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

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### Abstract

Recently, a Verwey-type transition in the mixed-valence alkali sesquioxide Cs$$_4$$O$$_6$$ was deduced from the charge ordering of molecular peroxide O$$_2^{2-}$$ 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 Rb$$_4$$O$$_6$$ 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 $$T_{\rm CO}=290$$ 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 theory computations suggest the emergence of a peculiar type of $$\pi^*$$-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 similarly as in 3$$d$$ transition metal compounds, also in 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.

### Author and article information

###### Journal
27 November 2019
###### Article
1911.12049