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Abstract:
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 pi(*) 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.
