Elusive Valence Transition in Mixed-Valence Sesquioxide Cs4O6

Published

November 4, 2019

Author(s)

Ross H. Colman, H. Esma Okur, Winfried Kockelmann, Craig Brown, Annette Sans, Claudia Felser, Martin Jansen, Kosmas Prassides

Abstract

Cs₄O₆ is a mixed valence molecular oxide with a cubic structure, comprising valency-delocalized O₂ 4/3- units and with properties highly sensitive to cooling protocols. Here we use neutron powder diffraction to authenticate that, while upon deep quenching the cubic phase is kinetically arrested down to cryogenic temperatures, ultraslow cooling results in an incomplete structural transition to a contracted tetragonal phase. Two dioxygen anions in a 1:2 ratio are identified, providing evidence that the transition is accompanied by charge and orbital order and stabilizes a Robin-Day Class-II mixed valency state, comprising O₂^2- and O₂^- anions. The phenomenology of the phase change is consistent with that of a martensitic transition. The response of the low-temperature phase assemblage to heating is complex involving a series of successive interconversions between the co-existing phases. Notably a broad interconversion plateau is present near 260 K signifying a reentrant kinetic arrest of the tetragonal phase on heating due to the combined effects of increased steric hindrance for molecular rotation and melting change and orbital order. The geometrically frustrated pyrochlore lattice adopted by the paramagnetic S=1/2 O₂^-1 units provides an intimate link between crystal and magnetic properties of charge ordered Cs₄O₆, naturally accounting for the absence of magnetic order.

Citation

Inorganic Chemistry

Volume

58

Issue

21

Pub Type

Journals

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Keywords

mixed valence oxide, neutron diffraction, phase transition

Citation

Colman, R. , Okur, H. , Kockelmann, W. , Brown, C. , Sans, A. , Felser, C. , Jansen, M. and Prassides, K. (2019), Elusive Valence Transition in Mixed-Valence Sesquioxide Cs₄O₆, Inorganic Chemistry, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=928484 (Accessed October 25, 2025)