Exotic magnetism in the alkali sesquoxides Rb4O6 and Cs4O6

TL;DR

This paper investigates the exotic magnetic properties of alkali sesquioxides Rb4O6 and Cs4O6, revealing magnetic frustration and charge ordering driven by p-electron correlations, contrasting previous theoretical predictions.

Contribution

It provides the first experimental comparison of Rb4O6 and Cs4O6, confirming their mixed valency, magnetic frustration, and the significance of electronic correlations in their magnetic behavior.

Findings

Rb4O6 and Cs4O6 exhibit magnetic frustration.

Charge ordering confirmed via Raman spectroscopy.

Experimental results contrast with initial theoretical predictions.

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 (ZFC) and field-cooled (FC) measurements. Such behavior is known from spin glasses and frustrated systems. Rb4O6 and Cs4O6 comprise 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.