| Abstract
| - Charge, orbital, and magnetic ordering of NdBaFe2O5 and HoBaFe2O5, the two end-membersof the double-cell perovskite series RBaFe2O5, have been characterized over the temperature range 2−450K, using differential scanning calorimetry, neutron thermodiffractometry and high-resolution neutron powderdiffraction. Upon cooling, both compounds transform from a class-III mixed valence (MV) compound, whereall iron atoms exist as equivalent MV Fe2.5+ ions, through a “premonitory” charge ordering into a class-IIMV compound, and finally to a class-I MV phase at low-temperature. The latter phase is characterized byFe2+/Fe3+ charge ordering as well as orbital ordering of the doubly occupied Fe2+ dxz orbitals. The relativesimplicity of the crystal and magnetic structure of the low-temperature charge-ordered state provide anunusual opportunity to fully characterize the classical Verwey transition, first observed in magnetite, Fe3O4.Despite isotypism of the title compounds at high temperature, neutron diffraction analysis reveals strikingdifferences in their phase transitions. In HoBaFe2O5, the Verwey transition is accompanied by a reversalof the direct Fe−Fe magnetic coupling across the rare earth layer, from ferromagnetic in the class-II and-III MV phases to antiferromagnetic in the low-temperature class-I MV phase. In NdBaFe2O5, the largerNd3+ ion increases the Fe−Fe distance, thereby weakening the Fe−Fe magnetic interaction. This decouplesthe charge and magnetic ordering so that the Fe−Fe interaction remains ferromagnetic to low temperature.Furthermore, the symmetry of the charge-ordered class-I MV phase is reduced from Pmma to P21ma andthe magnitude of the orbital ordering is diminished. These changes destabilize the charge-ordered stateand suppress the temperature at which the Verwey transition occurs. A comparison of the magnetic andstructural features of RBaFe2O5 compounds is included in order to illustrate how structural tuning, viachanges in the radius of the rare-earth ion, can be used to alter the physical properties of these double-cell perovskites.
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