Chi-Shun Tu

Dielectric, hypersonic, and domain anomalies of (PbMg1/3Nb2/3O3)1-x(PbTiO3)x single crystals

Dielectric permittivities, Brillouin backscattering spectra, polarization-electric field hysteresis loops, and domain structures have been measured as a function of temperature in relaxor-based ferroelectric single crystals (PbMg1/3Nb2/3O3)1−x(PbTiO3)x(PMN-xPT) for x=0.24 and 0.34. For PMN-24%PT, a diffuse phase transition which is associated with a broad frequency-dependent dielectric maximum was observed near 380 K. As the temperature increases, the PMN-24%PT crystal gradually develops cubic regions and is fully converted into the cubic state near 375 K. An extra dielectric anomaly appears at 370 K, possibly due to the percolating polar cluster induced by an external electric field. PMN-34%PT exhibits a nearly normal ferroelectric phase transition near 445 K from the tetragonal to the cubic phase. In addition, a weak diffuse phase transition observed near 280 K may result from partial conversion of rhombohedral phase to tetragonal phase. The dielectric thermal hysteresis confirms that the transitions ne...

Magnetoelectric coupling and phase transition in BiFeO3 and (BiFeO3)0.95(BaTiO3)0.05 ceramics

In situ high-resolution synchrotron x-ray diffraction reveals a local minimum in rhombohedral distortion angle αR (associated with an inflection in the lattice constant aR) near 400 and 350u2005°C in BiFeO3 (BFO) and (BiFeO3)0.95(BaTiO3)0.05 (BFO–5%BT), respectively. It suggests a coupling between ferroelectric and magnetic parameters near the antiferromagnetic–paramagnetic transition, which is responsible for the broad frequency-dependent dielectric maxima. A rhombohedral (R)–orthorhombic (O)–cubic (C) transition sequence takes place near 820 and 850u2005°C in BFO upon heating. BFO–5%BT exhibits a R–C transition near 830u2005°C. The BaTiO3 substitution can enhance dielectric and ferromagnetic responses and reduce electric leakage. The dielectric loss of BFO–5%BT remains less than 0.04 below 150u2005°C.

E-field-induced polarization rotation in Pb(Mg1/3Nb2/3)1−xTixO3 crystal

A sequence of field-induced phase transformations was observed by means of a polarizing microscope on a (111)-cut single crystal Pb(Mg1/3Nb2/3)0.67Ti0.33O3 at room temperature with an electric field applied along the [111] direction. As electric field increases, polarizations of the rhombohedral R domains whose polar directions are not along [111] rotate first toward a tetragonal T phase associated with 90° domain walls. Then the crystal gradually reaches total optical extinction for all polarizer angles at 12 kV/cm, indicating a single [111] rhombohedral R111 domain with polarization aligned with the electric field. These field-induced phase transformations are irreversible and proceed most likely through MA-type monoclinic distortions, i.e., R→MA→T→MA→R111.

Photovoltaic phenomena in BiFeO3 multiferroic ceramics

Open-circuit photovoltage and short-circuit photocurrent were investigated in BiFeO3 (BFO) ceramics as functions of laser wavelength (λ u2009=u2009 373 and 532u2009nm), illumination intensity, and sample thickness. BFO ceramics exhibit significant photovoltaic responses under near-ultraviolet illumination of λu2009=u2009373u2009nm. The photovoltaic responses strongly depend on wavelength, light intensity, and sample thickness. The relation between photovoltaic responses and light intensity can be described by exponential equations, EOCu2009=u2009ES[1 – exp(–I/α)] and JSCu2009=u2009JS[1 – exp(−I/β)]. EOC, JSC, ES, and JS are open-circuit photovoltage (V/cm), short-circuit photocurrent density (A/cm2), saturated photovoltage (V/cm), and saturated photocurrent density, respectively. This work suggests that BFO ceramic exhibits stronger photovoltaic responses than the ferroelectric WO3-doped Pb1-xLax(ZryTiz)1-x/4O3 ceramics and Pb(Mg1/3Nb2/3)1-xTixO3 crystals.

Temperature- and electric-field-dependent domain structures and phase transformations in (001)-cut tetragonal Pb(Mg1∕3Nb2∕3)1−xTixO3 (x=0.40) single crystal

Temperature- and electric (E)-field-dependent domain structures and phase transformations in a (001)-cut Pb(Mg1∕3Nb2∕3)0.6Ti0.4O3 (PMNT40%) single crystal have been investigated by polarizing microscopy and dielectric permittivity. The unpoled crystal has tetragonal phase domains (15–60μm wide) with polarizations along [100] and [010], which are separated by 90° domain walls. They coexist with a small fraction of the monoclinic phase domains near 200K. The tetragonal domains increase rapidly at the expense of the monoclinic domains as the temperature increases toward room temperature. This may account for the steplike dielectric anomaly in e′ from around 280K. The whole crystal reaches the cubic phase near 464K. The E-field-dependent domain structures at room temperature show a significant change near 11kV∕cm when the polarizations rotate to the tetragonal [001] (T001) direction through the monoclinic phase. Microcrackings begin to develop near 12.5kV∕cm. The entire crystal becomes T001 monodomain near 33...

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.

Polar nanoregions and dielectric properties in high-strain lead-free 0.93(Bi1/2Na1/2)TiO3-0.07BaTiO3 piezoelectric single crystals

A structural coexistence of rhombohedral (R) and tetragonal (T) phases has been revealed in the (001)c-cut lead-free 0.93(Bi1/2Na1/2)TiO3–0.07BaTiO3 (BNB7T) piezoelectric crystals, which grown by the self-flux method, in the lower temperatures by high-resolution synchrotron X-ray diffraction, reciprocal space mapping, and transmission electron microscopy. The dielectric permittivity exhibits a thermal hysteresis in the region of 120–260u2009°C, implying a first-order-like phase transition from R+T to T. The real part (e′) of dielectric permittivity begins to deviates from the Curie-Weiss equation, e′u2009=u2009C/(T − To), from the Burns temperature TBu2009=u2009460u2009°C, below which the polar nanoregions (or nanoclusters) develop and attenuate dielectric responses. The polar nanoregions of 5–10u2009nm were revealed by high-resolution transmission electron microscope. The normal piezoelectric coefficient d33 exhibits a rapid increase at Eu2009=u200915–20u2009kV/cm and reaches a maximum of d33 ∼450 pC/N. The high piezoelectric response and E-fi...

Phases and Domain Structures in Relaxor-Based Ferroelectric (PbMg1/3Nb2/3O3)0.69(PbTiO3)0.31 Single Crystal.

The Brillouin back-scattering spectra, dielectric permittivities, polarization–electric field (P–E) hysteresis loops and domain structures have been measured as a function of temperature in a relaxor-based ferroelectric single crystal (PbMg1/3Nb2/3O3)0.69(PbTiO3)0.31 (PMN–31%PT). In order of increasing temperature, PMN–31%PT undergoes successive phase transitions: rhombohedral phase (below ~370 K) →coexistence of rhombohedral and tetragonal phases (between ~370 and ~380 K) →tetragonal phase (between ~380 and ~400 K) →coexistence of tetragonal and cubic phases (betwee ~400 and ~420 K) →cubic phase (above ~420 K). An extra ferroelectric anomaly of the dielectric permittivity appears at 370 K possibly due to the percolating polar cluster induced by an external electric field. It was found that different individual domain regions have different transition temperatures. This phenomenon suggests an inhomogeneous distribution of Ti4+ concentration in the PMN–31%PT crystal. The dielectric permittivity e of PMN–31%PT obeys the empirical relation, em/eγ(f,T)=1+[T-Tm(f)]/2δγ2, above the temperature of permittivity maximum Tm.

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.

Polarization Rotation and Monoclinic Phase in Relaxor Ferroelectric PMN‐PT Crystal

A monoclinic phase is evidenced between rhombohedral and cubic phases in a 〈111〉‐cut single crystal PMN‐33%PT from polarizing microscope observation of domain structures. Some rules for interpreting these observations for various cuts are reviewed, to illustrate how particular cuts are useful for distinguishing among particular types of phases and domains. Near 360 K the structure begins to distort from the rhombohedral toward the tetragonal phase through monoclinic domains (probably MA‐type but perhaps MB‐type). However, the present 〈111〉‐cut crystal seems to disfavor the tetragonal phase and persists in the monoclinic phase up to T∼420 K, where the cubic phase begins to develop. Temperature‐dependent orientations of optical indicatrices of domains indicate polarization rotations within the monoclinic planes. In addition, a previous electric‐field‐cooled process enhances long‐range order in domain patterns observed upon heating. At room temperature, with increasing E‐field along [111], the crystal underg...