Phase Transition Dielectric Anomaly of PLZT Ceramic in the Temperature Range of 30–350°C

Abstract

The Pb 1Àx La x (Zr 1Ày Ti y ) 1Àx=4 O 3 , PLZT-x/100 À y/y, perovskite ceramic and thin film possessing a wide range of ferroelectric properties have promising potential for applications. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] The most typical properties of lead ziconatetitanate lantanium doped (PLZT) ceramics with Pb/La ratios (0 < x < 5%) and Zr/Ti ratios close to 95/5 are strong diffuse antiferroelectric-ferroelectric (AFE-FE) phase transition, exceptional big thermal hysteresis and a wide temperature range in which the AFE and FE phases coexist. 5-10) PLZT undergoes successive phase transition: at the upper transition point (T U ) from the cubic (Pm3m) high temperature paraelectric phase to the rhombohedral (R3c or R3m) intermediate-temperature ferroelectric phase, and at the lower transition point (T L ) to the orthorhombic (Pbam) low-temperature antiferroelectric phase. 8) Dielectric constant displays a slightly discontinuous drop at T L and sharp peak at T U . 5-8) The AFE-FE phase transition point are shifted downwards with increasing La content (0:5 < x < 5%). [5][6][7][8][9][10] Handerek et al. 9) reported dielectric, pyroelectric, and thermally stimulated depolarization current investigations for PLZT x/95/5 ceramics for 0:5 < x < 4:0%. They observed relaxor like dielectric dispersion (10 Hz-10 4 Hz) at T U , and attributed this anomaly behavior to space-charge-induced polarization by mobile ionic defects. However, detailed phase relations on La doping were not reported. 9,10) In PLZT-x/95/5 (x > 5) ceramics, relatively little has been done to study the phase transition above room temperature. We have investigated PLZT-2/95/5, 6/95/5, 8/95/5-type, that is with low and high La content phase transition using dielectric spectroscopy.PLZT-x/95/5 ceramics with La contents of x ¼ 2, 6, 8 were prepared using a conventional method of thermal synthesis of mixed oxides. The complex dielectric constant was measured at 1 MHz by using an Impedance Analyzer (SI 1260). The experiments were performed between room temperature and 350 C in ambient atmosphere and the temperature of the sample was measured with a platinumrhodium thermocouple. The heating and cooling rates were 0.2 C/min. Figures 1(a) and 1(b) show the real (" 0 ) and imaginary (" 00 ) dielectric constant as a function of temperature for PLZT-2/95/5. The values of " 0 increase with decreasing temperature. At T U ¼ 217 C, it becomes to decrease making a large -type peak on the dielectric constant vs temperature curve, on heating and cooling. As temperature decreases, if we take the phase transition temperature as the mid-point of the steepest of " 0 , then the lower transition occurs at T C,L ¼ 89 C on cooling and at T H,L ¼ 114 C on heating with a thermal hysteresis of 25 C. This results is similar to Xu et al. 10) Figures 2(a) and 2(b) show the " 0 and " 00 as a function of temperature for PLZT-6/95/5. A low temperature anomaly was observed at T H,L ¼ 95 C on heating and at T C,L ¼ 54 C on cooling with a thermal hysteresis of 40 C. At low temper...