Experimental and first-principles study of point defects, domain walls, and point-defect/domain-wall interactions in ferroelectric oxides

Ferroelectric oxides, such as lead zirconate titanate, have proved invaluable due to their excellent dielectric and piezoelectric properties. These classes of materials possess a large electric polarization below the Curie temperature. Regions of the crystal lattice having different directions of polarization are separated by nano-scale interfaces known as domain walls. These interfaces, which can be moved by electric and mechanical fields, are not only important for electromechanical properties but they also exhibit unique structural and electronic properties that may be exploited through novel application in nanoelectronics and photovoltaics. The mobility of domain walls is strongly dependent on the defect chemistry of the material due to defect-domain wall interactions. Indeed, aliovalent doping of these materials has been used to tune the properties of ferroelectric oxides for specific applications. In this work, we investigate the origin of the so-called âhardâ and âsoftâ behavior in lead zirconate titanate. Experimental evidence suggests that small concentrations of donor dopants (such as niobium) results in enhanced domain wall motion while doping with acceptor dopants (such as iron) leads to an inhibited domain wall response. The microscopic origin of this phenomena is best investigated using first-principles simulations of the atomistic properties of defects and domain wall in these materials to bring to light novel structural and electronic properties. First, we show that polar defect complexes are likely to exist in both acceptor-doped and undoped PbTiO3 . These defects and defect associates are attracted to 180o domain walls and cause pinning of such interfaces. Donor- doped materials, on the other hand, do not show the presence of polar defect complexes. A closer investigation of the 180o domain wall in PbTiO3 reveals the presence of a âBlochâ component of polarization at the domain wall. In other words, due to polarization rotation, there exists a ferroelectric phase within the domain wall itself. We characterize the strain dependence of this phenomena and calculate the piezoelectric properties of such domain walls. A complete study of domain walls in PbTiO3 also entails looking closer at the properties of the ferroelastic âhead-to-tailâ 90o domain boundary. We show the presence of an asymmetry in the variation of the lattice parameter across the domain wall. This asymmetry is verified using high resolution aberration corrected electron microscopy. We look at the energy landscape of oxygen vacancies in the vicinity of these walls to explain the pinning effect in terms of random-bond and random-field defects. Next we look at the electronic properties of the recently discovered charged domain walls. We characterize the band bending phenomena in 90o head-to-head and tail-to-tail domain wall in PbTiO3 . We then look at the long range effects of clusters of oxygen vacancies in BaTiO3; especially in terms of the formation of 180o tail-to-tail domain wall. In parallel to the first priciples work, we also prepared high quality samples of PZT 50/50 with different concentrations of donor dopant. We characterized the microstructure and hysteresis loops as a function of donor dopant concentration. Based on experimental observations and ab initio calculations we propose ideas on the origins of hardening and softening in PZT.

Softening and hardening transitions in ferroelectric Pb(Zr,Ti)O3 ceramics

Hysteretic and nonlinear dielectric behaviour in ferroelectric ceramics has been of interest since 1950s, when these materials found application in various electronic devices. Presently, these phenomena concern with important areas of science, technology and engineering. In particular, nonlinearity and hysteresis are the key factors in performance, precision and accuracy of modern devices. Many theoretical and experimental studies have been aimed at understanding the origins of hysteresis and nonlinearity in ferroelectrics. Nowadays, there are several models that describe major contributions to nonlinearity and hysteresis on phenomenological, microscopical or statistical levels. These models have a limited area of applicability due to the complexity of physical processes occurring in real materials. Empirically, hysteresis and nonlinearity in ferroelectrics can be controlled by softening and hardening of the material. This is the case of most widely used ferroelectric, lead zirconate titanate (PZT). The soft compositions possess large electro-mechanical coefficients but also large hysteresis and nonlinearity while the opposite is true for the hard compositions. After fifty years since introduction of these materials, the mechanisms of softening and hardening remain poorly understood. The present study is aimed at a better understanding of the processes leading to hardening and softening of Pb(Zr,Ti)O3 ceramics in order to verify the key principles required for a more universal physical model of hysteresis and nonlinearity. Based on the present state of knowledge, such model should consider domain wall contribution to nonlinear and hysteretic polarization response and at the same time account for hardening and softening of the ferroelectric. For this purpose the well known lead zirconate titanate (PZT) ceramics doped with various concentrations of niobium (soft materials) or iron (hard materials) are chosen as a prototype of the ferroelectric system. The starting hypothesis of the thesis' approach is that the softening and hardening are a result of electrostatic arrangement of charged defects in the ceramic bulk: the hard materials are characterized by the ordered and the soft by disordered defects. The thesis then develops in detail the idea that hardening-softening transitions in a ferroelectric system may occur under the influence of (i) dopants, depending on their type and concentration, (ii) a cyclically applied electric field, (iii) a thermal treatment, and (iv) time. The transition from microscopic order to microscopic disorder is confirmed experimentally using carefully analyzed phenomenological parameters of the macroscopic hysteresis and nonlinearity. Among the nonlinear and hysteretic parameters characterizing the polarization response of a ferroelectric material, some (e.g., third harmonic of polarization) are shown to be particularly sensitive to the softness and hardness of ferroelectric system and thus may serve as the characteristics of ferroelectric hardening-softening transitions. Contribution of domain walls to hysteresis and nonlinearity is analyzed in terms of domain wall energy potential and degree of ordering of pinning centres. It is shown that two existing models characterizing hard (V-potential) and soft (random potential) materials are ideal, limiting cases and that some real materials are described by an intermediary case, which can evolve with time and under influence of external factors. The dielectric characterization performed at wide range of frequencies has revealed an increase of the apparent frequency dispersion of the dielectric permittivity with the transition from the hard to soft state in PZT ceramics. The investigation of dielectric response over a wide temperature range has revealed the profound presence of hopping conductivity in iron doped PZT ceramics below the Curie temperatures and its absence in niobium doped PZT ceramics. The role of hopping charged species in ferroelectric hardening – softening transitions is analyzed and discussed. The thesis is organized in the following way. A brief introduction (Chapter 1) and a literature review of the theoretical description of domain wall contribution to dielectric nonlinearity and hysteresis in ferroelectrics (Chapter 2) is followed by the thesis outline and discussion of a unified model of hysteresis and nonlinearity in ferroelectrics with ordered and disordered states of domain wall pinning centres (Chapter 3). Processing of ceramics is described in Chapter 4 and mathematical and experimental background for the dielectric spectroscopy study in Chapter 5. The results and discussion of detailed experimental studies of polarization response in ferroelectric PZT ceramics under subswitching and switching conditions are given in Chapters 6 and 7. The summary of the main results and conclusions are given in the last thesis section.

EPFL2005

In-situ structural investigations of ferroelasticity in soft and hard rhombohedral and tetragonal PZT

Dragan Damjanovic

Despite the technological importance of hard and soft PZT, Pb(Zr,Ti)O3, ceramics, the mechanisms of ferroelectric hardening and softening remain widely discussed in the literature. The hardening and softening phenomena have traditionally been investigated in relation with dielectric manifestations such as aging of the dielectric susceptibility and constriction of the polarization electric field hysteresis loop. Here, we present a systematic investigation of the ferroelectric and ferroelastic properties of soft and hard PZT in both the tetragonal and rhombohedral phases. A particular focus has been devoted to ferroelastic domain switching by characterizing the macroscopic mechanical constitutive behavior and in-situ synchrotron X-ray diffraction during compression. It is demonstrated that variation of the ordering state of point defects in PZT ceramics affects the switching behavior of both ferroelectric and ferroelastic domains under mechanical or electrical fields. Softening of the mechanical and electrical properties of originally hard PZT ceramics was conferred by quenching the materials from above the Curie temperature. The present findings are discussed with respect to the current understanding of hardening-softening transitions in ferroelectric materials.

American Institute of Physics2015

Extrinsic contributions to the piezoelectric response of lead-based ferroelectrics

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.

EPFL2001

Softening and hardening transitions in ferroelectric Pb(Zr,Ti)O3 ceramics

EPFL2005

Extrinsic contributions to the piezoelectric response of lead-based ferroelectrics

EPFL2001