Variations of Structure and Dielectric Properties on Substituting A-site Cations for Sr 2+ in (Na 1/2 Bi 1/2 )TiO 3
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Variations of structure and dielectric properties on substituting A-site cations for Sr2+ in (Na1/2 Bi1/2 )TiO3 Seung-Eek Park and Kug Sun Hong Department of Inorganic Materials Engineering, College of Engineering, Seoul National University, Shinrim-dong, Kwanak-ku, Seoul 151-742, Korea (Received 16 February 1996; accepted 28 January 1997)
Changes in structure and phase transition behavior of (Na1/2 Bi1/2 )1–x Srx TiO3 were investigated using x-ray diffraction (XRD) analysis and dielectric measurements. A decrease in the degree of lattice distortion was observed when the structure was rhombohedral (x , 0.26). Further substitution of Sr2+ (x > 0.26) caused (Na1/2 Bi1/2 )1–x Srx TiO3 to show no macroscopic lattice distortion. However, all investigated samples (0.1 < x < 0.5) were ferroelectric at room temperature regardless of the lack of lattice distortion found in XRD patterns. Further XRD investigations revealed that nonuniform strains caused by nonuniform polarized islands were responsible for this behavior. A-site cation substitution of Sr2+ also resulted in the decrease of Tmax (the temperature where dielectric constant is maximum). Typical relaxor ferroelectric phase transition behavior, associated with the shift of Tmax , with respect to the measuring frequencies, was found above x 0.18. This could be also ascribed to the decrease of Tmax — this reveals the increase in the shift of Tmax with frequency, with associated decrease in ionic displacement and space charge contributions and associated increase in the relative contribution to the polarization by microdomains and domain walls at temperatures near Tmax . I. INTRODUCTION
(Na1/2 Bi1/2 )TiO3 (NBT) is ferroelectric at room temperature with a diffuse phase transition (DPT).1–10 The ferroelectric DPT for NBT is involved with a structural change of rhombohedral $ tetragonal at ,250 ±C.2–7 In spite of this DPT behavior, the expected shift of Tmax (the temperature where dielectric constant is maximum, ,320 ±C), with respect to the measuring frequencies, has not been found. Only polarization relaxation caused by space charge contributions have been reported at higher temperatures near tetragonal $ cubic transition (.450 ±C).10 Therefore, in spite of DPT, NBT was not always designated as a relaxor because the typical Tmax shift, with respect to measuring frequency, was not observed. A number of studies have been reported regarding the peculiarities accompanying the phase transition of NBT, such as structural changes,4,7 dielectric anomalies,1–10 optical isotropization,2,7 and the electrical states of each phase.2–10 It is especially difficult to identify the electrical states of each phase because of the high ionic conductivity at elevated temperatures. Substituting A-site cations for Sr2+ , and looking at the associated decrease in transition temperature, was attempted in order to identify the electrical state of the intermediate phase.11 Double hysteresis observed in the p fsEd curve of (Na1/2 Bi1/2 )1–x Srx TiO
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