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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.

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          Journal
          27 November 2019
          Article
          1911.12049

          http://arxiv.org/licenses/nonexclusive-distrib/1.0/

          Custom metadata
          cond-mat.str-el

          Condensed matter

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