anionic mixed-valence compound Rb4O6

A comprehensive study of the molecular mixed-valence compound Rb${}_{4}$O${}_{6}$ led the authors to the discovery of a peculiar $\ensuremath{\pi}*$ orbital ordering transition of magnetic O${}_{2}^{\ensuremath{-}}$ anions within the charge ordered state, which promotes the formation of an orbital order-driven quantum spin state, composed of spin dimers. The experiments suggest that a delicate interplay between an electron-lattice coupling and Kugel-Khomskii-type superexchange in $\ensuremath{\pi}$-electron systems determines the magnetostructural properties. These results establish a fruitful analogy between $\ensuremath{\pi}$- and more conventional $3d$-mixed-valence systems and push the research of such systems into new directions to tackle the intertwining of various active degrees of freedom.