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.

Correlation between dielectric anisotropy and positive or zero transverse piezoelectric coefficients in perovskite ferroelectric single crystals

Dragan Damjanovic, Matthew Davis, Nava Setter

The transverse piezoelectric coefficient (d(31)(star)) has been calculated for tetragonal barium titanate (BT) and lead titanate (PT) cut along nonpolar axes, over a range of temperatures, using the phenomenological Landau-Ginzburg-Devonshire theory. It is shown that negative values of d(31)(star) are favored when the transverse dielectric susceptibility eta(11) and shear coefficient d(15) are large, such that polarization rotation is more significant than the collinear piezoelectric effect. This occurs in tetragonal BT close to its ferroelectric-ferroelectric phase transition to an orthorhombic phase. In PT, however, where no phase transition occurs, d(15) is small and polarization "extension" due to d(33) dominates: small or positive values of d(31)(star) become prominent. This extends a previous result found for polycrystalline modified PT ceramics. (c) 2005 American Institute of Physics.

2005

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

Anand Chandrasekaran

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.

EPFL2016

Pulsed laser deposition of (SrBa)Nb2O6 thin films and their properties

Anna Infortuna

Strontium barium niobate (SrxBa1-xNb2O6, shortly SBN) is a solid solution system with tetragonal tungsten bronze crystal structure. It exhibits a ferroelectric phase with only one polar axis and a transition temperature depending on the Sr/Ba ratio. This thesis studies the growth of SBN thin films, in order to obtain a ferroelectric thin film model system for the study of 180° domain switching close to the ferroelectric phase transition. SBN has been chosen not only because uniaxial, but because it is possible to tune the temperature of phase transition varying the Sr/Ba ratio. It is thus possible to study electric properties of the phase transition close to room temperature, where conduction phenomena related to defects are normally lower. Thin films of SBN were grown by Pulsed Laser Deposition in a vacuum system constructed during this thesis work. For the formulation of a model it is desirable to have a system as close as possible to a single crystal. Therefore deposition parameters have been optimized to grow SBN thin films with the polar axis oriented perpendicular to the film plane. The films are integrated in parallel plate capacitor structures, in order to measure dielectric properties. The stability of the ferroelectric phase is found to be sensitive to impurities; contaminants with concentration below 1% induce a lowering of the transition temperature of about 100°C. Strain as well has influence on the phase transition; in the case of substrate with thermal expansion coefficients different from the SBN ones, the SBN film is under stress. A 0.6% tensile strain induces a lowering of the transition temperature of about 100°C, this is the case of silicon single crystal substrate. Grown on strontium titanate single crystals (STO), whose thermo-mechanical properties are close to the one of SBN, the films exhibit a phase transition temperature compatible with the one measured on single crystals. The obtained films suffer from leakage that is reduced by thermal treatment in oxygen rich atmosphere. The oxygen vacancies, created during the deposition process, are considered to be responsible of leakage. Vacancies recovery upon annealing does not eliminate the problem, and measurement of polarization at room temperature is difficult. However, piezoelectric measurements performed on individual grains, with atomic force microscope, prove that these have ferroelectric properties; by these experiment has been proved that it is possible to switch the polarization. Films with a disordered structure of grains are much less leaky than the ones with columnar grains spanning the whole cross section. These observations lead to the conclusion that conduction goes trough the grain boundaries. On STO single crystals, SBN grows epitaxial in Volmer-Weber mode. With a suitable preparation of the substrate surface, it is possible to induce the growth of films with different orientations of the polar axis: in the film plane, perpendicular to it or inclined of 45°. A model is proposed, explaining the epitaxial relation with the substrate on the basis of the perovskite kernel contained in the unit cell of the tetragonal tungsten bronze structure. The film grows from the nucleation of the perovskite structure that rules the orientation of the film; the high temperature at which the substrate is kept during the deposition, assure the necessary mobility for the arriving atoms to organize in the SBN structure around these nuclei. The study of literature data allows to state that such a model is valid not only for substrates with perovskite structure, but can be extended to substrates with a cubic crystal structure, like magnesium oxide.