Recently, a Verwey-type transition in the mixed-valence alkali sesquioxide ${\mathrm{Cs}}_4{\mathrm{O}}_6$ was deduced from the charge ordering of molecular peroxide ${\mathrm{O}}_2^{2-}$ and superoxide ${\mathrm{O}}_2^{-}$ 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 ${\mathrm{Rb}}_4{\mathrm{O}}_6$, a similar Verwey-type charge ordering transition is strongly linked to ${\mathrm{O}}_2^{-}$ orbital and spin dynamics. On cooling, a powder neutron diffraction experiment reveals a charge ordering and a cubic-to-tetragonal transition at $T_{\mathrm{CO}}=290$ K, which is followed by a further structural instability at $T_{\mathrm{s}}=92$ K that involves an additional reorientation of magnetic ${\mathrm{O}}_2^{-}$ 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 ${\mathrm{O}}_2^{-}$ units, which promotes the formation of a quantum spin state composed of weakly coupled spin dimers. These results reveal that as in $3d$ transition-metal compounds, also 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.

中文翻译：

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阴离子混合价化合物Rb4O6中连续的电荷和轨道有序跃迁驱动自旋二聚体基态

最近，混合价碱金属倍半氧化物的Verwey型过渡 ${\mathrm{Cs}}_4{\mathrm{Ø}}_6$ was deduced from the charge ordering of molecular peroxide ${\mathrm{O}}_2^{2-}$ and superoxide ${\mathrm{O}}_2^{-}$ 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 ${\mathrm{Rb}}_4{\mathrm{O}}_6$, a similar Verwey-type charge ordering transition is strongly linked to ${\mathrm{O}}_2^{-}$ orbital and spin dynamics. On cooling, a powder neutron diffraction experiment reveals a charge ordering and a cubic-to-tetragonal transition at $T_{\mathrm{CO}}=290$ K, which is followed by a further structural instability at $T_{\mathrm{s}}=92$ K that involves an additional reorientation of magnetic ${\mathrm{O}}_2^{-}$ anions. Magnetic resonance techniques supported by density functional theory computations suggest the emergence of a peculiar type of ${\pi }^{*}$磁激活的轨道定序 ${\mathrm{Ø}}_2^{-}$单位，这促进了由弱耦合的自旋二聚体组成的量子自旋态的形成。这些结果表明$3d$ 过渡金属化合物 ${\pi }^{*}$ 开壳碱倍半氧化物会像Jahn-Teller型电子-晶格耦合和Kugel-Khomskii型超交换之间的相互作用决定轨道有序性和磁性基态。