Browsing by Author "Mitchell, JF"

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    Local metallic and structural properties of the strongly correlated metal LaNiO3 using 8Li β–NMR
    (American Physical Society, 2019-10-07) Karner, VL; Chatzichristos, A; Cortie, DL; Dehn, MH; Foyevtsov, O; Foyevtsova, K; Fujimoto, D; Kiefl, RF; Levy, CDP; Li, R; McFadden, RML; Morris, GD; Pearson, MR; Stachura, M; Ticknor, JO; Cristiani, G; Logvenov, G; Wrobel, F; Keimer, B; Zhang, J; Mitchell, JF; MacFarlane, WA
    We report β-detected NMR of ion-implanted Li8 in a single crystal and thin film of the strongly correlated metal LaNiO3. Spin-lattice relaxation measurements reveal two distinct local environments, both metallic as evident from T-linear Korringa 1/T1 below 200K with slopes comparable to other metals. A small approximately temperature-independent Knight shift of ∼74ppm is observed, yielding a normalized Korringa product characteristic of substantial antiferromagnetic correlations. We find no evidence for a magnetic transition from 4 to 310K. The similarity of these features in the two very different samples indicates that they are intrinsic and unrelated to dilute oxygen vacancies. We attribute the two environments to two distinct but similar crystallographic Li8 sites and not to any form of phase inhomogeneity, but this is inconsistent with the conventional rhombohedral structure of LaNiO3, and also cannot be simply explained by the common alternative orthorhombic or monoclinic distortions. ©2024 American Physical Society.
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    Tetragonal YBaFe4O7.0: A stoichiometric polymorph of the "114" ferrite family
    (Elsevier, 2012-07-01) Duffort, V; Caignaert, V; Pralong, V; Barrier, N; Raveau, B; Avdeev, M; Zheng, H; Mitchell, JF
    The exploration of the phase diagram of the ferrite YBaFe(4)O(7+delta) versus the oxygen content delta and temperature shows the complex crystal chemistry of this system. Besides the cubic form (F (4) over bar 3m), which is observed up to 600 degrees C and for 0 < delta <= 0.65, a stoichiometric tetragonal form (delta= 0) is isolated below 300 degrees C that is stable only in the absence of oxidizing atmosphere. The resolution of the structure of this new YBaFe(4)O(7.0) form, from combined neutron and synchrotron data, in the space group I<(4)over bar>, shows significant displacements of the atoms with respect to the cubic form, especially concerning the oxygen atoms surrounding the barium cations. The decrease of several Ba-O distances around the under-bonded barium cations is explained by the existence of hybridized Ba((2-delta)+) -O(2-) -Fe((2+delta)+) bonds, in agreement with Mossbauer spectroscopy. The role of coulombic repulsions in the [Fe(4)O] and [Fe(4)] tetrahedra of the [Fe(4)](infinity) sublattice on the structural transition is also discussed. © 2012, Elsevier Ltd.