Abstract
Recently, a Verwey-type transition in the mixed-valence alkali sesquioxide was deduced from the charge ordering of molecular peroxide and superoxide 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 , a similar Verwey-type charge ordering transition is strongly linked to orbital and spin dynamics. On cooling, a powder neutron diffraction experiment reveals a charge ordering and a cubic-to-tetragonal transition at K, which is followed by a further structural instability at K that involves an additional reorientation of magnetic anions. Magnetic resonance techniques supported by density functional theory computations suggest the emergence of a peculiar type of -orbital ordering of the magnetically active units, which promotes the formation of a quantum spin state composed of weakly coupled spin dimers. These results reveal that as in transition-metal compounds, also in the 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
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