Crystalline structure and dielectric behavior of (Ce,Ba)TiO 3 ceramics
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Chen Anga) Department of Physics, The University of Akron, Akron, Ohio 44325, and Department of Physics, Zhejiang University, Hang Zhou, 311027, China (Received 9 August 2001; accepted 8 July 2002)
In this work, (Ba1−xCex)Ti1−x/4(VTi)x/4O3 ceramics with x ⳱ 0.02, 0.04, 0.06, and 0.08 (VTi denotes titanium vacancies) were synthesized by the mixed-oxide method. The results of x-ray diffraction analysis and scanning electron microscopy show that all the samples are monophasic. The crystalline structure can be indexed as tetragonal for the samples with x 艋 0.06, but as cubic for x ⳱ 0.08. Three phase transitions were observed in the temperature dependence of the dielectric permittivity, similar to those observed in pure BaTiO3, and the three phase transition temperatures (Tc, T1, and T2) shifted to lower temperatures with the rates of −18, −12, and −7 K per molar percentage of Ce3+, respectively. This is quite different from that observed in BaTiO3 with Ce substitution at the Ti-site, in which Tc shifted to a lower temperature, and T1 and T2 to higher temperatures. The permittivity maximum increased with increasing Ce content, which is mainly attributed to an increase in the grain size. I. INTRODUCTION
BaTiO3 doped with a variety of elements, such as Sn, Hf, Zr, La, Y, and Ce, shows interesting dielectric properties and has found wide applications.1–3 Among these dopants, cerium is a special element that can appear in two oxidation states: Ce3+ and Ce4+. Because the effective ionic radii of Ce3+ (0.134 nm) and Ce4+ (0.087 nm) are similar to those of Ba 2+ (0.161 nm) and Ti 4+ (0.0605 nm),4 respectively, Ce3+ can be incorporated at the Ba-site5 and Ce4+ at the Ti-site in BaTiO3.6–10 For Ce4+ substitution at the Ti-site in BaTiO3, the limit of solid solubility is approximately 30 at.%; i.e., y ∼ 0.3 in Ba(Ti1−yCey)O3. The evolution from a typical ferroelectric to a relaxor ferroelectric was reported in Ba(Ti1−yCey)O3. Slight relaxor ferroelectric behavior occurs at y ∼ 0.06, and the relaxor behavior becomes more and more obvious with further increasing Ce4+ concentration.6–10 Ce3+ partially doped BaTiO3 at the Ba site has recently received much attention11–18 due to its high dielectric permittivity and high endurance under electric field. It is a promising lead-free material for multilayer capacitors.11,12 Most studies of Ce3+-doped (at Ba-sites)
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 11, Nov 2002
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BaTiO3 have focused on the phase formation, microstructure, and permittivity versus temperature characteristics.11–18 Detailed ferroelectric and dielectric behavior has not received enough attention. In this work, we first report the phase assemblage and microstructure and then report the temperature dependence of the ferroelectric and dielectric properties of (Ba1−xCex)Ti1−x/4(VTi)x/4O3 (0.02 艋 x 艋 0.08) ceramics in a wide temperature range, 12–500 K. The results show that the effect of replacing
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