Abstract
The influence of impurities on piezoelectric parameters such as the radial coupling factor kr and mechanical quality factor Qm have been measured. A mechanism of internal friction is proposed. The effects of the interaction of 90 degrees domain walls with defects introduced by impurities are explained. A-site vacancies reduce the local stress in a domain, making it become remarkably mobile. The easy motion of the domain wall causes the internal friction in the ceramics to increase and thus Qm to decrease. O-site vacancies act as pinning points for the domain walls, so that domain wall motion is reduced when O-site vacancies are dominant, and Qm is raised due to the reduced internal friction. The effect of poling conditions is also discussed.