Knaflic, T.; Jeglic, P.; Komelj, M.; Zorko, A.; Biswas, P. K.; Ponomaryov, A. N.; Zvyagin, S. A.; Reehuis, M.; Hoser, A.; Geiss, M.; Janek, J.; Adler, P.; Felser, C.; Jansen, M.; Arcon, D.: Spin-dimer ground state driven by consecutive charge and orbital ordering transitions in the anionic mixed-valence compound Rb4O6. Physical Review B 101 (2020), p. 024419/1-13

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 anda cubic-to-tetragonal transition at TCO = 290 K, which is followed by a further structural instability at Ts = 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.