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Elusive Valence Transition in Mixed-Valence Sesquioxide Cs₄O₆.

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-valence 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 coexisting phases. Notably, a broad interconversion plateau is present near 260 K, signifying reentrant kinetic arrest of the tetragonal phase upon heating because of the combined effects of increased steric hindrance for molecular rotation and melting of charge and orbital order. The geometrically frustrated pyrochlore lattice adopted by the paramagnetic S = ¹/₂ O₂^- units provides an intimate link between the crystal and magnetic properties of charge-ordered Cs₄O₆, naturally accounting for the absence of magnetic order.

SUBMITTER: Colman RH

PROVIDER: S-EPMC7880551 | biostudies-literature | 2019 Nov

REPOSITORIES: biostudies-literature

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Publications

Elusive Valence Transition in Mixed-Valence Sesquioxide Cs4O6.

Colman Ross H RH Okur H Esma HE Kockelmann Winfried W Brown Craig M CM Sans Annette A Felser Claudia C Jansen Martin M Prassides Kosmas K

Inorganic chemistry 20191021 21

Cs4O6 is a mixed-valence molecular oxide with a cubic structure, comprising valency-delocalized O24/3- 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 r ...[more]

PMID: 31633914

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Project description:The intriguing functional nature of ceramics containing rare earth sesquioxide (RES) is associated with the type of polymorphic structure they crystallize into. They prefer to be in the cubic, monoclinic or hexagonal structure in the increasing order of cation size, RRE. Since the functional properties of these ceramics varies with RRE, temperature and pressure, a systematic investigation delineating the cation size effect is indispensable. In the present work we report the structural stability and compressibility behaviour of the RES ceramics, (Eu1-xLax)2O3, of RESs with dissimilar structure and significant difference in cationic radii. The selected compositions of (Eu1-xLax)2O3 have been studied using the in-situ high pressure synchrotron X-ray diffraction and the structural parameters obtained through Rietveld refinement. The cubic structure, which is stable for 0.95 Å [Formula: see text] RRE [Formula: see text] Å at ambient temperature and pressure (ATP), prefers a cubic to hexagonal transition at high pressures. The biphasic region of cubic and monoclinic structure, which is stable for 0.98 Å [Formula: see text] RRE [Formula: see text] Å at ATP, prefers a cubic/monoclinic to hexagonal transition at high pressures. Further, in the biphasic region of monoclinic and hexagonal structure, observed for 1.025 Å [Formula: see text]RRE [Formula: see text] Å, the monoclinic phase is found to be progressing towards the hexagonal phase with increasing pressure. The pure hexagonal phase obtained for 1.055 Å [Formula: see text] RRE [Formula: see text] 1.10 Å is found to be structurally stable at high pressures. The bulk moduli are obtained from the Birch-Murnaghan equation of state fit to the compressibility data and its dependance on the cation size is discussed. The microstrain induced by the difference in cation size causes an internal pressure in the crystal structure leading to a reduction in the bulk modulus of [Formula: see text] and 0.6. A pressure-concentration (P-x) phase diagram upto a pressure of 25 GPa is constructed for (Eu1-xLax)2O3. This would provide an insight to the fundamental and technological aspects of these materials and the RESs in general.

Dataset Information

Elusive Valence Transition in Mixed-Valence Sesquioxide Cs₄O₆.

Publications

Elusive Valence Transition in Mixed-Valence Sesquioxide Cs<sub>4</sub>O<sub>6</sub>.

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