Ferroelectric BaTiO3 Single Crystals Research Articles

Bulk Photovoltage Effect in Ferroelectric BaTiO3.

Due to the unusual charge separation mechanism and anomalous photovoltaic effects, the bulk photovoltage effect in ferroelectric semiconductors has attracted a great deal of attention in solar energy conversion, especially in attempts to utilize nonthermalized carriers. Among the various mechanisms that have been proposed for interpreting the photovoltaic effect, a shift mechanism was derived from quantum phenomena, which have been modeled and studied for many years. However, the concurrent shift and ballistic mechanism make investigating the ferroelectric bulk photovoltage effect complex and challenging. Here, taking a tetragonal ferroelectric BaTiO3 single crystal as a prototype, we report an approach for distinguishing the shift and ballistic mechanism-induced surface photovoltage. The results indicate different effects on the charge separation of the ballistic mechanism and shift mechanisms, as evidenced by surface photovoltage measurement. Interestingly, the shift and ballistic mechanisms afford charge separation in two opposite directions but on the same order of magnitude under monochromatic superband illumination. Our results provide facile and efficient methods for clarifying the shift and ballistic mechanisms in ferroelectrics.

Bipolar charge collecting structure enables overall water splitting on ferroelectric photocatalysts

Ferroelectrics are considered excellent photocatalytic candidates for solar fuel production because of the unidirectional charge separation and above-gap photovoltage. Nevertheless, the performance of ferroelectric photocatalysts is often moderate. A few studies showed that these types of photocatalysts could achieve overall water splitting. This paper proposes an approach to fabricating interfacial charge-collecting nanostructures on positive and negative domains of ferroelectric, enabling water splitting in ferroelectric photocatalysts. The present study observes efficient accumulations of photogenerated electrons and holes within their thermalization length (~50 nm) around Au nanoparticles located in the positive and negative domains of a BaTiO3 single crystal. Photocatalytic overall water splitting is observed on a ferroelectric BaTiO3 single crystal after assembling oxidation and reduction cocatalysts on the positively and negatively charged Au nanoparticles, respectively. The fabrication of bipolar charge-collecting structures on ferroelectrics to achieve overall water splitting offers a way to utilize the energetic photogenerated charges in solar energy conversion.

Imaging microstructural dynamics and strain fields in electro-active materials in situ with dark field x-ray microscopy.

The electric-field-induced and temperature induced dynamics of domains, defects, and phases play an important role in determining the macroscopic functional response of ferroelectric and piezoelectric materials. However, distinguishing and quantifying these phenomena remains a persistent challenge that inhibits our understanding of the fundamental structure-property relationships. In situ dark field x-ray microscopy is a new experimental technique for the real space mapping of lattice strain and orientation in bulk materials. In this paper, we describe an apparatus and methodology for conducting in situ studies of thermally and electrically induced structural dynamics and demonstrate their use on ferroelectric BaTiO3 single crystals. The stable temperature and electric field apparatus enables simultaneous control of electric fields up to ≈2 kV/mm at temperatures up to 200 °C with a stability of ΔT = ±0.01 K and a ramp rate of up to 0.5 K/min. This capability facilitates studies of critical phenomena, such as phase transitions, which we observe via the microstructural change occurring during the electric-field-induced cubic to tetragonal phase transition in BaTiO3 at its Curie temperature. With such systematic control, we show how the growth of the polar phase front and its associated ferroelastic domains fall along unexpected directions and, after several cycles of electric field application, result in a non-reversible lattice strain at the electrode-crystal interface. These capabilities pave the way for new insights into the temperature and electric field dependent electromechanical transitions and the critical influence of subtle defects and interfaces.

Synchrotron radiation analyses of domain switching behaviors for ferroelectric BaTiO3 single crystals under electric fields

The domain structures and their dynamics of BaTiO3 single crystals under in-situ electric fields along the 〈101〉 direction were investigated by using synchrotron radiation single-crystal X-ray diffractions. Diffraction patterns clearly show the presence of a 90° domain structure in 〈101〉-poled crystals. The linear characteristics of the strain and the polarization with respect to unipolar electric fields along the 〈101〉 direction suggest that the 90° domain structures established by poling treatments are not changed by applying electric fields. The diffraction analysis, however, provides direct evidence for a reversible change in the volume fractions of two kinds of 90° domains under unipolar in-situ electric fields.

Ferroelectric BaTiO3 single crystal under superstrong electric fields up to 55 MV/m: A comparative experimental and theoretical study

We investigated the effect of huge electric field up to E=55 MV/m on tetragonal ferroelectric phase of BaTiO3 single crystal. Both lattice parameters and the dielectric susceptibility along the polar axis were determined. The induced polarization shows a continuous increase up to 20% of the spontaneous polarization. The longitudinal d33 and transverse d31 piezoelectric coefficients were also determined. The experimental data are described by a phenomenological approach. Interestingly, an intermediate critical electric field value of E∼25 MV/m was evidenced suggesting that the physics below and above this value are different.

Influence of structural transitions of BaTiO3 on the magnetic properties of Fe nanoparticles

Ferroelectric BaTiO3 single crystals with (001) orientation were implanted with Fe at room temperature with an energy of 100 keV and a fluence of 1×1017 cm-2. For the implanted crystals, the analysis of the magnetic properties shows that Fe nanoparticles are formed and display characteristic superparamagnetic behaviour. The thermal dependence of the nanoparticles magnetisation shows the signature of the structural phase transitions of BaTiO3 with changes in the magnetisations values and thermal hysteresis observed at related temperatures.

Dynamic ferroelectric hysteresis scaling of BaTiO3 single crystals

The scaling behavior of the dynamic hysteresis of ferroelectric BaTiO3 single crystals was investigated. Two sets of the scaling relation of hysteresis area ⟨A⟩ against frequency f and field amplitude E0 were clearly established. Above the coercive field, the scaling took a form of ⟨A⟩∝f−0.195E00.950. On the other hand, the scaling in the form of ⟨A⟩∝f1.667E0−2.804E04.157 was obtained under subcoercive field condition. While these scaling relations were generally comparable to previously reported ones, it was found that the f and E0 exponents depended on E0 and f, respectively, which was in contrast to the prior theoretical prediction and experimental investigations.

Reorientation of (MnTi″−VO••)× defect dipoles in acceptor-modified BaTiO3 single crystals: An electron paramagnetic resonance study

The effect of external electric fields on the orientation of (MnTi″−VO••)× defect dipoles in ferroelectric BaTiO3 single crystals and its interplay with the domain structure were investigated by means of electron paramagnetic resonance (EPR) spectroscopy and optical microscopy. The results show that in specimens aged in the ferroelectric state for a defined time, so-termed ferroelectrically aged, the (MnTi″−VO••)× defect dipoles orient along the direction of spontaneous polarization and follow the domain switching upon poling with correspondingly high electric fields. A comparison of the EPR signal in poled aged single-domain samples to that in naturally aged multidomain samples indicates a similar reorientation process of the defect dipole, which means that dipole reorientation rather requires long time, thermal energy, and high electric fields, i.e., more energy.

Dielectric Constant Evaluation of Dielectric Powders by Complex Impedance Analysis with Model of RC–RC Parallel Circuits Connected in Series

A new evaluation method for the high dielectric constants of SrTiO3 and BaTiO3 powders has been developed using the complex impedance measurement analyzed by a model of RC–RC parallel circuits that are individually connected in series. The model properly represented the various dispersions of the powders in the slurry specimens. This evaluation method was experimentally examined using SrTiO3 powder with an average particle size of 300 nm and BaTiO3 powder, 500 nm. The dielectric constants were obtained by (1) fitting the complex impedance of the models of RC parallel circuits connected in series to the measured complex impedance of the slurry specimens and (2) applying simultaneous equations to calculate each parameter of the model of RC–RC parallel circuits connected in series. The obtained dielectric constants of the SrTiO3 and BaTiO3 powders were estimated to be 232±3 and 2,610±190, respectively. They were in good agreement with the dielectric constant of the paraelectric SrTiO3 single crystal and the average value of the ferroelectric BaTiO3 single crystal.

Scanning electron acoustic microscopy for the evaluation of domain structures in BaTiO3 single crystal and ceramics

As a non-destructive and subsurface detecting technique, the scanning electron acoustic microscopy (SEAM) method has been applied to study the ferrodomain structures in ferroelectric BaTiO3 single crystal and ceramics. The domain arrangements and the orientations of domain walls at different geometry structures in the BaTiO3 single crystal, and the relationships of domain structures with surface grains in the BaTiO3 ceramics, have been discussed by analysing the experimental results obtained at different operation conditions. The distributions of electron acoustic signals with modulation frequencies up to 1 MHz have been obtained. The relationship of the electron acoustic signal with incident electron energy has also been studied. Although the thermal wave coupling mechanism makes a certain contribution to the acoustic signal generations, the image contrast of ferrodomains is dominated by differences in the electrical properties of ferroelectric materials. © 1998 Kluwer Academic Publishers

Low frequency electroluminescence in ferroelectric BaTiO3 single crystals

Electroluminescence has been studied in ferroelectric BaTiO3 single crystals. The variation of EL with potential temperature and d.c. biasing suggests the existence of complex surface charge layer on the surface of these crystals. The mode of growth of EL pulses reveals the nature of these layers.