Browsing by Author "Raveau, B"

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    Lifting the geometric frustration through a monoclinic distortion in “114” YBaFe4O7.0: Magnetism and transport
    (Elsevier, 2013-09-01) Duffort, V; Sarkar, T; Caignaert, V; Pralong, V; Raveau, B; Avdeev, M; Cervellino, A; Waerenborgh, JC; Tsipis, EV
    The possibility to lift the geometric frustration in the “114” stoichiomeric tetragonal oxide YBaFe4O7.0 by decreasing the temperature has been investigated using neutron and synchrotron powder diffraction techniques. Besides the structural transition from tetragonal to monoclinic symmetry that appears at TS=180 K, a magnetic transition is observed below TN=95 K. The latter corresponds to a lifting of the 3D geometric frustration toward an antiferromagnetic long range ordering, never observed to date in a cubic based “114’” oxide. The magnetic structure, characterized by the propagation vector k1=(0,0,½), shows that one iron Fe2 exhibits a larger magnetic moment than the three others, suggesting a possible charge ordering according to the formula YBaFe 3 + Fe 3 2 + O 7.0 . The magnetic M(T) and χ′(T) curves, in agreement with neutron data, confirm the structural and magnetic transitions and evidence the coexistence of residual magnetic frustration. Moreover, the transport measurements show a resistive transition from a thermally activated conduction mechanism to a variable range hopping mechanism at TS=180 K, with a significant increase of the dependence of the resistivity vs. temperature. Mössbauer spectroscopy clearly evidences a change in the electronic configuration of the iron framework at the structural transition as well as coexistence of several oxidation states. The role of barium underbonding in these transitions is discussed. © 2013, Elsevier Ltd.
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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.