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

TL;DR

This study uncovers a charge and orbital ordering transition in Rb$_4$O$_6$ that leads to a spin-dimer ground state, highlighting complex electron-lattice interactions in mixed-valence alkali sesquioxides.

Contribution

It demonstrates the link between charge, orbital, and spin dynamics in Rb$_4$O$_6$, revealing a novel spin-dimer ground state driven by consecutive ordering transitions.

Findings

Charge ordering occurs at 290 K with a cubic-to-tetragonal transition.

Additional structural reorientation at 92 K involves magnetic O$_2^-$ units.

Emergence of a quantum spin state of weakly coupled dimers.

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.