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Title: Ross Colman: Caesium sesquioxide, Cs4O6 – a complex p-electron magnet with interplaying structural, charge and orbital degrees of freedom – probed by diffraction.
Number: 2/20
Status: Closing date exceeded
Begin: Středa, 15.01. 2020, 14:10
Tutor: Vladimír Sechovský
Location: lecture room F2, first floor Ke Karlovu 5


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We have a pleasure to invite you to attend the joint seminar
of the Department of Condensed Matter Physics (DCMP)
and the Materials Growth and Measurement Laboratory (MGML) http://mgml.eu.

 

Program

 

Caesium sesquioxide, Cs4O6 – a complex p-electron magnet with interplaying structural, charge and orbital degrees of freedom – probed by diffraction.

lecture given by:

Ross H. Colman

Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic

The seminar takes place in the lecture room F2 
of the Faculty of Mathematics and Physics, Ke Karlovu 5, Praha 2
on Wednesday, 15.1. 2019 from 14:10 

Vladimír Sechovský
On behalf of the DCMP and MGML


Abstract 

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. Material examples of magnetism arising from p-electrons is relatively rare (compared to d- or f- electron magnets). I will present our recently published structural analysis, tuning the observed behaviour using both temperature and pressure. Here we observed that, while upon deep quenching the cubic phase is kinetically arrested down to cryogenic temperatures, ultraslow cooling at ambient pressure results in an incomplete structural transition, at ~200 K to a contracted tetragonal phase [1]. Pressure is capable of tuning this transition to room temperature [2]. 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 mixed-valence 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 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.

[1] Colman, R. H.; Okur, H. E.; Kockelmann, W.; Brown, C. M.; Sans, A.; Felser, C.; Jansen, M.; Prassides, K. Elusive Valence Transition in Mixed-Valence Sesquioxide Cs4O6. Inorg. Chem.2019, 58 (21), 14532–14541. https://doi.org/10.1021/acs.inorgchem.9b02122.

[2] Okur, H. E.; Colman, R. H.; Ohishi, Y.; Sans, A.; Felser, C.; Jansen, M.; Prassides, K. Pressure-Induced Charge Disorder–Order Transition in the Cs4O6 Sesquioxide. Inorg. Chem.2020, acs.inorgchem.9b02974. https://doi.org/10.1021/acs.inorgchem.9b02974.