F.-T. Wang

Electric-field effects of dielectric and optical properties in Pb(Mg1∕3Nb2∕3)0.65Ti0.35O3 crystal

Dielectric properties and domain structure have been measured as functions of temperature in a (001)-oriented Pb(Mg1∕3Nb2∕3)0.65Ti0.35O3 (PMNT35%) single crystal with and without a prior dc electric (E)-field poling. Without E-field poling, the dielectric loss exhibits a frequency-dependent maximum in the region of 120–180K, which can be described by a Vogel–Fulcher equation and fractal cluster model which implies structural irregularities within domains. With a prior poling a long-range monoclinic (tetragonal)→tetragonal (monoclinic) transition takes place near 212K upon heating. “Monoclinic (tetragonal)” represents that dominant monoclinic phase domains coexist with a small fraction of tetragonal phase domains. Optical transmission and birefringence were significantly enhanced by a prior E-field poling. The Cauchy equations for ordinary no and extraordinary ne refractive indices were determined between 0.45 and 1.4μm. However, the phase-matching criterion for second-harmonic generation was not found.

Nanotwins and phases in high-strain Pb(Mg1/3Nb2/3)1−xTixO3 crystal

This work is a study of the thermal stability of (001)-cut Pb(Mg1/3Nb2/3)1−xTixO3 (x=0.30) single crystals before and after an electric (E)-field poling by means of dielectric permittivity, hysteresis loop, domain structure, polarization current, and x-ray diffraction. An R(RNT)-R(RNT)/T(TNT)-T(TNT)-C transition sequence was observed upon heating in the unpoled sample. R, RNT, T, TNT, and C are the rhombohedral, rhombohedral nanotwin, tetragonal, tetragonal nanotwin, and cubic phases, respectively. R/T indicates coexistence of the R and T phases. R(RNT) and T(TNT) indicate that the RNT and TNT structures mimic monoclinic phases in the R and T matrices, respectively. After a prior E-field poling, an R(RNT)-T(TNT)-C phase sequence takes place upon heating. The dielectric permittivity and current density evidenced an additional polarization at 355 K, which is associated with the vanishing of the dielectric dispersion, which reappears near 410 K and remains up to the Burns temperature TB=510 K. This study sug...

Phase transformations in poled PZN-4.5%PT single crystal revealed by combined property measurements and high-resolution diffraction technique

The phase transformation of annealed and poled 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 (PZN-4.5%PT) single crystals has been studied by means of combined bulk property measurements (dielectric permittivity and thermal current density measurements) and microstructural characterization (polarized light microscope and high-resolution x-ray diffraction). Our results showed that upon heating under zero-field condition, poled PZN-4.5%PT undergoes a R-(R+RNT+T+TNT)-(T+TNT)-C transformation sequence, where (R+RNT) and (T+TNT) denote that the rhombohedral and tetragonal phases present consist of a mixture of microscopic (R,T) and nanotwin (RNT,TNT) domains and C denotes the cubic phase. The RNT and TNT nanotwin domains were detected only in the temperature vicinity of the R-T transformation, possibly as a means to alleviate the transformation stress in the material. These nanotwin diffractions can be easily mistaken as monoclinic diffractions.

Surface layer in relaxor ferroelectric PZN-4.5%PT single crystals

The origin of the extremely broad peak at the lower 2θ side of the main (002) rhombohedral x-ray peak in an unpoled (001)-cut Pb(Zn1/3Nb2/3)0.955Ti0.045O3 (PZN-4.5%PT) single crystal has been examined. This peak, located at about 2θ∼43°–44°, was absent on fractured surfaces of the crystal but reappeared when the fractured surfaces were polished mechanically. High-resolution synchrotron x-ray mesh scans revealed that the as-polished surface was covered with a “deformed layer” of heavily stressed rhombohedral phase of monoclinic symmetry having its c axis lying perpendicular to the surface. This deformed phase can be structurally likened to one full of “incipient monoclinic nuclei/phases,” which are microscopic in size and subject to intense compression in the plane of the surface. With a given polishing direction, this surface layer showed parallel domain patterns when viewed under the polarized light microscope. This surface layer could be largely eliminated by appropriate poling at 0.7–1.5 kV/mm at room ...

Dielectric and photovoltaic phenomena in tungsten-doped Pb(Mg1/3Nb2/3)1-xTixO3 crystal

This work investigates dielectric and photovoltaic behaviors in Pb(Mg1∕3Nb2∕3)0.64Ti0.36O3 single crystal doped with 0.5mol% WO3. Dielectric permittivities measured as functions of temperature and frequency reveal two first-order-type phase transitions upon heating and cooling. The photovoltaic response strongly depends on illumination wavelength, sample thickness, and prior electric-field poling. The relation of photovoltage and light intensity under near-ultraviolet (λ=406nm) illumination for the poled samples can be expressed by an exponential equation. Optical transmission reveals that the cutoff wavelength is near 400nm and indicates a minimum electronic energy gap of ∼3.0eV.

Electric-field-induced and temperature-induced phase transitions in high-strain ferroelectric Pb(Mg1/3Nb2/3)0.67Ti0.33O3 single crystal

This work is to study electric (E)-field-induced and temperature-induced phase transitions in (001)-cut Pb(Mg1/3Nb2/3)0.67Ti0.33O3 (PMNT33%) single crystal, which are critical concerns for piezoelectric applications. Dielectric properties and domain structures (by polarizing microscope) are measured as functions of temperature and E field. The hysteresis loop of the polarization versus E field at room temperature is also measured. Without any E-field application, upon heating a first-order-type phase transition sequence rhombohedral (C) takes place near 350 and 430 K, respectively. Under a dc E-field application along [001] at room temperature, [001] tetragonal (T001) phase domains are induced by various phase transition sequences, i.e. , , , and , as the E-field strength increases. In addition, E-field-induced microcracking is observed in this work.

Electric-field-induced dielectric anomalies and optical birefringence in Pb(Zn1∕3Nb2∕3)1−xTixO3 (x=0.10) single crystal

This work aims to study thermal stability, frequency response, and refractive indices in a (001)-cut Pb(Zn1∕3Nb2∕3)0.90Ti0.10O3 (PZNT10%) crystal before and after an electric (E)-field poling, which are important for piezoelectric and optical uses. Dielectric permittivities have been measured as functions of temperature, frequency, and poling strength. The dielectric absorption reveals a broad dipolar relaxation in the region of 260–310K in the unpoled sample. Piezoelectric resonances were observed in the poled sample and show significant changes while phase transitions are taking place. The unpoled crystal shows almost no birefringence, indicating that the structure averaged over the optically anisotropic ferroelectric domains is optically isotropic. Birefringence appears after an E-field poling and shows a “negative” uniaxial distortion. The Cauchy equations for ordinary no and extraordinary ne refractive indices were determined between 0.45 and 1.4μm. However, the phase-matching criterion for second ha...

Phase transformations in annealed PZN-4.5%PT single crystals

Phase transformations of annealed bulk 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 (PZN-4.5%PT) single crystals have been examined by means of dielectric permittivity, thermal current, domain structure, and high-resolution synchrotron x-ray diffraction as a function of temperature. The result shows that annealed (unpoled) bulk PZN-4.5%PT single crystal undergoes an R-T-C transformation sequence upon zero-field heating, where R, T, and C are the rhombohedral, tetragonal, and cubic phases, respectively. Diffractions pertaining to rhombohedral and tetragonal nanotwins are detected. These diffractions can easily be mistaken as the monoclinic diffractions, which, however, are not supported by our thermal current measurement. The nanotwin structure plays an important role in bridging the R-T and T-C transformations, possibly by alleviating the accompanying transformation stresses.

Dielectric, domain, and optical studies in high-Curie-temperature Pb(In1∕2Nb1∕2)1−xTixO3 (x=0.40) single crystal

Dielectric permittivity, domain structure, and hysteresis loop have been measured as a function of temperature in a (001)-cut Pb(In1∕2Nb1∕2)0.6Ti0.4O3 (PINT40%) single crystal. A first-order-type tetragonal/monoclinic→cubic phase transition takes place at TC≅486K, below which tetragonal and monoclinic domains coexist. Frequency-dependent maxima in dielectric absorption appear in the region of 180–260K and can be described by the Vogel-Fulcher equation. Optical transmission and refractive indices were obtained as a function of wavelength. The crystal shows a broad transparency (0.4–6.0μm) and no obvious birefringence, indicating that the crystal structure averaged over the optically anisotropic ferroelectric domains is optically isotropic.

Prior poling effect on thermal phase stability in (110)-cut Pb(Mg1/3Nb2/3)0.74Ti0.26O3 single crystal

Phases and domains in a (110)-cut Pb(Mg1∕3Nb2∕3)0.74Ti0.26O3 (PMNT26%) single crystal have been investigated as functions of temperature and dc electric (E) field by dielectric permittivity, polarizing microscopy, and electric polarization. The unpoled sample has a dominant rhombohedral (R) phase, which coexists with monoclinic (M) phase domains, i.e., R∕M at room temperature. With 45kV∕cm dc poling applied along [110] at room temperature, a single domain of R phase with polar orientation perpendicular to the poling field, i.e., R, was obtained. After the poling was removed, the poled sample has R∕M microdomains, where the M distortion is close to the R phase. The zero-field-heating domain patterns in both unpoled and poled samples exhibit continuous polarization rotation via an intrinsic M phase in the regions of 355-375 and 365-380 K, respectively. Orthohombic and tetragonal phases were not observed in the temperature-dependent study. In general, a rhombohedral/monoclinic→monoclinic→cubic transition s...