Tungsten Bronze Crystals Research Articles

Mechanism and Kinetics of the Phase Formation and Dissolution of NaxWO3 on a Pt Electrode in a Na2WO4-WO3 Melt.

A comprehensive study concerning the phase formation mechanism and growth/dissolution kinetics of sodium tungsten bronze crystals during the electrolysis of a 0.8Na2WO4-0.2WO3 melt was carried out. The regularities of deposit formation on a Pt(111) working electrode were investigated experimentally using cyclic voltammetry, chronoamperometry, scanning electron microscopy, and X-ray diffraction analysis. Models have been developed to calculate the current response during the formation, growth and dissolution of a two-phase deposit consisting of NaxWO3 and metallic tungsten or two oxide tungsten bronzes with different sodium content. These models consider mass transfer to the electrode and nuclei; chemical and electrochemical reactions with the participation of polytungstate ions, Na+, Na0, and O2-; as well as the ohmic drop effect. The approach was proposed to describe the dissolution of an NaxWO3 crystal with a nonuniform sodium distribution. The fitting of cyclic voltammograms was performed using the Levenberg-Marquardt algorithm. The NaxWO3 formation/growth/dissolution mechanism was determined. Concentration profiles and diffusion coefficients of [WnO3n]-, reaction rate constants, number density of nuclei, and time dependencies of crystal size were calculated. The proposed approaches and models can be used in other systems for the cyclic voltammogram analysis and study of the mechanism and kinetics of electrode processes complicated by phase formation; parallel and sequential electrochemical and chemical reactions; as well as the formation of a deposit characterized by a nonuniform phase and/or chemical composition.

Development of Functional Ceramics by Colloidal Processing in Rotating High Magnetic Field

In this paper, regarding colloidal shaping in a rotating magnetic field, the fabrication and evaluation were described for various crystal-oriented ceramics including c-axis oriented ZnO, piezoelectric ceramics with tungsten bronze crystals as Sr2KNb5O15, (Sr,Ca)2NaNb5O15 (SCNN) and (Sr,Ba)Nb2O6. In order to shorten the shaping time, which is the problem of this method, a shaping method was proposed in which a slurry was prepared with a solution containing an ultraviolet (UV) curing monomer, and the slurry was solidified by UV irradiation after orientation treatment in a magnetic field. Crystal-oriented SCNN and Ca10(PO4)6(OH)2 compacts were prepared by holding for several tens of seconds. The experimental results were examined based on the theory regarding the UV curing depth and rotation time, and it was confirmed that the experimental results follow the theory. In addition, regarding the fabrication of laminated ceramics by the stereolithography method, c-axis oriented SCNN ceramics with a thickness of 600 μm were produced, and the piezoelectric property were also confirmed. In the future, particle orientation in a low magnetic field were achieved, which would be thought to enhance versatility.

Acoustic phonons in unfilled tetragonal tungsten-bronze crystals

ABSTRACT We have studied several unfilled tetragonal tungsten-bronze crystals (SBN-35, CBN-28, CBN-30, CBN-32) by differential scanning calorimetry, Brillouin spectroscopy and piezoresponse-force microscopy. The Brillouin backscattering configuration in the c-plates revealed the longitudinal acoustic phonon, which has a frequency near 50 GHz and displays softening when approaching the phase transition in all crystals. The ferroelectric domain structure and the domain size are dependent on the Sr/Ba or Ca/Ba ratios, and on the occupation rate in the channels as shown by the piezoresponse-force microscopy images. A splitting of the longitudinal acoustic phonon was found in SBN-35 in the multidomain sample, but poling of the crystal removed this splitting and just left the phonon with higher frequency.

Anisotropic Polarization Switching in Tungsten Bronze Ferroelectrics in Binary (Bi1/2Na1/2)Nb2O6–BaNb2O6 System

Randomly oriented (1-x)BaNb2O6–x(Bi1/2Na1/2)Nb2O6 ceramics were prepared by conventional solid state sintering. The ceramics (0.2≤x≤0.7) sintered at 1250 °C consisted of a single phase of a tungsten bronze structure, while the end-member materials did not crystallize into the tungsten bronze structure. The crystal systems of tungsten bronze phases were orthorhombic for 0.2≤x≤0.25 and tetragonal for 0.3≤x≤0.7. The temperatures for the maximum dielectric constant (Tm) measured for orthorhombic crystals were 300 and 190 °C for x=0.2 and 0.25, respectively, and showed no frequency dispersion. However, the Tm for tetragonal crystals was suppressed below 160 °C and showed an obvious frequency dispersion. To evaluate the orientation of spontaneous polarization, (001)- and (hk0)-oriented fiber-textured ceramics were prepared for x=0.2, 0.25, and 0.3 by slip casting under a strong magnetic field. Clear hysteresis loops of polarization–electric field were observed for (001)-oriented orthorhombic ceramics (x=0.2 and 0.25) at room temperature, while the polarization induced in the (hk0)-oriented or tetragonal ceramics (x=0.3) varied proportionally to the external electric field. It was concluded that the tungsten bronze crystals, (1-x)BaNb2O6–x(Bi1/2Na1/2)Nb2O6 (x=0.2 and 0.25), are ferroelectric at room temperature with the spontaneous polarization parallel to the c-axis.

Light‐induced self‐defocusing property in tetragonal tungsten bronze crystals

AbstractIn open‐circuit boundary conditions, tetragonal tungsten bronze (CSBN75) crystals are settled on one of arms of Michelson's interferometer in such a way that even minor perturbation of the penetrated beam can be enlarged and observed through the Michelson's interferometer rings. The experimental results shows that CSBN75 (Δn < 0) are self‐defocusing photorefractive crystals. In this paper, qualitative and quantitative discussion on the light‐induced change in the index of refraction is given. The self‐defocusing character in CSBN75 suggests that it is possible to observe the dark spatial solitons with this crystal in further investigation. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-temperature transverse dielectric and pyroelectric anomalies of uniaxial tungsten bronze crystals

Low-temperature dielectric and pyroelectric properties were investigated for filled and unfilled tungsten bronze (TB) crystals, i.e., (Sr0.61Ba0.39)5Nb10O30, (K0.5Na0.5)1.0(Sr0.75Ba0.25)4.5Nb10O30, K5.80Li3.82Nb10.12O30, and K5.20Li2.34Nb10.88O30 with the spontaneous polarizations along the c axis at room temperature except for the last one. In all samples, transverse dielectric constant along the a axis showed a universal dielectric dispersion in the low-temperature range resembling a diffuse phase transition. Characteristic relaxation times obtained from the dielectric loss peaks followed the Arrhenius law, signifying thermal activation processes. The distribution of relaxation times seems to be caused by the distribution of activation energy, which has been successfully obtained by scaling the temperature- and frequency-dependent dielectric losses. This broadening may be attributed to the intrinsic randomness due to the charge disorder and quenched random fields caused by the unfilled structure and the off-stoichiometry of the TB compounds. Observed low-temperature dynamics were isotropic in the ab plane, and were insensitive to the poling conditions, degree of disorder, and high-temperature phase transitions. Structural phase transition accompanying a polarization tilt from the c axis toward the [110] axis, which was suggested to be the origin of the low-temperature anomalies, was not confirmed in our study. Some dynamics which are still active in the ferroelectric phase, like the concerted rotations of the oxygen octahedra, have been suggested as one of the possible origins of the observed phenomena.

Crystal structure analysis and polarization mechanisms of ferroelectric tetragonal tungsten bronze lead barium niobate

Lead barium niobate single crystals of the composition Pb0.596Ba0.404Nb2.037O6, near the tetragonal: orthorhombic morphotropic phase boundary with tetragonal symmetry at room temperature, were the subject of this study. The crystallographic details, including the ionic coordination, the site occupancies and the thermal anisotropy of a lead-containing tetragonal tungsten bronze crystal are determined. This paper reports the structural refinement results, the site-preference by cations and vacancies, the macroscopic polarization properties derived from the cation displacement, and the apparent valences determined by the bonding distance. Unlike other non-lead-containing ferroelectric tungsten bronzes, significant polarization contribution can be attributed to cations (Pb2+ and Ba2+) at the A2-site. Large anisotropic thermal coefficients found for cations at the A2-site further strengthens the local polarization fluctuation model and suggests the local polarization components are perpendicular to the mirror plane along the (110) direction.

Piezoelectricity in tungsten bronze crystals

The range of piezoelectric properties available in tungsten bronze crystals reviewed includes both longitudinal and shear mode responses. Specific crystal compositions investigated include Sr1-xBaxNb2O6 (SBN), Ba2-xSrxK1-yNayNb5O15 (BSKNN), K3Li2Nb5O15 (KLN), (Ba, Sr)6Ti2Nb8O30 (BSTN), Sr2-x-CaxNaNb5O15 (SCNN), Sr2KNb5O15 (SKN) and Pb1-xBaxNb2O6 (PBN). SBN, SCNN and SKN exhibit large piezoelectric d33 coefficients, while BSKNN, PBN:60 and KLN have large d15 shear coefficients. In La3+ -modified SBN:60, d33 can exceed 1000 pC/N, although its relaxor character increases with increasing La3+ content. The role of dopants in the 12-, 9- and 6-fold coordinated crystallographic sites is described with respect to the relaxor character of SBN crystals. The high piezoelectric and dielectric constants in these single crystals suggest their use in high frequency ultrasonic tomography. Both piezoelectric and electrostrictive options would appear to be available in the bronzes and the very high perfection of the available crystals could permit the generation of composite ultrasonic senders and receivers at scales which would be impossible in conventional polycrystalline piezoelectric ceramics.

Enhancing the photorefractive effect

The transient response for beam fanning in photorefractive tungsten bronze crystals with an applied electric field and a focused laser beam is studied. Response times on the order of 1 ms for an incident power of 1 mW observed for incident beams focused to a 30 μm diameter in the crystal. This improved response time is accomplished without the significant reduction in the magnitude of the photorefractive effect normally observed for focused beams.

ANISOTROPIC DIFFRACTION IN DOPED KNSBN PHOTOREFRACTIVE CRYSTAL

Two wave coupling equations on anisotropic diffraction in photorefractive crystal is deri-ved according to coupling wave theory. Phase gratig diffraction results of a He-Ne laser beam (λ= 632.8 nm) passing through tungsten bronze crystals KNSBN doped respectively Ca, Ce and Co are given. The anisotropic diffraction patterns of these three kinds of photorefractive crystals is formally analyzed using these coupling formulae. It is concluded that the anisotro-pic diffraction ring pattern will present in one of the two kinds of diffraction (e→o or o→e diffraction) for different (positive or negtive) unixial crystal. Theory analysis and experi-mental results agree fair by well.

Glassy polarization in the ferroelectric tungsten bronze (Ba,Sr)Nb2O6

We report accurate temperature dependent measurements of optic indices of refraction, the birefringence, and the strain in the ferroelectric tungsten bronze crystals Ba0.25Sr0.75Nb2O6 and two compositions of (Ba2−xSrx)2(K1−yNay)2(NbO3)10. These results are compared to our previous results in Ba0.4Sr0.6Nb2O6. From the experimental data, it appears that far above the ferroelectric Tc, up to a temperature Td, these crystals possess a local, randomly oriented polarization, Pd, with similar Td values, irrespective of their chemical composition and Tc. Various aspects of our understanding of the polarization behavior and other effects in this ferroelectric system are discussed.

Calculation of mean projected range and range straggling of heavy ions in polyatomic targets

Based on Biersack's angular diffusion model for the slowing down of ions in matter, an efficient method has been developed for calculating the mean projected range and range straggling of heavy ions in polyatomic targets. The method differs from both the LSS procedure and Monte Carlo simulation. Heavy ions such as Br+, Au+, Hg+ and Bi+ have been chosen. The target ranged from diatomic to heptatomic, and included amorphous SiO2, quartz, polymers, Li0.16Na0.84NbO3 crystals, tungsten bronze crystals and seven-component glasses. In each case, calculated values are compared with experimental data at energies from 50 to 400 keV. Good agreement between calculated and experimental values of the mean projected range is found for the first-order treatment in Biersack's model; for the range straggling, the calculated values are much lower than experimental values in the first-order treatment. However, after considering the second-order energy loss, marked improvements are obtained.

Electric field-induced orthogonal polarization switching in morphotropic phase boundary Pb_057Ba_043Nb_2O_6 (PBN57) single crystals

Tungsten bronze crystals of Pb(0.57)Ba(0.43)Nb(2)O(6) composition near the morphotropic phase boundary were examined for possible switching of the polarization vector. It is demonstrated that for the single crystal lead barium niobate of tetragonal symmetry, the polarization vector can be switched orthogonally to an orthorhombic phase simply by the application of an electric field.

Ferroelectric tungsten bronze crystals and their photorefractive applications

Abstract The photorefractive properties of tungsten bronze ferroelectric crystals are reviewed with respect to host character and dopant. A classification into four groups is proposed for tungsten bronze materials based on their structural, ferroelectric and optical properties. The use of dopants is found to control the speed of response and coupling strength as well as spectral range. In particular, the availability of different crystallographic sites is shown to provide opportunities to separately adjust these properties. The best performance has been established for Ce3+ and Cr3+ doped SBN:60 crystals in the visible and IR regions, respectively.

GROWTH OF TUNGSTEN BRONZE CRYSTALS

Some phenomena of tungsten bronze crystals growth were observed carefully. The effects of electrolytic temperature, current and time on the growth of crystals were studied and the optimum conditions for crystal growth were found. KxWO3,NayWO3 and KxNayWO3 crystals were produced successfully by fused salt electrolysis.

Tungsten bronze crystals for photorefractive self-pumped phase conjugation

Abstract We review the current status of photorefractive self-pumped optical phase conjugation in the tungsten bronze family of ferroelectric crystals. The tungsten bronze ferroelectrics have emerged as an important class of materials for performing optical phase conjugation in addition to the well known pyroelectric, piezoelectric, and electro-optic applications. The large electro-optic coefficients, open lattice structure, compositional flexibility, and good optical quality of these materials has made possible a new set of self-pumped phase conjugate mirrors. Our initial interest in this class of crystals arose from a number of enlightening discussions with Professor L. Eric Cross who pointed out the multitude of applications possible with the tungsten bronzes. We would like to acknowledge his assistance and encouragement by dedicating this paper to Professor Cross on the occasion of his 65th birthday.

Preparation and Electrical Properties of Highly Conductive Glass-Ceramic Films Containing Tungsten Bronze

A series of alkali tungsten bronzes are non-stioichometric compounds having a general formula RxWO3, where R is H, Li, Na, K and Rb and x is less than unity. They show metallic conduction. The present work was done in order to obtain highly conductive glass-ceramic. films containing tungsten bronze and to investigate their electric conductivites. A series of glasses consisted of the compositions containing 30.0-37.5 Na2O⋅57.5-67.5 WO3⋅2.5-10.0 B2O3 (in mol%). About 70 grams batches corresponding to the nominal compositions were melted in alumina crucibles at 1000°C for 1h. The melts were rapidly quenched by the twin roller (3000rpm) to obtain clear glasses without devitrification. Glasses were ground into powders of particle size less 37μm. Films were prepared by two methods. One was the method by dipping of a quartz glass plate into water dissolved Na2O-WO3-B2O3 glasses. The other was the method by melting the glass powders uniformly dispersed on a quartz glass plate in ethanol. Tungsten bronze crystals precipitated from these films when heat-treated in a H2 atomosphere. These glass-ceramic films were shown to be much higher electric conductivities than the sintered tablet. The maximum conductivity of these glass-ceramic films was about 103S/cm at 25°C. The charge carrier of glass-ceramic films was confirmed to be electron by the Seebeck coefficient measurements. The conduction mechanism of glass-ceramics was considered to be electronic conduction due to the large polaron.

Growth and ferroelectric properties of tungsten bronze Ba 2− xSr xK 1− yNa yNb 5O 15 (BSKNN) single crystals

The undoped and Ce-doped ferroelectric tungsten bronze crystals, specifically Ba 1.5Sr 0.5K 0.75Na 0.25Nb 5O 15 (BSKNN-1) and Ba 0.5Sr 1.5K 0.5Na 0.5Nb 5O 15 (BSKNN-2), have been grown in optical quality using the Czochralski technique. Although both of these crystals are tetragonal with point group 4mm, their growth habit along the [001] direction differs: BSKNN-1 crystals grow in a square shape with four well-defined facets, while BSKNN-2 crystals have an octahedral growth habit with eight well-defined facets. The longitudinal dielectric (ϵ 11) and linear electro-optic ( r 51) constants show strong enhancement when expanding from BSKNN-1 and BSKNN-2 crystals, indicating that the latter could be of major interest for electro-optic, photorefractive and pyroelectric applications.

Development And Modification Of Photorefractive Properties In The Tungsten Bronze Family Crystals

The Sr1-xBaxNb2O6 (SBN) and Ba2-xSrxK1-yNayNb5O15 (BSKNN) tungsten bronze solid-solution systems are shown to be promising photo-refractive materials. Because of the versatility of the bronze structure, both the response time and spectral response can be controlled by altering the type of dopant and its crystallographic site preference. This paper reviews the current status of the tungsten bronze crystals SBN and BSKNN for photorefractive applications in terms of their growth, electro-optic character, and the role of cerium dopants. Ferroelectric morphotropic phase boundary (MPB) bronze materials are also discussed as potentially important for future development.

Piezoelectric tungsten bronze crystals for SAW device applications

Piezoelectric tungsten bronze solid solution crystals Sr 1−xBa xNb 2O 6, x = 0.60, and 0.50, have successfully been grown by the Czochralski technique, and their piezoelectric and acoustical properties were investigated. The crystals have ferroelectric tetragonal tungsten bronze structure with excellent piezoelectric properties. The Sr 1−xBa xNb 2O 6 solid solution crystals possess temperature-compensated orientations, and high SAW electromechanical coupling has been reported for the (001)-plate propagating along the (100) direction; its value is measured to be 180 × 10 −4. This value is similar to the coupling found in other important orthorhombic tungsten bronze crystals such as Pb 2KNb 5O 15 indicating that crystals in this family are potentially important for future SAW device applications.