Tetragonal YBaFe4O7.0: A stoichiometric polymorph of the “114” ferrite family

The exploration of the phase diagram of the ferrite YBaFe4O7+δ versus the oxygen content δ and temperature shows the complex crystal chemistry of this system. Besides the cubic form (F4¯3m), which is observed up to 600 °C and for 0<δ≤0.65, a stoichiometric tetragonal form (δ=0) is isolated below 300 °C that is stable only in the absence of oxidizing atmosphere. The resolution of the structure of this new YBaFe4O7.0 form, from combined neutron and synchrotron data, in the space group I4¯, 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 underbonded barium cations is explained by the existence of hybridized Ba(2−δ)+–O2−−Fe(2+δ)+ bonds, in agreement with Mössbauer spectroscopy. The role of coulombic repulsions in the [Fe4O] and [Fe4] tetrahedra of the [Fe4]∞ sublattice on the structural transition is also discussed.

Graphical abstractA new form of YBaFe4O7 “114” ferrite which is only stable in oxygen free atmosphere, even at room temperature has been prepared. This new ferrite has its structure closely related to the cubic form YBaFe4O7+δ but differently from the latter which is stable for 0<δ<0.65, it requires a rigorous oxygen “O7” stoichiometry, i.e., δ=0. We describe herein the phase diagram of the oxides YBaFe4O7+δ and we determine the structure of this new YBaFe4O7.0 form, from combined neutron and synchrotron data.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Oxygen stoichiometry control in “114” YBaFe4O7+d ferrites. ► A new structural transition leading to a new form of YBaFe4O7+d “114” ferrite for d=0. ► Structural resolution from combined neutron and synchrotron data. ► Phase diagram of the oxides YBaFe4O7+d