STRUCTURE ORDER-DISORDER AND DIELECTRIC RESPONSE IN PEROVSKITE-RELATED SRO-FE2O3-NB2O5 SYSTEM - THE OXYGEN-DEFICIENT COMPOSITION SR(FE1-XNBX)O2.5+Y (X = 0.17, 0.25, 0.50)

Microstructures of materials with composition Sr(Fe1-xNbx)O2.5+y (x=0. 17, 0.25, 0.50) have been investigated by powder x-ray diffraction (XR D) and transmission electron microscopy (TEM) examinations. XRD data f or ceramic samples annealed under different P(O2) conditions could be indexed to a cubic perovskite. Superlattice diffraction spots have bee n observed by TEM investigation for the samples which were re-equilibr ated by a subsequent annealing in a reducing atmosphere. A microdomain model is proposed to describe the observed phenomena. According to th e model, the structure of reduced Sr(Fe1-xNbx)O2.5+y with x=0.17, 0.25 is considered to be a result of disordered intergrowth between brownm illerite-type Sr2Fe2O5 domains and perovskite-type Sr(Fe0.5Nb0.5)O3 do mains. Brownmillerite-type domains are formed because of vacancy order ing. The order is extremely short ranged. Electron diffraction pattern s constructed in terms of this microdomain model agree well with the o bserved results. The order-disorder transition can be altered by varyi ng ambient oxygen partial pressures. For both ordered and disordered s amples the frequency-domain dielectric response as a function of tempe rature is reported. Directly measured complex conductivity Y(omega) ha s been transformed by Kramers-Kronig relations in order to subtract th e disturbances of G(0) and C(infinity). The transformed temperature de pendences have been further normalized and re-expressed in the form of master curves. For all the samples measured, the frequency-domain die lectric response deviates from the Debye model but follows the so-call ed universal power law behavior. The dielectric spectra for the sample s in which brownmillerite-type ordering is identified show an anomalou s low-frequency dispersion, while for the disordered samples, the rela xation behavior with a loss peak is observed. The variation of spectra shape with composition indicates that increasing Nb content tends to weaken the cooperative many-body interactions and causes the dielectri c behavior to approach the Debye model.