The temperature dependence of the piezoelectric constant d 33 , electromechanical coupling factor k 33 , dielectric constant " T 33 and elastic compliance s E 33 for a single crystal of KF-doped barium titanate with a KF content near the tricritical point (TCP), Ba 0:91 K 0:09 TiO 2:91 F 0:09 (KF-BT/0.09), was measured by a resonance-antiresonance method in a temperature range between 270 and 340 K. The single crystals used were grown by a KF-flux method and the largest single crystal was a cube of 5 mm size. A large piezoelectric constant and a large dielectric constant were obtained at 300 K: d 33 ¼ 300 pC/N, k 33 ¼ 0:59, and " T 33 ¼ 3;000. As the temperature increases, " T 33 gradually increases toward T C but does not show any discontinuous changes at T C ¼ 329 K; moreover, s E 33 and d 33 increase with temperature between 270 and 310 K. However, above 310 K, they decrease toward T C . The fluctuation of the order parameter near the TCP induces these peculiar behaviors.
10% KF-doped barium titanate powders, Ba0.9K0.1TiO2.9F0.1, were synthesized through a sol–gel process. The powders, calcined at 650 °C, consist of cubic crystalline particles of ∼70 nm in length; the particle size increases to ∼200 nm as the firing temperature increases to ca. 800 °C, above which F2 begins to evaporate. Dense ceramics were fabricated by the spark plasma sintering (SPS) method; the average grain size is ∼2 µm in lengths. The ceramics, well annealed at 1,000 °C in an O2 gas flow, have good dielectric and piezoelectric properties; the piezoelectric d
33 value is 230 pC/N at room temperature. At the ferroelectric Curie temperature T
C = 47 °C, the dielectric constant and loss tan
δ are 10,000 and <5% at 10 kHz, respectively. The Curie–Weiss relation holds in the fully disordered cubic and ordered rhombohedral phases, showing the second order 1:2 relation. Below 10 kHz, large dielectric dispersion caused by a domain-wall motion appears at the temperature range of -50 to 107 °C. Some discussions are made for these dielectric properties of the ceramics.
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