Impurity defects in Cr-doped Bi12SiO20 and Bi12TiO20 crystals

Bi₁₂SiO₂₀ and Bi₁₂TiO₂₀ crystals doped with l0^-5 to 2 x 10^-2 wt % Cr were grown by the Czochralski technique. The Cr content of the crystals and the valence state of Cr ions were determined by a new analytical technique with the use of 1,5-diphenylcarbazide. Correlations were revealed between the segregation coefficient of the dopant, the charge state of Cr ions, and the absorption coefficient of the crystals. Doping with Cr was found to shift the intrinsic edge to longer wavelengths and to give rise to a new absorption band in the near-IR region. The results are interpreted as indicating that the Cr ions in the sillenite crystals studied are in at least two oxidation states and occupy inequivalent sites in the sillenite cell.     

Noise filtering in the interference pattern by dynamic holographic recording in photorefractive Bi(12)TiO(20) crystals

The possibilities of stabilization of the interference pattern by filtration of a random-phase noise caused by vibrations, turbulence, and other local changes in the wave front in interferometric measurements are investigated. Dynamic holographic recording in photorefractive Bi(12)TiO(20) crystals is used. The parameters of the holographic recording are presented for determination of the dynamic range for filtering. Noise filtering takes place in real time and contributes to the enhancement of the contrast and the signal-to-noise ratio of the interference pattern. This results in a considerable increase in the sensitivity and the accuracy of the interferometric measurements.     

光折变Bi12SiO20晶体中的波耦合性质

研究在光折变Bi12SiO20晶体中二波耦合扩散全息记录的信号光能量增益和偏振态改变。在3种制备可得的晶体切割面上给出任意光栅取向的能量和偏振态耦合量的取值范围,分析了信号光能量增益和偏振态改变量分别达到最大值时各向同性和各向异性耦合的作用,此外研究旋光效应，压电及弹光效应在3种晶体切割面上对矢量波耦合的影响。     

Influence of W5+ ions on the optical properties of doped Bi12TiO20

There have been studied single crystals of undoped and doped Bi₁₂TiO₂₀ with two concentrations of W⁵⁺ (2.62 × 10¹⁷ cm⁻³ and 2.62 × 10¹⁸ cm⁻³). There have been obtained absorption spectra in the energy range of 10,482–15,408 cm⁻¹ by classical measurements. There have been determined the cross-section (σ_a) of the impurity absorption and the oscillator strength of d–d transitions. There have been calculated the refractive index of doped crystals and the concentration of Ti³⁺ ions in an undoped sample through an experiment.     

Fiber-optic voltage sensor based on a Bi12TiO20 crystal

A fiber-optic voltage sensor based on the longitudinal Pockels effect in a Bi(12)TiO(20) crystal is described. The use of a special backreflecting prism as a phase-retarding element is shown to improve the sensitivity and temperature stability of the sensor. A comparison between the temperature properties of the glass backreflecting prism and that of a quarter-wave plate is derived. The sensor demonstrates temperature stability of +/-1.5% from -20 degrees C to 60 degrees C and sensitivity of 0.145% per 1 V(rms) at 850 nm without the use of an additional temperature control channel.     

Sub-millisecond photorefractive two-wave coupling in Bi12TiO20 at 633 nm

Abstract

The unusually fast response of the two-wave mixing process in fibre-like Bi₁₂TiO₂₀ crystals on transient change of the external electric field was experimentally observed. Pulses of the amplification of the signal wave with a rise time of 0.2 ms and a width of 0.5 ms at a total input intensity of 6 mW mm^?2 are obtained when applying a rising or decreasing front of an external electric field to the crystal. The two-wave coupling without external field has a response time of 200 ms. A simple calculation based on the bandtransport model qualitatively explains the fast transient amplification but fails to explain the slow response without the electric field.     

Optical properties of Bi12SiO20 (BSO) and Bi12TiO20 (BTO) obtained by mechanical alloying

Mechanical alloying has been used successfully to produce nanocrystalline powders of BTO and BSO. The milled BTO and BSO were studied by x-ray powder diffraction, DTA, infrared and Raman scattering spectroscopy. After 7 hours of milling the formation of BTO and BSO was confirmed by x-ray powder diffraction. The infrared and Raman scattering spectroscopy results suggest that the increase of the milling time lead to the formation of ferroelectric BTO and BSO, as seen by x-ray diffraction analysis. These materials are attractive for various electro-optical devices, including optical data processing. They present a number of attractive features as reversible recording materials for real-time holography and image processing applications. This milling process presents the advantage, that melting is not necessary, and the powder obtained is nanocrystalline with extraordinary mechanical properties. The material, can be compacted and transformed in solid piezoelectric ceramic samples. The high efficiency of the process, opens a way to produce commercial amount of nanocrystalline piezoelectric powders. Due to the nanocrystalline character of this powder, their mechanical properties have changed and for this reason a pressure of 1 GPa is enough to shape the sample into any geometry.     

Hierarchical architecture of Bi12TiO20 via ethylene glycol-mediated synthesis route

Hierarchical architecture of cubic sillenite bismuth titanate (Bi₁₂TiO₂₀) is successfully synthesized using simple ethylene glycol-mediated self-assembly; followed by calcination under air at 600 °C for 30 min. The products are characterized using field emission scanning electron microscopy (FE-SEM), fourier transform infrared spectroscopy (FT-IR), thermogravimetric-differential thermal analysis (TG-DTA), and X-ray diffraction (XRD). The reaction time has an effect on the product morphology, which changed from a leaf- and circular plate-like structure to a completely formed hierarchical structure within 2 h. The hierarchical structure of Bi₁₂TiO₂₀, having a size around 3 μm, is composed of 2D twist nanoplates, and each twist nanoplate has a thickness around 10–30 nm.     

Comparison of space- and ground-grown Ce: Bi12SiO20 single crystals

Ce doped Bi₁₂SiO₂₀ single crystals were grown either on board of the Chinese Spacecraft-Shenzhou No.3 (SZ-3) or on the ground at the same conditions with the exception of microgravity. The surface morphology of crystals clearly showed significant differences between the space- and ground-grown portions. The space- and ground-grown crystals have been measured by X-ray rocking curve, Ce concentration distribution in growth direction, dislocation density, absorption spectrums. These results show that the compositional homogeneity and structural perfection of Ce doped crystal grown in space are obviously improved.     

Experimental study of the temperature stability of magnetic field sensors based on Bi12SiO20 crystals

The temperature characteristics of the conversion coefficient of the sensitive element on a fiber-optic magnetic field probe based on a Bi₁₂SiO₂₀ crystal are studied experimentally. A laboratory model of the sensitive element of a magnetic field sensor is built, and the temperature drift of the conversion coefficient is found to be ∼0.15% over the temperature interval from +15 to +70 °C. Pis’ma Zh. Tekh. Fiz. 24, 26–33 (June 12, 1998)     

Growing high-quality Bi12SiO20 crystals of large diameter (85 mm)

Conditions that ensure reproducible growth of ??110??-oriented perfect Bi₁₂SiO₂₀ (BSO) crystals by the low-thermal-gradient Czochralski technique, whereby the entire crystallization front is occupied by the (110) crystal face, have been determined with the aid of numerical simulations. Using the established regime, BSO crystals have been obtained with a diameter of 85 mm, a length of 200 mm, and a mass of 10 kg. The density of dislocations in the crystals does not exceed 10 cm^?2, and the refractive index inhomogeneity is below 10^?3.     

Effect of the purity of starting materials on the structure and properties of permanent magnets

The need for using pure starting components in the production of Alnico and Nd-Fe-B system alloys for permanent magnets of high quality is substantiated. The content of gas-forming impurities and nonmetallic inclusions was shown to affect substantially the formation of perfect structure of Alnico-type magnets; optimum conditions of preliminary treatment of the melt to decrease the content of gas-forming elements in ingots were determined. The negative effect of the contamination of starting materials with respect to gas-forming elements was determined and correlations between the oxygen content in Nd-Fe-B magnets and their properties were analyzed.     

Polarization recording of holograms in partially polarized recording light

A theoretical approach is considered for media whose scalar and anisotropic responses are of opposite sign. The nondiffracted beam, and the imaginary and real images formed by a polarization hologram are analyzed under these conditions. It is shown that the imaginary image has its polarization transformed compared with the object field while a pseudoscopic object field reconstructed in terms of polarization state and degree is formed in the real image. Pis’ma Zh. Tekh. Fiz. 23, 38–42 (February 12, 1997)     

3D Bi12TiO20/TiO2 hierarchical heterostructure: synthesis and enhanced visible-light photocatalytic activities

A three-dimensional (3D) multicomponent oxide, Bi(12)TiO(20)/TiO(2) hierarchical heterostructure was successfully synthesized via a one-step and template-free hydrothermal route. X-ray diffraction and X-ray photoelectron spectroscopy measurements confirm that the composition of the as-fabricated sample is Bi(12)TiO(20)/TiO(2) composite. Scanning and transmission electron microscopy observation reveals that the as-synthesized sample is microsized flower-like hierarchical networks consisted of Bi(12)TiO(20) nanorods decorated with the primary TiO(2) nanoparticles. Extension of the light absorption from the ultraviolet region to the visible-light region was confirmed by UV-vis absorption spectra. Due to the structure-property relationships, the 3D Bi(12)TiO(20)/TiO(2) heterostructure exhibited enhanced visible photocatalytic activity over that of Bi(12)TiO(20) and TiO(2) samples in the decomposition of Rhodamine B in water which is a typical model pollutant. The enhanced photocatalytic activity can be attributed to the extended absorption in the visible light region resulting from the 3D Bi(12)TiO(20)/TiO(2) heterostructures, and the effective separation of photogenerated carriers driven by the photoinduced potential difference generated at the Bi(12)TiO(20)/TiO(2) junction interface, demonstrating that the Bi(12)TiO(20)/TiO(2) heterostructure is a promising candidate as a visible light photocatalyst.     

Growth crystallography of Bi12SiO20 single crystals solidified by a floating zone method

Relations between the growth directions of Bi₁₂SiO₂₀ (BSO) single-crystal rods solidified by a floating zone method and the pulling parameters were studied using an X-ray back-reflection Laue technique. It was found that when a platinum wire is used as a seed, the growth direction of the produced BSO single-crystal rod is related to the pulling rate. Statistically, the probability of the growth direction approaching 〈0 1 1〉, 〈1 1 2〉 or 〈0 0 1〉 appears to increase in this order with increase of the pulling rate. In addition when a BSO crystal is used as a seed, the growth direction of the produced BSO crystal rod has the same orientation as the seed crystal if the pulling rate is less than 30 mm h^-1. If the pulling rate is higher than 30 mm h^-1, the growth appears to incline mostly to 〈1 1 2〉 if the seed orientation is near 〈0 1 1〉 or 〈1 1 1〉 but far from 〈0 0 1〉, or to 〈0 0 1〉 if the seed orientation is near 〈0 0 1〉 but far from both 〈0 1 1〉 and 〈1 1 1〉. The angle of inclination increases with the pulling rate, and also with the difference in angle between the orientation of seed crystal and 〈1 1 2〉 or 〈0 0 1〉. The facet morphology of the BSO single-crystal rod is related to its growth direction. The cross-section of the BSO single-crystal rod grown along 〈0 0 1〉 is an octagon with tetrad-rotational symmetry, that grown along 〈0 1 1〉 is an ellipsoid with diad-rotational symmetry, and that along 〈1 1 1〉 is a hexagon with triad-rotational symmetry. The cross-sections of the BSO single-crystal rods grown in other directions are not regular, because there is no clear symmetry. This revised version was published online in November 2006 with corrections to the Cover Date.