Abstract
The different properties of acceptor-doped (hard) and donor-doped (soft) lead zirconate titanate (PZT) ceramics are often attributed to different amounts of oxygen vacancies introduced by the dopant. Acceptor doping is believed to cause high oxygen vacancy concentrations, while donors are expected to strongly suppress their amount. In this study, La3+ donor-doped, Fe3+ acceptor-doped and La3+/Fe3+-co-doped PZT samples were investigated by oxygen tracer exchange and electrochemical impedance spectroscopy in order to analyse the effect of doping on oxygen vacancy concentrations. Relative changes in the tracer diffusion coefficients for different doping and quantitative relations between defect concentrations allowed estimates of oxygen vacancy concentrations. Donor doping does not completely suppress the formation of oxygen vacancies; rather, it concentrates them in the grain boundary region. Acceptor doping enhances the amount of oxygen vacancies but estimates suggest that bulk concentrations are still in the ppm range, even for 1% acceptor doping. Trapped holes might thus considerably contribute to the charge balancing of the acceptor dopants. This could also be of relevance in understanding the properties of hard and soft PZT.
Highlights
The ferroelectric and piezoelectric properties of perovskite-type titanates are employed in many technological devices such as positive temperature coefficient (PTC) resistors and multilayer capacitors [1], generators, motors, ultrasonic transductors, actuators, capacitors, or non-volatile memories [2,3]
Undoped PZT did not show clear indication of fast grain boundary diffusion in tracer distribution images but still a kind of tail in the depth profiles at 560 ◦ C that resembles the shape of profiles for donor doping
The resulting bulk diffusion coefficients are in reasonable agreement with those obtained from the depth profiles and this supports the validity of the analysis described above
Summary
The ferroelectric and piezoelectric properties of perovskite-type titanates are employed in many technological devices such as positive temperature coefficient (PTC) resistors and multilayer capacitors [1], generators, motors, ultrasonic transductors, actuators, capacitors, or non-volatile memories [2,3]. Soft PZT ceramics result, for example, by doping with Nb5+ and exhibit properties such as square hysteresis loops, low coercive fields, high remnant polarization, high dielectric constants, maximum coupling factors, higher dielectric loss, high mechanical compliance, and reduced aging Essential in this context is the fact that mainly immobile cation vacancies result from donor doping. EIS and the partial pressure dependence of the conductivity allow the analysis of the conducting species and relative concentration changes of the electronic charge carriers with doping Applying this combination of techniques to sample series of La3+ donor-doped, Fe3+ acceptor-doped, and Fe/La co-doped PZT allowed us to draw conclusions on oxygen vacancy concentrations in differently doped PZT
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