Nonlinear electro-mechanical coupling and thermally stimulated currents in relaxor ferroelectrics

In this work, the emergence of polarization and electro-mechanical coupling in Pb(Mg1/3Nb2/3)O3 and Pb(Mg1/3Nb2/3)O3 â PbTiO3 was investigated by means of thermally stimulated current, and nonlinear dielectric and electro-mechanical measurements. The presence of complex time-dependent and interrelated mechanisms, and their influence on macroscopic functional properties was demonstrated. Widely accepted interpretations related to the nature of relaxor ferroelectrics were questioned. Persistent macroscopic polarization in the nominally ergodic and centrosymmetric relaxor state of pristine Pb(Mg1/3Nb2/3)O3 single crystals was revealed by dynamic pyroelectric measurements. Annealing experiments below and above Burns temperature related this polarization then to the presence of nano-sized polar entities. Poling experiments of Pb(Mg1/3Nb2/3)O3 associated a maximum in the electric-field dependent nonlinear dielectric permittivity and an attendant anomaly in the second harmonic polarization response to the reorientation of nano-sized polar entities.In the following, changes in the polar structure of Pb(Mg1/3Nb2/3)O3 were characterizedin-situ with thermally stimulated depolarization current measurements. It was shown thatanomalies can be induced by electrical or mechanical poling at temperatures related to staticand dynamic nano-sized polar regions. An aging effect was revealed with dynamic dielectricand thermally stimulated current measurements. A current reversal was observed around thecoherence temperature in electrical polarized and unpolarized aged samples, suggesting a director indirect correlation between oxygen vacancies and nano-sized polar regions.Based on these observations, the effect of aging on macroscopic functional properties wasinvestigated in Pb(Mg1/3Nb2/3)O3 â PbTiO3 by means of thermally stimulated depolarization current, and dielectric and electro-mechanical measurements. For this purpose, the response ofunpoled, direct current poled and alternating current poled samples was compared for annealedand aged samples. Aging was related to reduced piezoelectric properties and depolarizationtemperatures. An apparent contradicting duality of polarization stabilization and property enhancement was observed for alternating current poling. It was proposed that the applied bipolar poling field leads to a distribution of oriented defect clusters throughout the volume, which could improve dielectric and piezoelectric properties through locally broken symmetry and mediated reversible polarization rotation. The existence of an ergodic relaxor state and the model of a single polar entity below TB are not consistent with the results presented in this work. It is proposed that functional and structural properties of undoped Pb(Mg1/3Nb2/3)O3 and undoped Pb(Mg1/3Nb2/3)O3 â PbTiO3 are considerably affected by defect structures, which depend on the thermal, electric field and mechanical field history, and on time. It was speculated that the multitude of sometimes contradicting experimental results reported in the literature are related to these findings. The outcomes of this work are expected to further the fundamental understanding of relaxor ferroelectrics and to contribute to an enhanced reproducibility of experimental results found in the literature.

Origins of the macroscopic symmetry breaking in centrosymmetric phases of perovskite oxides

Sina Hashemi Zadeh

Many perovskite materials experience a temperature-driven phase transition at the Curie temperature from a non-centrosymmetric polar ferroelectric phase to a paraelectric phase, where polarization is lost. The paraelectric phase is usually centrosymmetric and therefore non-piezoelectric. However, ferroelectrics still show some forms of electro-mechanical coupling in their paraelectric phase like flexoelectricity, which is not symmetry limited and is practically strong in ferroelectrics. It has been shown that the flexoelectric coefficient of perovskite (Ba0.67,Sr0.33)TiO3 in its centrosymmetric paraelectric state at room temperature is up to three to four orders of magnitude larger than its theoretically predicted values. Such enormous discrepancy demonstrates that either the theory is unable to fully explain the flexoelectric response, or that another electro-mechanical mechanism is playing a role in the apparently large flexoelectric response of the material. It was found that Ba0.60Sr0.40TiO3 (BST6040) samples with apparently large flexoelectric coefficient exhibit symmetry breaking and consequently exhibit forbidden piezoelectric- and pyroelectric-like behavior in its paraelectric phase. In this thesis work, our interest lies in describing and interpreting the conditions where breaking of the symmetry is not associated with the external field nor with the flexoelectricity, but is proper to the material. Origins and mechanism of symmetry breaking were investigated in two different cases: local breaking of centrosymmetry; and macroscopic symmetry breaking. The local breaking of the centrosymmetry may refer to the existence of microscopic polar entities and/or disordered displacement of ions in the unit cell. The global symmetry breaking could be associated with a non-uniform distribution of charged defects and/or the coupling of polar nanoâregions with the strain gradient in the sample. We compared dielectric, elastic, and pyroelectric properties of the Ba1-xSrxTiO3 (BST) family, pure BaTiO3 in perovskite (p-BTO) and hexagonal (h-BTO) forms, and SrTiO3 to study the governing mechanisms of symmetry breaking. In addition, the atomic structure of BST6040 was studied by high resolution scanning transmission electron microscopy for direct evidence of polar regions and to understand their exact nature and origin. According to experimental data, the paraelectric phase of p-BTO shows local and global breaking of nominal centrosymmetricity and possesses microscopic and macroscopic polarizations, which are associated with precursors of the ferroelectric phase ordered by the strain gradient in the sample. On the other hand, h-BTO does not exhibit macroscopic pyroelectric response because the precursors are paraelectric-ferroelastic at room temperature, and therefore non-polar. Studying the material response to dynamic electric field showed that the Rayleigh-like relation, which describes dynamics of domain walls in a random pinning environment, cannot describe the dynamics of polar entities in these materials. Results of our high-resolution atomic studies showed distinct static polar clusters with the average size of few nanometers where A-site, B-site and O ions are displaced from their ideal cubic positions. We believe that all presented experimental data can be consistently explained by the presence of microscopic precursors of the ferroelectric phase and their interaction with the strain gradient in the material.

EPFL2017

Electromechanical properties and self-polarization in relaxor Pb(Mg1/3Nb2/3)O-3 thin films

Dragan Damjanovic, Zian Kighelman, Nava Setter

Pyrochlore free Pb(Mg1/3Nb2/3)O-3 (PMN) thin films were prepared from alkoxide-based solution precursors. The influence of different seeding layers and chemical solution on the microstructures is shown. Dielectric, electrostrictive, and piezoelectric properties of the films were characterized in detail. Films show relaxor-like behavior, but with dielectric permittivity which is low (around 4000 at peak) compared to bulk ceramics and single crystals. Several parameters which might be responsible for this lower permittivity are suggested. Electrostrictive coefficients, M and Q, were determined by measuring strain S and polarization P as a function of the electric field (E-ac). At large fields (>2.6x10(6) V/m), S vs P-2 appears to deviate from linear behavior possibly suggesting that the electrostrictive coefficient Q becomes nonlinear in this field range. Investigated as-prepared PMN films exhibit piezoelectric response in the absence of a dc electric field (d(33)=8-20 pm/V). The value of the associated self-polarization in the films is estimated and its presence confirmed by zero-field pyroelectric measurements. The self-polarization and the piezoelectric coefficient are strong functions of the ac field amplitude. Asymmetry of S vs E-ac and d(33) vs E-dc loops are related to the self-polarization. An ac field induces shift in P vs E loops along the field axis. This increase in the coercive field is associated, through a simple model, with the presence of the self-polarization. (C) 2001 American Institute of Physics.

2001

Dielectric and electromechanical properties of ferroelectric-relaxor 0.9 Pb(Mg1/3Nb2/3)O-3-0.1PbTiO(3) thin films

Dragan Damjanovic, Zian Kighelman, Nava Setter

Pyrochlore free 0.9Pb(Mg1/3Nb2/3)O-3-0.1PbTiO(3) thin films were prepared from alkoxide-based solution precursors. Preferential (111) crystallographic orientation was obtained on TiO2/Pt/TiO2/SiO2/Si substrates by spin coating. Dielectric, electrostrictive, and piezoelectric properties of the films were characterized in detail. Films show relaxor-like behavior, but with dielectric permittivity which is low (around 4300 at peak and at 340 Hz with E-ac=1.6x10(6) V/m) compared to bulk ceramics and single crystals. Several parameters which might be responsible for this lower permittivity are suggested. The ac and dc field dependences of the dielectric response have been investigated. Electrostrictive coefficients, M-11(7.76x10(-18) m(2)/V-2) and Q(11)(1.9x10(-2) m(4) C-2), were determined by measuring strain and polarization as a function of the electric field (E-ac). The maximum field induced piezoelectric d(33) coefficient is 100 pm/V and electrostrictive strains up to 1.2x10(-3) (with an ac electric field of 140 kV/cm) were measured. (C) 2001 American Institute of Physics.

2001

Origins of the macroscopic symmetry breaking in centrosymmetric phases of perovskite oxides

Sina Hashemi Zadeh

EPFL2017