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

T. Knaflič; P. Jeglič; M. Komelj; A. Zorko; P. K. Biswas; A. N. Ponomaryov; S. A. Zvyagin; M. Reehuis; A. Hoser; M. Geiß; J. Janek; P. Adler; C. Felser; M. Jansen; D. Arčon

doi:10.1103/PhysRevB.101.024419

paper

Spin-dimer ground state driven by consecutive charge and orbital ordering transitions in the anionic mixed-valence compound Rb $_{4}$ O $_{6}$

arXiv:1911.12049 · doi:10.1103/PhysRevB.101.024419

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 $T_{CO} = 290$ 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 similarly as in 3 $d$ transition metal compounds, also in 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.

Spin-dimer ground state driven by consecutive charge and orbital ordering transitions in the anionic mixed-valence compound Rb4​O6​

Abstract

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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 $_{6}$