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    Elusive Valence Transition in Mixed-Valence Sesquioxide Cs4O6
    (Amer Chemical Soc, 2019) Colman, Ross H.; Okur Kutay, Havva Esma; Kockelmann, Winfried; Brown, Craig M.; Sans, Annette; Felser, Claudia
    Cs4O6 is a mixed-valence molecular oxide with a cubic structure, comprising valency-delocalized O-2(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 O-2(2-) and O-2(-) 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 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 O-2(-) units provides an intimate link between the crystal and magnetic properties of charge-ordered Cs4O6, naturally accounting for the absence of magnetic order.
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    Fulleride superconductivity tuned by elastic strain due to cation compositional disorder
    (Royal Soc Chemistry, 2024) Okur, H. Esma; Colman, Ross H.; Takabayashi, Yasuhiro; Jeglic, Peter; Ohishi, Yasuo; Kato, Kenichi; Prassides, Kosmas
    Dynamical fluctuations of the elastic strain in strongly correlated systems are known to affect the onset of metal-to-insulator or superconducting transitions. Here we report their effect on the properties of a family of bandwidth-controlled alkali-intercalated fullerene superconductors. We introduce elastic strain through static local structural disorder in a systematic and controllable way in the fcc-structured KxCs3-xC60 (with potassium content, 0.22 <= x(K) <= 2) series of compositions by utilizing the difference in size between the K+ and Cs+ co-dopants. The occurrence of the crossover from the Mott-Jahn-Teller insulating (MJTI) state into the strongly correlated Jahn-Teller metal (JTM) on cooling is evidenced for the compositions with x(K) < 1.28 by both synchrotron X-ray powder diffraction (SXRPD) - anomalous reduction of the unit cell volume - and Cs-133 NMR spectroscopy - sudden suppression in the Cs-133 spin-lattice relaxation rates. The emerging superconducting state with a maximum critical temperature, T-c = 30.9 K shows a characteristic dome-like dependence on the unit-cell volume or equivalently, on the ratio between the on-site Coulomb repulsion, U, and the bandwidth, W. However, compared to the parent Cs3C60 composition in which cation disorder effects are completely absent, the maximum T-c is lower by similar to 12%. The reduction in T-c displays a linear dependence on the variance of the tetrahedral-site cation size, sigma(2)(T), thus establishing a clear link between structural-disorder-induced attenuation of critical elastic strain fluctuations and the electronic ground state.
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    Pressure effects on the crystal structure of the cubic metallofullerene salt [Li@C60][PF6] to 12 GPa
    (Elsevier, 2022) Colman, Ross H.; Okur, H. Esma; Garbarino, Gaston; Ohishi, Yasuo; Aoyagi, Shinobu; Shinohara, Hisanori; Prassides, Kosmas
    The pressure-dependent structural properties of the metallofullerene salt, Li+@C-60][PF6](-), which has been shown before to adopt a primitive cubic structure at ambient conditions, have been studied by synchrotron X-ray powder diffraction at ambient temperature and at 7 K up to similar to 12 GPa. We find no evidence for a phase transition across the accessed pressure range at either temperature with the structure always remaining strictly primitive cubic (space group Pa (3) over bar). The extracted bulk moduli, K-0 of 17.51(13) GPa and 16.6(2) GPa at 7 K and ambient temperature, respectively are comparable to those of other cubic close-packed fullerene solids implying little influence of the interaction between the endohedral Li+ and the interstitial [PF6](-) unit. Rietveld analysis of the diffraction data shows that the adopted primitive cubic structure incorporates orientationally ordered [Li+@C-60] units - they are rotated anticlockwise by similar to 101 degrees about the three-fold [111] rotation axis. This finding together with the lack of observation of a monomer -> polymer phase transition upon pressurization contrast sharply with the structural behaviour of isostructural pristine C-60 and its intercalated salts, (deely quenched) CsC60 and Na2CsC60, which both incorporate orientationally disordered C60 units and have a tendency to easily polymerize via intermolecular C-C bond formation. Such differences can be understood in terms of the expanded nature of the salt and the steric interactions introduced by the large [PF6](-) unit residing in the octahedral interstices.
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    Pressure-Induced Charge Disorder-Order Transition in the Cs4O6 Sesquioxide
    (Amer Chemical Soc, 2020) Okur Kutay, Havva Esma; Colman, Ross H.; Ohish, Yasuo; Sans, Annette; Felser, Claudia; Jansen, Martin
    Cs4O6 adopts two distinct crystal structures at ambient pressure. At temperatures below similar to 200 K, its ground state structure is tetragonal, incorporating two symmetry-distinct dioxygen anions, diamagnetic peroxide, O-2(2-), and paramagnetic superoxide, O-2(-), units in a 1:2 ratio, consistent with the presence of charge and orbital order. At high temperatures, its ground state structure is cubic, comprising symmetry-equivalent dioxygen units with an average oxidation state of -4/3, consistent with the adoption of a charge-disordered state. The pressure dependence of the structure of solid Cs4O6 at 300 K and at 13.4 K was followed up to similar to 12 GPa by synchrotron X-ray powder diffraction. When a pressure of similar to 2 GPa is reached at ambient temperature, an incomplete phase transition that is accompanied by a significant volume reduction (similar to 2%) to a more densely packed highly anisotropic tetragonal structure, isostructural with the low-temperature ambient-pressure phase of Cs4O6, is encountered. A complete transformation of the cubic (charge-disordered) to the tetragonal (charge-ordered) phase of Cs4O6 is achieved when the hydrostatic pressure exceeds 6 GPa. In contrast, the pressure response of the Cs4O6 cubic/tetragonal phase assemblage at 13.4 K is distinctly different with the cubic and tetragonal phases coexisting over the entire pressure range (to similar to 12 GPa) accessed in the present experiments and with only a small fraction of the cubic phase converting to tetragonal. Pressure turns out to be an inefficient stimulus to drive the charge disorder-order transition in Cs4O6 at cryogenic temperatures, presumably due to the high activation barriers (much larger than the thermal energy at 13.4 K) associated with the severe steric hindrance for a rotation of the molecular oxygen units necessitated in the course of the structural transformation.