Xiaoli Tan

Compositional disorder, polar nanoregions and dipole dynamics in Pb(Mg1/3Nb2/3)O3-based relaxor ferroelectrics

Abstract The complex structure of relaxor ferroelectrics comprises polar nanoregions (PNRs) which appear upon cooling below the Burns temperature and quenched compositional (chemical) disorder. The relation between the polar nanostructure and compositionally ordered regions (CORs) often observed in relaxors has been the subject of extensive theoretical investigations; however, the experimental data, especially concerning Pb(B′1/3B″2/3)O3-type complex perovskite relaxors, are rather limited. In this paper, we analyse and discuss the results of our recent investigations of the morphology of CORs and the dynamics of PNRs in Pb(Mg1/3Nb2/3)O3 based solid solutions in which the degree of compositional disorder was varied by means of changing the composition and/or by means of high-temperature annealing. The samples were characterised using X-ray diffraction, transmission electron microscopy, piezoresponse force microscopy, Brillouin light scattering, dielectric spectroscopy, as well as by measuring pyroelectric effect and ferroelectric hysteresis loops. No influence of the size of CORs on the PNRs relaxation in the ergodic relaxor phase is found. Instead, the CORs size influences significantly the diffuseness of the transition from the field-induced ferroelectric phase to the ergodic relaxor state. The results are interpreted in the framework of a model suggesting the coexistence of static and dynamic PNRs in the ergodic relaxor phase.

Impact of phase transition sequence on the electrocaloric effect in Pb(Nb,Zr,Sn,Ti)O3 ceramics

The phase transition sequence in PbZrO3-based ceramics can be readily altered by chemical modification. In Pb0.99Nb0.02[(Zr0.57Sn0.43)0.92Ti0.08]0.98O3 (PNZST 43/8/2), the sequence is ferroelectric–antiferroelectric–paraelectric during heating, while in Pb0.99Nb0.02(Zr0.85Sn0.13Ti0.02)0.98O3 (PNZST 13/2/2), it is antiferroelectric–ferroelectric–paraelectric during heating. The electrocaloric effect associated with the antiferroelectric ↔ ferroelectric phase transition is studied in both ceramics via indirect measurement. PNZST 43/8/2 is observed to display a positive electrocaloric effect; in contrast, PNZST 13/2/2 exhibits a negative effect.

Enhanced ordered structure and relaxor behaviour of 0.98Pb(Mg1/3Nb2/3)O3–0.02La(Mg2/3Nb1/3)O3 single crystals

High quality Pb(Mg1/3Nb2/3)O3 (PMN) and La-doped PMN (PLMN) single crystals were grown via a top-seeded solution growth method. At a doping level as low as 2 at.% of La, the 1:1 B-site cation (chemical) order was significantly enhanced as revealed by the presence of superlattice peaks in the x-ray diffraction pattern and comparatively large (~100 nm) chemically ordered regions (CORs) in the transmission electron microscopy dark field images. The average chemical order parameter was calculated (from the intensity of superlattice peaks) to be S≈0.44. Besides, x-ray diffraction revealed the presence of a low-symmetry (presumably rhombohedral) phase. The effect of La doping on the dielectric relaxation was not substantial. Though the magnitude of the dielectric constant peak in PLMN was reduced by half as compared to pure PMN, the peak temperature and diffuseness as well as the shape of the dielectric relaxation spectra remained almost unchanged. The analysis of the temperature dependence of the characteristic time for the main relaxation process in terms of the Vogel–Fulcher law revealed the freezing of dipole dynamics at temperature Tf = 192 K, which was only slightly lower than the Tf = 213 K in PMN. To reconcile the structural and dielectric data we suggest that the rhombohedral phase forms inside CORs while the chemically disordered matrix remains cubic and is populated by dynamic polar nanoregions that give rise to the characteristic dielectric response.

In situ transmission electron microscopy study of the nanodomain growth in a Sc-doped lead magnesium niobate ceramic

Sc doping enhances the B-site 1:1 cation ordering in the Pb(Mg1∕3Nb2∕3)O3 ceramic. At low doping levels, the electrical polar domains remain at the nanometer scale and the relaxor ferroelectric behavior persists. The electric field-induced relaxor to normal ferroelectric phase transition process was directly observed with an in situ transmission electron microscopy technique in a Sc-doped Pb(Mg1∕3Nb2∕3)O3 polycrystalline sample. It was found that the phase transition started at the grain boundary and took two steps to complete: the gradual coalescence of the polar nanodomains and the abrupt formation of the large wedge-shaped ferroelectric domains.

Acoustic emission and dielectric studies of phase transitions within the morphotropic phase boundary of xPb(Zr1/2Ti1/2)O3-(1−x)Pb(Ni1/3Nb2/3)O3 relaxor ferroelectrics

We have carried out a combined acoustic emission (AE) and dielectric permittivity study of the xPb(Zr1/2Ti1/2)O3-(1−x)Pb(Ni1/3Nb2/3)O3 relaxor ferroelectric ceramics with compositions x=0.7–0.9 corresponding to its morphotropic phase boundary. Temperatures of all phase transitions occurring on heating are identified accurately by AE, and a direct transition between the low-temperature (rhombohedral) and high-temperature (pseudocubic) relaxor phases is found. The AE peak intensity is generally proportional to the temperature derivative of the dielectric permittivity, in agreement with a model proposed for a thermally cycled small elastic dipole.

Double hysteresis loops at room temperature in NaNbO3-based lead-free antiferroelectric ceramics

ABSTRACT Polarization-field double hysteresis loops have hardly ever been observed at room temperature in polycrystalline NaNbO3, one of a few lead-free antiferroelectric compounds. In this Letter, it is shown that the exposure of a modified NaNbO3 ceramic to bipolar electric fields of ±160 kV/cm at 100°C can preserve the double hysteresis loops at room temperature. These double hysteresis loops can still be observed after 125 days room temperature aging with some decay in maximum polarization. Frequency dependence analysis suggests that the double hysteresis loops originate from the antiferroelectric ↔ ferroelectric phase transition, not the charged defects. GRAPHICAL ABSTRACT IMPACT STATEMENT A novel electrical treatment of a NaNbO3-based polycrystalline ceramic to achieve double hysteresis loops at room temperature is reported for the first time.

Suppression of the antiferroelectric phase during polarization cycling of an induced ferroelectric phase

The ceramic Pb0.99Nb0.02[(Zr0.57Sn0.43)0.92Ti0.08]0.98O3 can exist in either an antiferroelectric or a ferroelectric phase at room temperature, depending on the thermal and electrical history. The antiferroelectric phase can be partially recovered from the induced ferroelectric phase when the applied field reverses polarity. Therefore, polarization cycling of the ferroelectric phase in the ceramic under bipolar fields at room temperature is accompanied with repeated phase transitions. In this letter, the stability of the recovered antiferroelectric phase upon electrical cycling of the ceramic is investigated. Ex-situ X-ray diffraction reveals that bipolar cycling suppresses the antiferroelectric phase; this is indirectly supported by piezoelectric coefficient d33 measurements. It is speculated that the accumulated charged point defects during polarization cycling stabilize the polar ferroelectric phase. The findings presented are important to the fundamental studies of electric fatigue and field-induced p...

Domain disruption and defect accumulation during unipolar electric fatigue in a BZT-BCT ceramic

0.5Ba(Zr0.2Ti0.8)O30.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) is a promising lead-free piezoelectric ceramic with excellent piezoelectric properties (e.g., d33 > 600 pC/N). As potential device applications are considered, the electric fatigue resistance of the ceramic must be evaluated. In this Letter, electric-field in situ transmission electron microscopy is employed to study the microstructural evolution in the BZT-BCT polycrystalline ceramic during unipolar cycling. It is shown that the large ferroelectric domains are disrupted and replaced with accumulated defect clusters and fragmented domains after 5 × 104 unipolar cycles. In this fatigued state, the grain becomes nonresponsive to applied voltages.0.5Ba(Zr0.2Ti0.8)O30.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) is a promising lead-free piezoelectric ceramic with excellent piezoelectric properties (e.g., d33 > 600 pC/N). As potential device applications are considered, the electric fatigue resistance of the ceramic must be evaluated. In this Letter, electric-field in situ transmission electron microscopy is employed to study the microstructural evolution in the BZT-BCT polycrystalline ceramic during unipolar cycling. It is shown that the large ferroelectric domains are disrupted and replaced with accumulated defect clusters and fragmented domains after 5 × 104 unipolar cycles. In this fatigued state, the grain becomes nonresponsive to applied voltages.

Introduction to the IEEE International Symposium on Applications of Ferroelectrics and International Symposium on Piezoresponse Force Microscopy and Nanoscale Phenomena in Polar Materials

The 20th IEEE International Symposium on Applications of Ferroelectrics (ISAF) was held on July 24-27, 2011, in Vancouver, British Columbia, Canada, jointly with the International Symposium on Piezoresponse Force Microscopy and Nanoscale Phenomena in Polar Materials (PFM). Over a period of four days, approximately 400 scientists, engineers, and students from around the world presented their work and discussed the latest developments in the field of ferroelectrics, related materials, and their applications. It is particularly encouraging to see that a large number of students (115) were attracted to the joint conference and presented high-quality research works. This trend is not only important to this conference series, but more importantly, it is vital to the future of the ferroelectrics field.