The low-temperature - high-temperature rhombohedral-ferroelectric phase transition and the rhombohedral-ferroelectric - cubic-paraelectric phase transition in Nb-doped 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 , while that of PZT(Nb)75/25 deviates from the linear Curie - Weiss law in a temperature range of above , and can be described by the relation with the exponent in the range . 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 phase boundary region, the temperature dependences of the resonance frequencies for both compositions show remarkable anomalies. The piezoelectric constant 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.
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Dong et al. (1997) studied this question.
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