Phase Transition in Ba6−3x(Sm1−yLay)6+2xTi18O54 (x=0.5) Ceramics
The effect of La substitution for Sm was investigated in Ba6−3x(Sm1−yLay)6+2xTi18O54 (x=0.5) ceramics in the entire range of y (0∼1.0). X-ray diffraction (XRD) analysis indicated that the phase constitution of the present ceramics was sensitive to y, and an abnormal phase transition from the tungsten bronze-type to a new Ba16Ce32Ti64O192-type solid solution was determined at y=0.6–0.9. The crystal structure of the new phase was refined by Rietveld refinement. The stability of the Ba16Ce32Ti64O192-type structure was discussed with the ordering of cations and the average radius of Ln3+. The Ba16Ce32Ti64O192-type phase was observed only in the sintered pellets but not in the loose powders in the investigated temperature range of 1200°–1350°C. A differential scanning calorimetric analysis (DSC) was performed for investigating the thermal effect of the phase transition. Discontinuous dielectric properties were observed corresponding to the phase transition. The Ba16Ce32Ti64O192-type phase has a much higher dielectric constant, a lower quality factor, and a larger positive temperature coefficient of resonant frequency at microwave frequencies in comparison with the tungsten bronze-type phase. Besides, the dielectric loss of the Ba16Ce32Ti64O192-type phase is much lower (tan ∼10−4) at frequencies below 108 Hz, while the dielectric constant is insensitive to the frequency.
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Document Type: Research Article
Affiliations: Department of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, China
Publication date: 2006-09-01