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)

Abstract

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 (XRD) and transmission electron microscopy (TEM) examinations. XRD data for ceramic samples annealed under different P(O2) conditions could be indexed to a cubic perovskite. Superlattice diffraction spots have been observed by TEM investigation for the samples which were re-equilibrated by a subsequent annealing in a reducing atmosphere. A microdomain model is proposed to describe the observed phenomena. According to the 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 brownmillerite-type Sr2Fe2O5 domains and perovskite-type Sr(Fe0.5Nb0.5)O3 domains. Brownmillerite-type domains are formed because of vacancy ordering. The order is extremely short ranged. Electron diffraction patterns constructed in terms of this microdomain model agree well with the observed results. The order-disorder transition can be altered by varying ambient oxygen partial pressures. For both ordered and disordered samples the frequency-domain dielectric response as a function of temperature is reported. Directly measured complex conductivity Y(ω) has been transformed by Kramers–Kronig relations in order to subtract the disturbances of G(0) and C(∞). The transformed temperature dependences have been further normalized and re-expressed in the form of master curves. For all the samples measured, the frequency-domain dielectric response deviates from the Debye model but follows the so-called universal power law behavior. The dielectric spectra for the samples in which brownmillerite-type ordering is identified show an anomalous low-frequency dispersion, while for the disordered samples, the relaxation 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 dielectric behavior to approach the Debye model.