6533b835fe1ef96bd129fced

RESEARCH PRODUCT

Exotic magnetism in the alkali sesquioxidesRb4O6andCs4O6

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Among the various alkali oxides the sesquioxides ${\text{Rb}}_{4}{\text{O}}_{6}$ and ${\text{Cs}}_{4}{\text{O}}_{6}$ are of special interest. Electronic-structure calculations using the local spin-density approximation predicted that ${\text{Rb}}_{4}{\text{O}}_{6}$ should be a half-metallic ferromagnet, which was later contradicted when an experimental investigation of the temperature-dependent magnetization of ${\text{Rb}}_{4}{\text{O}}_{6}$ showed a low-temperature magnetic transition and differences between zero-field-cooled and field-cooled measurements. Such behavior is known from spin glasses and frustrated systems. ${\text{Rb}}_{4}{\text{O}}_{6}$ and ${\text{Cs}}_{4}{\text{O}}_{6}$ comprise of two different types of dioxygen anions, the hyperoxide and the peroxide anions. The nonmagnetic peroxide anions do not contain unpaired electrons while the hyperoxide anions contain unpaired electrons in antibonding ${\ensuremath{\pi}}^{\ensuremath{\ast}}$ orbitals. High electron localization (narrow bands) suggests that electronic correlations are of major importance in these open-shell $p$-electron systems. Correlations and charge ordering due to the mixed valency render $p$-electron-based anionogenic magnetic order possible in the sesquioxides. In this work we present an experimental comparison of ${\text{Rb}}_{4}{\text{O}}_{6}$ and the related ${\text{Cs}}_{4}{\text{O}}_{6}$. The crystal structures are verified using powder x-ray diffraction. The mixed valency of both compounds is confirmed using Raman spectroscopy, and time-dependent magnetization experiments indicate that both compounds show magnetic frustration, a feature only previously known from $d$- and $f$-electron systems.