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

• Received 13 November 2019

DOI:https://doi.org/10.1103/PhysRevB.101.024419