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Recently, a Verwey-type transition in the mixed-valence alkali sesquioxideCs$_4$O$_6$ was deduced from the charge ordering of molecular peroxideO$_2^{2-}$ and superoxide O$_2^-$ anions accompanied by the structuraltransformation and a dramatic change in electronic conductivity [Adler et al,Sci. Adv 4, eaap7581 (2018)]. Here, we report that in the sister compoundRb$_4$O$_6$ a similar Verwey-type charge ordering transition is strongly linkedto O$_2^-$ orbital and spin dynamics. On cooling, a powder neutron diffractionexperiment reveals a charge ordering and a cubic-to-tetragonal transition at$T_{\rm CO}=290$ K, which is followed by a further structural instability at$T_{\rm s}=92$ K that involves an additional reorientation of magnetic O$_2^-$anions. Magnetic resonance techniques supported by density functional theorycomputations suggest the emergence of a peculiar type of $\pi^*$-orbitalordering of the magnetically active O$_2^-$ units, which promotes the formationof a quantum spin state composed of weakly coupled spin dimers. These resultsreveal that similarly as in 3$d$ transition metal compounds, also in in the$\pi^*$ open-shell alkali sesquioxides the interplay between Jahn-Teller-likeelectron-lattice coupling and Kugel-Khomskii-type superexchange determines thenature of orbital ordering and the magnetic ground state.

Spin-dimer ground state driven by consecutive charge and orbital ordering transitions in the anionic mixed-valence compound Rb$_4$O$_6$