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Highly sodium deficient Na 0.6 NbO 3 (NN) ceramics were prepared and characterized. Their mean structure could be described by a mixture of the predominant P polymorph (S.G. Pbma) and the Q phase (S.G. P 2 1 ma), at room temperature, as deduced from Rietveld refinements of Powder X-ray diffraction and Raman scattering data. Besides, microstructural and compositional heterogeneity was evidenced from HRTEM and XPS analyses. Dielectric measurements on this highly vacancies-containing NN material indicated a hysteretic transition taking place at ~12 and −18 °C on the heating and cooling regimes, respectively. This means a considerable shift to lower temperatures of the transition temperatures concerning the P/Q transformations previously described for the NaNbO 3 material. At an intermediate frequency of 10 kHz, maximum permittivity and tangent loss values of ε' = 1250 and tgδ = 2 were obtained on heating, whereas lower values of ε' = 65 and tgδ = 0.04 were displayed on cooling regime. These anomalies were interpreted in terms of ferrolectric-antiferroelectric-paralectric phase-transitions implying a PTCR behavior within certain temperature range. This response, observed in this complex material for the first time, has interesting implications for the possible application of NN ceramics in sensing and dielectric materials fields. [ABSTRACT FROM AUTHOR]

[Display omitted] The BiFeO 3 –NaNbO 3 electroceramics, synthesized by the ceramic method, are studied aiming to obtain materials with a well-defined thermistor response coexisting with a relevant magnetic response. XRD data and Raman analysis reveal a structural transition as a function of composition. Compositional features explored from ICP, XPS and EDS measurements, suggest compositional heterogeneity leading to a cluster-type scenario implying NNO-rich and BFO-rich regions in the samples. Impedance spectroscopy data reveal the development of a PTCR thermistor response for x ≥ 0.5 near room temperature. The x = 0.9 ceramic shows resistivity changes of about six orders of magnitude in the first thermal cycle and maximum permittivity values of ∼ 105, much higher than those previously reported for BFO-doped ceramics. Magnetization data are interpreted in terms of the stabilization of superparamagnetic clusters. The response displayed by the x = 0.9 ceramic makes it a promising multifunctional material for device applications. [ABSTRACT FROM AUTHOR]

Two new phases in the (SrTiO 3 )-(BiMnO 3 ) perovskite type system were studied. These SrBiMn 2−x Ti x O 6 ( x =0.5 and 0.75) ceramics were prepared by the liquid-mix method as polycrystalline powders. They were characterized by neutron diffraction (ND), electrical and magnetic measurements and these results were analysed in connection with the x =0.25 phase, previously described by us. Their crystal structures were fitted in the tetragonal I 4/ mcm space group, with additional superstructure reflections suggesting local ordering. Electron microscopy results revealed these ordering effects are actually present concerning both perovskite sublattices. The electrical response pointed to a mechanism consisting in variable range hopping of polarons. A relaxor ferroelectric behaviour dependent on the pellets thickness was found with T m in the 140–240 K range. The magnetic structures imply an incommensurable modulation of the magnetic moments along the c or a directions, depending on the substitution degree x . A cluster picture in terms of chemically different nanoregions responsible of the macroscopic response is discussed. [ABSTRACT FROM AUTHOR]

Microcrystalline powders of the Sr 2− x Bi x Ti 2− x Fe x O 6± δ ( x =0.25; 0.50; 0.60; 0.75; 1; 1.25) perovskites have been prepared by the liquid-mix technique and the SrBiTi 2− x Fe x O 6 ( x =0.85) sample was obtained by the ceramic method. Rietveld refinements from X-ray powder diffraction data showed a structural transition as a function of x : cubic symmetry (S.G. Pm 3 m ) is observed for x ≤1 whereas the x =1.25 sample presents rhombohedral symmetry (S.G. R 3_ c ). Refined occupations and EDX analyses indicate that the obtained cationic compositions are close to the nominal ones for the Sr 2− x Bi x Ti 2− x Fe x O 6± δ samples but a small cationic deficiency was detected in the sample obtained by the ceramic method. Particle sizes of about 200 nm were obtained for the samples obtained by the liquid mix technique whereas they were clearly micrometric for the x =0.85 sample, obtained by the ceramic method. HRTEM analysis showed that these greater in size crystals present high degree of microstructural disorder. The dielectric response seems to be related to a relaxor-type ferroelectric behavior. Capacitance, conductivity and tangent loss values indicate the prevalence of intergrain conductive contribution in the electrical response. The results are comparatively analyzed in relation to microstructural features and the synthesis method employed. [ABSTRACT FROM AUTHOR]

Polycrystalline Sr 2− x Bi x Ti 2− x Fe x O 6 ( x =0, 0.5, 1 and 1.5) perovskites have been synthesized by the ceramic method. Structural characterization from X-ray diffraction data showed that the symmetry evolves from centrosymmetric cubic ( Pm 3 ¯ m ) to non centrosymmetyric rhombohedral ( R3c ) as iron and bismuth content increase. For the intermediate composition, x =1, a mixed phase is obtained, which mean symmetry matches the best with the orthorhombic one ( Pbnm ). EDS analyses showed that the effective compositions differ from the nominal ones, and they were confirmed from Rietveld refinements of XRD data. HRTEM experiments indicated that in the case of x =1.5, a superstructure along [001] c and [111] c is observed. The dielectric response is modified depending on the composition range x . The x =0 compound shows a Debye type relaxation implying oxygen vacancy cluster, whereas a relaxor ferroelectric behavior is obtained for the x =0.5 and x =1 samples. Finally, in the case of x =1.5, the dielectric behaviour is characterized by strong dependence of permittivity with temperature. [ABSTRACT FROM AUTHOR]