Extrinsic contributions to the piezoelectric response of lead-based ferroelectrics

Abstract

The present study aims at a better understanding of the high piezoelectric properties encountered in lead-based ferroelectrics by focusing on the extrinsic contributions to the response. The main characteristics of these materials are the highly nonlinear character of the electro-mechanical response and the presence of a morphotropic phase boundary (MPB) where properties are reaching a maximum. Thus, our approach was first to develop a new description for the piezoelectric hysteresis and nonlinearities and second to investigate MPB effects on the extrinsic contributions to the piezoelectric response. For these purposes, lead titanate (PT), lead zirconate titanate (PZT), and lead nickel niobate-lead zirconate titanate solid solution (PNN-PZT) were chosen as prototype compositions. The sample preparation was classical for the first two compounds whereas two new synthesis routes of PNN-PZT were developed. The first consists in the preparation of a B-site precursor combining all the perovskite B-site cations before calcination with lead oxide, enhancing chemical homogeneity and insuring good reproducibility in the final properties. The second processing method aimed at more time efficiency: the use of a nickel hydroxy-carbonate instead of nickel oxide permitted to obtain in one single calcination step a pure perovskite phase exhibiting properties as high as those obtained for two firing steps procedures. The piezoelectric hysteresis and nonlinearity description was undertaken using the Preisach formalism, first developed in ferromagnetism. This approach considers that each hysteretic system satisfying the wiping-out and the congruency properties can be seen as composed of bistable units characterized by distributed coercive and bias fields. In our case, the use of a general expression for the unit parameters distribution function permitted to describe the most significant piezoelectric coefficient nonlinearities such as applied field dependence, saturation or threshold fields. Moreover, a minimal expression describing the piezoelectric nonlinearity in lead-based compounds was derived from experimental determination of the distribution function topography. Advanced piezoelectric hysteresis modelling was attained by coupling the classical viscous descriptions with Preisach-inspired loops expressions. This allowed to separate piezoelectric losses into viscous and field dependent parts. Moreover, the possibility of extracting distribution function parameters from the loops was established. The versatility of the Preisach hysteresis description was also demonstrated by deriving pinched ferroelectric loops from the supposed microscopical mechanisms in hard-doped ferroelectrics. Morphotropic phase boundary study started with the derivation of a composition-temperature phase diagram for PNN-PZT using dielectric and pyroelectric measurements along with X-ray diffraction. The MPB of this solid solution was shown to be strongly curved toward the rhombohedral side. Therefore, certain compositions undergo a tetragonal to rhombohedral transition upon cooling. The influence of electric field and thermal history on this morphotropic transition was first investigated in view of optimizing the poling conditions of such materials. Then, the piezoelectric extrinsic contributions at MPB for both PZT and PNN-PZT were studied as a function of composition and temperature. The peak of properties occurring in this region was assigned to intrinsic effects coupled with an extension of the irreversible contributions. Using the developed hysteresis and nonlinearity formalism, this extrinsic contributions increase could be related to an extended response of each defect rather than to an increase in the defects density.