Exotic magnetism in the alkali sesquioxides Rb4O6 and Cs4O6

Among the various alkali oxides the sesquioxides Rb₄O₆ and Cs₄O₆ are of special interest. Electronic-structure calculations using the local spin-density approximation predicted that Rb₄O₆ should be a half-metallic ferromagnet, which was later contradicted when an experimental investigation of the temperature-dependent magnetization of Rb₄O₆ 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. Rb₄O₆ and Cs₄O₆ 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 π^∗ 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 Rb₄O₆ and the related Cs₄O₆ . 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.