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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.