BaTiO3 Single Crystals Research Articles

Wavelength dependence of refractive index in lead-free Na0.5Bi0.5TiO3–BaTiO3 single crystals

Refractive indices of (1−x)Na0.5Bi0.5TiO3–xBaTiO3 (NBT–xBT, x=0, 0.06 and 0.08) single crystals were measured at room temperature after poled along pseudo-cubic crystallographic direction [001]. The refractive indices decrease dramatically when the wavelength increases for all crystals. At the same wavelength, refractive indices of NBT–xBT single crystals decrease with increasing BT content. Sellmeier dispersion equations were obtained by least square fitting, which can be used to calculate the refractive indices in low absorption wavelength range. Parameters connected to the energy band structure were determined by fitting single-oscillator dispersion equation. Similar to most oxygen-octahedral ferroelectrics, NBT–xBT crystals have the same dispersion behavior described by the refractive-index dispersion parameter. Dispersion energies take on covalent crystal values.

Nanomaterials and Nanodevices for Energy Applications

Energy generation and storage are perhaps two of the most imperative, ongoing, and challenging issues facing the world to date. The rapidly growing global energy demand is placing increasingly greater burdens on the incumbent fossilfuel based infrastructure necessitating the development and mobilization of alternative technologies, particularly those focused on exploiting or scavenging new and clean energy sources alongside the development of highly efficient energy storage devices. Though advanced energy storage concepts and regenerative energy systems have been the focus of considerable attention over the past decade, the advent of the nanoage added traction to field by promising the potential to fabricate, for the first time, a wide range of truly novel devices that will enable this paradigm shift in energy applications. However, breakthroughs are needed to synthesize, engineer, and manufacture, en masse, nanomaterials and microand macrostructures based thereon. Central to this is a new understanding of the atomistic assembly of such materials in order to study energy transduction at nanoscale and to elucidate the underlying chemical and physical mechanism involved in high efficient energy conversion and storage. This special issue—which focuses on the science and engineering of nanomaterials and nanodevices used in all forms of energy harvesting, conversion, storage, and utilization—consists of 13 papers spanning topics detailing the empirical exploitation of nanowires for enhanced supercapacitors to thermoelectric generators, photocatalytic nanoparticles, and small molecule based organic solar cells. L. Chen et al. discuss the use of scanning force microscopy to investigate frictional images of newly cleaved surface domains of single crystal BaTiO 3 under ambipolar biases, providing a new method to determine domain polarization direction, whereas H.-S. Tsai et al. synthesized novel onedimensional SiO 2 /Ta 2 O 5 core-shell nanostructures, proposing a Ta-O diffusion-controlled growth process. P. Hiralal et al. report on the tailored growth of carbon nanostructures for application in high-frequency supercapacitors formed from engineered vertically aligned nanocarbon electrodes which are shown to enhance the porosity and electrolyte permeability within the device, resulting in a novel fabrication route to realise a functionally advanced compact energy storage platform. In a similar approach, H. Xu et al. summarise their work towards the preparation of Co 3 O 4 nanoparticles which facilitate long-lifetime and electrochemically superior supercapacitors by undertaking detailed studies of the electrode metrology. Z. Ma et al. report on energy recovery of coolant heat for internal combustion engines using thermoelectric generation technology. They showed that cooling improved with increased fan speeds and that the Seebeck coefficient was reduced with an increase in temperature difference between the two ends of thermoelectric generator.

Abnormal thermal conductivity of [720]‐oriented BaTiO3 single crystal

AbstractThe 3ω ‐scanning thermal microscopy and piezoresponse force microscopy techniques were combined to characterize the local thermal conductivity (λ) of ferroelastic domains in [720]‐oriented and [001]‐oriented BaTiO3 single crystal. The [720]‐oriented BaTiO3 showed a local λ value of 3.39 W/m·K, while it was 5.95 W/m·K for [001]‐oriented BaTiO3. The underlying mechanism for such drastic thermal conductivity reduction in [720]‐oriented BaTiO3 is ascribed to a stronger phonon‐scattering effect due to the high strains appearing in [720]‐oriented BaTiO3 single crystals. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Landau expansion parameters for BaTiO3

The 6th order Taylor expansion of the free energy in terms of the order parameter has been generally accepted to describe first order structural phase transitions. In recently years, some researchers added the 8th order terms in order to explain the two branches in dielectric loops observed in the BaTiO3 single crystal. We show that the Landau free energy terminated at the 6th orders is sufficient to fit the experimental observations if the rich information in the field-induced phase transitions is explored. By using the typical characteristics of ferroelectric and dielectric double loops above the Curie temperature Tc, the temperature-dependent expansion coefficients can be determined with high accuracy.

In-situ observation of crack growth and domain switching around vickers indentation on BaTiO3 single crystal under sustained electric field

The crack propagation and domain switching process around the indentation on the surface of barium titanate single crystal under the external electric field was investigated by atomic force microscope and polarized light microscope. The evolutions of domain switching and crack propagation were in-situ observed when a 90°a–c domain wall moved across the indentation which was driven by external electric field. The results show that the incompatible strain induced by domain switching in the residual stress zone around the indentation is the driving force of the anisotropic crack propagation. The crack propagation results in the changes of the fine domain stripes around the crack tip.

The behaviour of 180° polarization switching in BaTiO3 from first principles calculations

In this paper, the behaviour of 180° polarization switching in tetragonal BaTiO3 single crystal was investigated by first principles density functional theory calculations to understand the atomic scale mechanisms of 180° switching in this ferroelectric material. The calculation results indicate that the switching energy barrier per unit cell for direct 180° switching (5.11×10−3eV) is over 4.5times higher than the barrier for two-step 90° switching (1.10×10−3eV). Consequently, there are two critical electric fields for 180° switching in tetragonal BaTiO3 single crystal and which can be evaluated as E90°c=744V/mm for two-step 90° switching and E180°c=2445V/mm for direct 180° switching in an ideal condition, respectively. In other words, if the applied antiparallel field Ea≤E90°c, the switching cannot happen. If E90°c≤Ea≤E180°c, the 180° switching occurs by two 90°-switching steps. If Ea≥E180°c, the direct 180° switching can occur. These calculation results can explain the 180° switching behaviours experimentally observed under various electric field intensities.

In situ observation of correlations between domain switching and crack propagation in BaTiO3 single crystals under coupling of mechanical and electric loads

Domain switching and crack propagation in BaTiO 3 single crystals under external mechanical and electric load are observed directly by polarized light microscopy. The results show that the driving force for crack propagation is the incompatible strain induced by 90° domain switching due to electric load only or the coupling of mechanical and electric loads. The sequence and path of the crack propagation are related to the initial domain structure and the characteristics of the external load.

Polarization Sensitive Surface Band Structure of DopedBaTiO3(001)

We present a spatial and wave-vector resolved study of the electronic structure of micron sized ferroelectric domains at the surface of a BaTiO(3)(001) single crystal. The n-type doping of the BaTiO(3) is controlled by in situ vacuum and oxygen annealing, providing experimental evidence of a surface paraelectric-ferroelectric transition below a critical doping level. Real space imaging of photoemission threshold, core level and valence band spectra show contrast due to domain polarization. Reciprocal space imaging of the electronic structure using linearly polarized light provides unambiguous evidence for the presence of both in- and out-of-plane polarization with two- and fourfold symmetry, respectively. The results agree well with first principles calculations.

Dielectric properties in lead-free piezoelectric (Bi0.5Na0.5)TiO3–BaTiO3 single crystals and ceramics

The 0.93(Bi0.5Na0.5)TiO3–0.07BaTiO3 (BNB7T) piezoelectric single crystals and ceramics have been grown respectively by using the self-flux and solid-state-reaction methods. The real (ε′) and imaginary (ε″) parts of the dielectric permittivity of BNB7T crystals and ceramics were investigated with and without an electric (E) poling as functions of temperature and frequency. The BNB7T crystal shows a stronger dielectric maximum at Tm~240°C than the ceramic at Tm~300°C. The dielectric permittivity of BNB7T ceramic shows an extra peak after poling at an electric field E=40kV/cm in the region of 80–100°C designated as the depolarization temperature (Td). A wide-range dielectric thermal hysteresis was observed in BNB7T crystal and ceramic, suggesting a first-order-like phase transition. The dielectric permittivity ε′ obeys the Curie–Weiss equation, ε′=C/(T−To), above 500°C, which is considered as the Burns temperature (TB), below which polar nanoregions begin to develop and attenuate dielectric responses.

Influence of Adsorbate-Induced Charge Screening, Depolarization Factor, Mobile Carrier Concentration, and Defect-Induced Microstrain on the Size Effect of a BaTiO3Nanoparticle

It has been experimentally reported that the crystal structure of a BaTiO3 single crystal becomes cubic when the crystal size is smaller than a critical one at room temperature, while the crystal structure of the macroscopic BaTiO3 single crystal is tetragonal. The critical size is, however, largely different for different experiments ranging from about 5 to 200 nm. In the present paper, numerical calculations of the free energy have been performed to study the mechanism of the large diversity in the critical size of a BaTiO3 single crystal. The results have indicated that the critical size becomes considerably smaller as the degree of adsorbate-induced charge screening increases because the depolarization energy is reduced. In addition, higher mobile-carrier concentration in a particle as well as smaller depolarization factor and smaller defect-induced microstrain considerably decrease the critical size. Furthermore, it is suggested that above about 30 nm in particle size the domain structure with 90° domain walls or the composite structure, which consists of an inner tetragonal core and a surface cubic layer, is more stable than a single domain below the critical degree of adsorbate-induced charge screening or below a critical mobile-carrier concentration in a particle.

Crystal Structural Analyses of Ferrielectric Tetragonal (Bi1/2Na1/2)TiO3–7%BaTiO3 Powders and Single Crystals

We have investigated the crystal structure of (Bi1/2Na1/2)TiO3–7%BaTiO3 (BNT–7%BT) by high-resolution neutron powder diffraction (NPD) and high-energy synchrotron radiation X-ray diffraction (SR-XRD) analyses. The NPD study revealed that the BNT–7%BT crystals have a single-phase tetragonal structure with P4bm symmetry. The crystal structure refined by the Rietveld method was found to be similar to the ferrielectric P4bm phase reported for BNT at a high temperature of 673 K. The SR-XRD analyses for single crystals of BNT–7%BT demonstrated that the P4bm phase remains as a stable phase in the crystals even after a high electric field is applied for poling, which is different from the structural analysis of ceramics by Ma et al. [Phys. Rev. Lett. 109 (2012) 107602].

Dipole-Dipole Interaction Model for Oriented Attachment of BaTiO<sub>3</sub> Nanocrystals Revisited

Numerical simulations of the oriented aggregation of BaTiO3 nanocrystals using the dipole-dipole interaction model have been performed under conditions corresponding to those used in ultrasound-assisted synthesis. The results suggest that BaTiO3 nanocrystals with a diameter of 5 nm have a spontaneous polarization that is not more than one order of magnitude lower than the value of ~0.15 C/m2 for a macroscopic BaTiO3 single crystal.

Domain Dynamics under Unipolar Electric Fields for BaTiO<sub>3</sub> Single Crystals

Domain structures and dynamics of BaTiO3 single crystals under in-situ electric fields along <110>c were investigated by using synchrotron radiation single-crystal X-ray diffractions. Diffraction patterns clearly show the presence of a 90 ° domain structure in the crystals poled along <110>c. The diffraction analysis provides direct evidence of a reversible change in the volume fractions of two kinds of the 90 o domains under unipolar in-situ electric fields. This reversible change in the domain structures under unipolar fields is suggested to originate from the interaction between spontaneous polarization and defect dipoles composed of acceptor and oxygen vacancy.

Strain actuation of barium titanate single crystals under electromechanical loading in the non-polar [110] direction

The switching characteristics of BaTiO3 single crystals subjected to uniaxial electromechanical loading in the non-polar [110] direction at room temperature and 55 ° C were investigated. Polarization and strain hysteresis measurements in the [110] direction revealed that under the combination of large in-plane switching coercivities and a strong out-of-plane depolarization field (induced by the shape irregularity and large unshielded surfaces of the crystal sample), strain-inducing 90° in-plane to out-of-plane (or vice versa) switching was the dominant switching behavior even in the absence of mechanical bias. By lowering the in-plane switching coercivities, which was achieved by increasing the loading temperature from room temperature to 55 ° C, a contrasting non-strain-inducing switching behavior characterized by 90° and/or 180° switches between the in-plane variants was observed instead. The contrasting switching behavior at two different temperatures indicates that apart from the bias stress magnitude, the combined effect of the depolarization field and switching coercivity is another critical factor governing the strain actuation of BaTiO3 single crystals. The [110] electromechanical loading responses reveal the possibility of increasing the strain capacity of BaTiO3 single crystals by introducing accompanying depolarization fields which promote strain-inducing 90° switching. Such an approach could be potentially useful when bias stress-activated ferroelastic switching is not attainable.

3D dependence of the dielectric dispersion in a BaTiO3 single crystal

The 3D dependences ɛ′(log f, T) and tanδ(logf, T) of a perfect BaTiO3 single crystal grown by the Remeika method have been studied in the ranges f = 1–2 × 107 Hz and T = −80–130°C. These dependences characterize a transition from the paraelectric phase (121.5°C) as a near-antiferroelectric transition followed by the transition to the tetragonal phase at ∼79.5°C. According to a number of signs, the range 121.5–79.5°C corresponds to a metastable phase typical of first-order phase transitions. The unexpected result of this work has been discussed with invoking the hypothesis on the BaTiO3 structure in the paraelectric phase, according to which it consists of three antiferroelectric states oriented along the crystallographic axes. Using the dielectric properties of BaTiO3 as an example, the method of direct correct determination of the temperatures of the structural transformations from the anomaly of tanδ(logf, T) has also been demonstrated.

High-resolution electrocaloric and heat capacity measurements in barium titanate

The electrocaloric (EC) effect in a BaTiO3 single crystal oriented along the [001] direction has been studied by direct high-resolution EC measurements in the vicinity of the field-induced critical point. It is shown that the observed behavior of the EC temperature change , as well as the heat capacity anomalies are in good agreement with the predictions of the Landau theory. We also present the electric field-temperature phase diagram for the field-induced paraelectric-to-ferroelectric phase transition line, derived from the calculated latent heat.

Unusual domain configurations and local piezoresponse in the [720]‐cut BaTiO3 single crystal

AbstractThe domain configurations and local piezoresponse property of the [720]‐cut BaTiO3 single crystal and [001]‐oriented BaTiO3 crystal were investigated by high‐resolution piezoresponse force microscopy. Large differences in their surface topography features, domain configurations and local piezoresponse were found between [720]‐cut and [001]‐oriented BaTiO3 crystals. The large surface bending angle due to ferroelastic domain walls leads to a high strain energy appearing in the [720]‐cut BaTiO3, further resulting in unique band‐like topographic features, needle domains for stress compensation and locally ultrahigh piezoelectric response as well. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical dispersion and interband transition in Na0.5Bi0.5TiO3-x%BaTiO3 lead-free relaxor ferroelectric single crystals

Optical properties of A-site disordered Na0.5Bi0.5TiO3-x%BaTiO3 (NBT-xBT) single crystals with three typical compositions were studied systematically. Refractive index as a function of wavelength was measured by Spectroscopic ellipsometry, and modified Sellmeier dispersion equations were obtained by the least square fitting. High optical transmittance (65%) over a wide transparent region (from 400 to 6000 nm) has been found in NBT-7.5BT, much higher than that of NBT and NBT-5BT. Optical band gap energies were calculated using the absorption coefficient through Tauc equation. With increasing BT content, refractive index and absorption coefficient decrease, while transmission and band gap energy increase.

Polarization rotation theory for field-induced-phase transitions in BaTiO3 single crystal

We have proposed the polarization rotation theory to understand the underlying physics of the large piezoelectric effect in ferroelectric crystals by referring to the coherent rotation model in ferromagnetism. When both the electric field energy and crystalline anisotropy energy are taken into account, the polarization in BaTiO3 crystal can be rotated toward the field direction. The numerical results are in agreement with the experimental observations and have indicated the highly anisotropic polarization rotations and the field-induced-phases with different symmetries in the crystal. This theory is helpful for understanding the polarization reversal and the electromechanical effect in ferroelectric materials.

A theory for structural phase transitions in BaTiO3 single crystal and PbZrO3 -xPbTiO3 solid solution

A phenomenological theory has been proposed for the diffusionless structuralphase transitions in BaTiO3single crystal and PbZrO3 -xPbTiO3 solid solution here. It has been found that for BaTiO3single crystal, both the phase transitionsand the crystal structurescan be predicted with the crystalline anisotropy constants and strain constants that depend on the temperature. For PbZrO3 -xPbTiO3 solution, the morphotropic phase boundaries arise from the strong dependence of the crystalline anisotropy constants on the composition. The good agreement between the numerical results and experimental observations has shown that our theory is effective for the diffusionless structuralphase transitions.