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

Recently, a Verwey-type transition in the mixed-valence alkali sesquioxide Cs₄O₆ was deduced from the charge ordering of molecular peroxide O²⁻ ₂ and superoxide 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 Rb₄O₆, a similar Verwey-type charge ordering transition is strongly linked to 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_CO = 290 K, which is followed by a further structural instability at T_s = 92 K that involves an additional reorientation of magnetic O⁻ ₂ anions. Magnetic resonance techniques supported by density functional theory computations suggest the emergence of a peculiar type of π*-orbital ordering of the magnetically active 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 π* 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.