Browsing by Author "Barrier, N"

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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.