Dielectric and resonance frequency investigations of phase transitions in Nb-doped PZT95/5 and 75/25 ceramics

The low-temperature-high-temperature rhombohedral-ferroelectric phase transition (F-R(LT)-F-R(HT)) and the rbombohedral-ferroelectric-cubic-paraelectric (F-R(HT)-PE) phase transition in Nb-doped Pb(Zr1-xTix)O-3 ceramics (x = 0.05 and 0.25) were investigated using dielectric and resonance frequency measurements. The dielectric behaviour in PZT(Nb)95/5 is found to obey the Curie-Weiss law above T-c, while that of PZT(Nb)75/25 deviates from the linear Curie-Weiss law in a temperature range of similar to 50 K above T-c, and can be described by the relation 1/epsilon similar to (T-T-c)(gamma) with the exponent gamma in the range 1 < gamma < 2. Dielectric dispersions are observed in the rhombohedral-ferroelectric region for both compositions. Dielectric anomalies and thermal hysteresis are observed in PZT(Nb)95/5; in contrast, no anomaly appears in PZT(Nb)75/25. Around the F-R(LT)-F-R(HT) phase boundary region, the temperature dependences of the resonance frequencies for both compositions show remarkable anomalies. The piezoelectric constant d(31) of PZT(Nb)95/5 rapidly decreases, but that of PZT(Nb)75/25 gradually increases. These results are discussed on the basis of the coupling of M-type and R-type oxygen octahedra tilts to the polarization and strain.