Elusive Valence Transition in Mixed-Valence Sesquioxide Cs4O6
Ross H. Colman, H. Esma Okur, Winfried Kockelmann, Craig Brown, Annette Sans, Claudia Felser, Martin Jansen, Kosmas Prassides
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 O22- 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.
, Okur, H.
, Kockelmann, W.
, Brown, C.
, Sans, A.
, Felser, C.
, Jansen, M.
and Prassides, K.
Elusive Valence Transition in Mixed-Valence Sesquioxide Cs<sub>4</sub>O<sub>6</sub>, Inorganic Chemistry, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=928484
(Accessed November 30, 2023)