Propagation of elastic waves near acoustic axes in crystals under the action of extreme external forces: II. Centrosymmetric crystal in an electric field; III. Crystal under mechanical stress

The problem of degeneracy of elastic waves propagating in the vicinity of acoustic axes in centrosymmetric crystals in an external electric field and in crystals with arbitrary symmetry under uniaxial mechanical stress is considered. For the case of acoustic axes coinciding in the absence of external forces with the symmetry axes, equations describing the directions of external forces at which the difference in the velocities of initially degenerate transverse waves or angles of splitting and shift of the initial degeneracies reach maximum values are obtained. By the example of acoustic axes in LiNbO₃, La₃Ga₅SiO₁₄, Bi₁₂SiO₂₀, and SrTiO₃ crystals, extreme directions of uniaxial mechanical stress and the corresponding changes in the characteristics of transverse waves are calculated. The behavior of acoustic axes under uniaxial compression is analyzed.     

Microhardness of Bi12SiO20, Bi12TiO20, and Bi12GeO20 Monocrystals

The microhardness, H, of sillenite-type monocrystals of undoped Bi₁₂SiO₂₀, Bi₁₂TiO₂₀, Bi₁₂GeO₂₀ and Cr-, Fe-, Mn- doped Bi₁₂SiO₂₀ has been investigated. The dependences of H on the surface treatment, dopants and orientation of the materials have been established. It was found that the H and dislocation density, N_d, distribution along the diameter of the sample for undoped (100)-Bi₁₂SiO₂₀ are of M- and W-shape, respectively.     

X-ray Photoelectron Spectroscopic Studies of Sellinite and Eulytite BGO and BSO Crystals

Single crystals of Bi₁₂GeO₂₀, Bi₁₂SiO₂₀, Bi₄Ge₃O₁₂, and Bi₄Si₃O₁₂ are grown by Floating zone and Czochralski techniques. The X-ray photoelectron spectroscopic (XPS) studies have been carried out on BSO and BGO crystals. XPS is employed to characterise the surface quality and bulk nature of the crystals. The surface contamination on both types of crystals are identified. In addition to the contamination, some amount of Bi atoms are observed with Bi ions on sellinite. It is demonstrated that the eulytite crystals are chemically more robust to degradation than the sellinite crystals. The predominant covalent and ionic character of Bi₄Ge₃O₁₂ and Bi₄Si₃O₁₂ respectively is explained from their oxygen 1s core level spectra.     

Spectroscopic study on iron doped silica-bismuthate glasses and glass ceramics

Fourier transform infrared (FTIR) and electron paramagnetic resonance (EPR) spectroscopic data obtained for iron doped silica-bismuthate glasses were used to investigate the changes induced in the local structure of samples as the ratio between Bi₂O₃ and SiO₂ content changes from 3 to 0.66. The environment of constituent cations was investigated both in vitreous and partially crystallized samples of same composition. Progressive substitution of Bi₂O₃ by SiO₂ contributes to the structural relaxation of vitreous network and enhances glass stability. By crystallization heat treatment the structural units appear to be more uniform as it results from the narrowing of corresponding IR bands and EPR lines. FTIR data show that more affected by composition and heat treatment is the environment of bismuth than that of silicon. EPR data indicate an ordering tendency with SiO₂ content in glasses, while in vitroceramics the iron environment is much uniform in samples with less SiO₂ content, where Bi₁₂SiO₂₀ phase is developed.     

Effect of axial vibration on Bi12SiO20 crystal growth by vertical Bridgman technique

Bi₁₂SiO₂₀ single crystals have been grown successfully by vertical Bridgman technique. During the crystal growth process, different axial vibration amplitudes of 50 μm, 70 μm and 100 μm were applied with the same vibration frequency of 50 Hz. The effect of different axial vibration amplitudes on quality of the as‐grown Bi₁₂SiO₂₀ single crystals was discussed. The crystals have been characterized by X‐ray rocking curve optical and absorption spectrum. The experimental results reveal that the axial vibration amplitude has a pronounced effect on quality of the as‐grown Bi₁₂SiO₂₀ single crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of Bi<Subscript>12</Subscript>GeO<Subscript>20</Subscript> and Bi<Subscript>12</Subscript>SiO<Subscript>20</Subscript> crystals by the low-thermal gradient Czochralski technique

Bi₁₂SiO₂₀ crystals have been grown for the first time by the low-thermal gradient Czochralski technique in the 〈111〉 and 〈110〉 directions. The conditions for reproducible crystal growth with a high-quality polyhedral faceted front are found. The systematic features of shaping Bi₁₂SiO₂₀ and Bi₁₂GeO₂₀ crystals, grown by the low-thermal gradient Czochralski technique, are compared. The defect formation in these crystals is studied and their optical homogeneity is analyzed by interferometry.     

Raman scattering in sillenite-type crystals

The Raman spectra of Bi₂₄AlPO₄₀ and Bi₃₈ZnO₅₈ crystals have been studied for the first time. The polarization Raman spectra of Bi₁₂GeO₂₀, Bi₁₂TiO₂₀, Bi₂₄AlPO₄₀, and Bi₃₈ZnO₅₈ crystals are also studied. The lines in the Raman spectra of Bi₁₂TiO₂₀, Bi₂₄AlPO₄₀, and Bi₃₈ZnO₅₈ are identified and compared with the lines of the Bi₁₂GeO₂₀ spectra studied earlier. It is shown that the differences in the Raman spectra of the crystals studied are associated with the specific features of their atomic structures.     

Growth,structure, and some transport properties of topological insulators based on bismuth chalcogenide single crystals

Single crystals of topological insulators—bismuth chalcogenides Bi₂Te₃, Bi_{2 ? x} Sn _x Te₃, Bi₂Se₃, and Bi_{2 ? x} Cu _x Se₃ with different charge-carrier densities—are grown by the modified Bridgman method. Their composition and structure are investigated and temperature dependences of the electric resistance and magnetic field dependences of the Hall voltage are obtained.     

Correlation Between the Impurity Content,the Average Charge State of Chromium Cations and Optical Absorption in Sillenite Crystals Doped by Chromium in Wide Range of Concentrations

Single crystals of sillenites (Bi₁₂SiO₂₀, Bi₁₂TiO₂₀) doped with chromium in a wide range of concentrations from 1 x 10^‐5 up to 1.8 x 10^‐2 wt. % were grown by the Czochralski and top‐seed solution growth (TSSG) techniques. To estimate the content and the average charge state of chromium in the grown crystals the chemical analysis by the modified diphenylcarbazide method was applied. The dependencies of the chromium distribution coefficient, the average charge state of Cr cations, and optical absorption on the Cr concentration were found. Both the shift of the absorption edge toward lower frequencies and the appearance of an additional absorption band in the near IR were observed when the chromium concentration in the crystals was increased. The experimental data suggest that chromium has at least two charge states and occupies probably different positions in the sillenite unit cell.     

The Czochralski growth of optical quality bismuth silicon oxide (Bi12SiO20)

Bismuth silicon oxide (Bi₁₂SiO₂₀) exhibits several unique characteristics for application to various optical devices, including the Pockels Readout Optical Memory (PROM), photorefractive read-write holographic memory, and thin film optical waveguides. These applications place stringent requirements on the optical quality of the material. A number of difficulties associated with Czochralski growth of optical quality Bi₁₂SiO₂₀ are discussed, including photochromic effects, selective decoration, growth striations, refractive index homogeneity, and strain-induced birefringence. The photochromic decoration effect is shown to exhibit selectivity between polar antipodes as well as crystallographic orientations.     

Photoacoustic transformation of Bessel light beams in magnetoactive superlattices

Photoacoustic transformation of the TE mode of a Bessel light beam (BLB) has been studied for piezoelectric detection in short-period superlattices formed by magnetoactive crystals of bismuth germanate (Bi₁₂GeO₂₀) and bismuth silicate (Bi₁₂SiO₂₀) types. It is shown that the resulting signal amplitude can be controlled using optical schemes of BLB formation with a tunable cone angle. A resonant increase in the signal amplitude has been found in the megahertz range of modulation frequencies and its dependences on the BLB modulation frequency, geometric sizes of the two-layer structure and piezoelectric transducer, radial coordinate of the polarization BLB mode, and dissipative superlattice parameters are analyzed.     

Substitution effects on ferroelectric,leakage current and anti‐fatigue characteristic of Bi4‐xSbxTi3O12 thin films

Bi_4‐xSb_xTi₃O₁₂ (BSTO) (x = 0, 0.03, 0.04, 0.05, 0.06 and 0.07) thin films have been fabricated on Pt/Ti/SiO₂/Si substrates by sol‐gel method. The effects of various Sb³⁺ content on microstructure and ferroelectric properties of systems are investigated. XRD show that Bi_4‐xSb_xTi₃O₁₂ (x≠0) thin films prefer (117) orientation. The substitution Sb³⁺ for Bi³⁺ reduces the grain size of the film surface. Compared to the BTO (x = 0) film, Bi_4‐xSb_xTi₃O₁₂ films display exciting electric properties. Especially when x = 0.04, the film Bi_3.96Sb_0.04Ti₃O₁₂ has achieved the max 2Pr value of 87μC/cm². This film also has a better anti‐fatigue characteristic, which can be up to 10¹⁰ switching cycles without fatigue. The leakage current density improved with J = 8×10⁻⁸ A/cm².     

Thermal properties and glass formation in the SiO2–B2O3–Bi2O3–ZnO quaternary system

Lead-free glasses in the SiO₂–B₂O₃–Bi₂O₃–ZnO quaternary system were studied. The glass formation region, as determined by XRD patterns of bulk samples, was limited to glasses having more than 40 mol% of the glass-forming oxides SiO₂ and B₂O₃. Crystalline phases of Zn₂SiO₄ (willemite) were detected in compositions of 30SiO₂ · 10B₂O₃ · 20Bi₂O₃ · 40ZnO and 20SiO₂ · 10B₂O₃ · 25Bi₂O₃ · 45ZnO. Glass transition temperatures (T_g), dilatometric softening points (T_d) and linear coefficients of expansion in the temperatures range of 25–300 °C (α_25–300) were measured for subsystems along the B₂O₃ join of 10, 20 and 30 mol%. For these subsystems, T_g ranged from 411 to 522 °C, and T_d ranged from 453 to 563 °C, both decreasing with increasing Bi₂O₃ content. The measured α_25–300 ranged from 53 to 95 × 10⁻⁷ °C⁻¹, with values increasing with increasing Bi₂O₃ content. The ZnO content had the opposite effect to the Bi₂O₃ content. It appears that Bi³⁺ acts as a glass-modifier in this quaternary system.     

Crystallization of bismuth oxide nano-crystallites in a SiO2–PbO–Bi2O3 glass matrix

SiO₂–PbO–Bi₂O₃ glasses having the composition of 35SiO₂–xPbO–(65 ? x)Bi₂O₃ (where x = 5, 20 and 45; in mol%) have been prepared using the conventional melting and annealing method. Differential scanning calorimetry (DSC) was employed to characterize the thermal behavior of the prepared glasses in order to determine their crystallization temperatures (T_cr). It has been found that T_cr decreases with the decrease of Bi₂O₃ content. The amorphous nature of the prepared glasses as well as the crystallinity of the produced glass–ceramics were confirmed by X-ray powder diffraction (XRD) analysis. SiPbBi₂O₆ glass nano-composites, comprising bismuth oxides nano-crystallites, were obtained by controlled heat-treatment of the glasses at their (T_cr) for 10 h. Transmission electron microscopy (TEM) of the glass nano-crystal composites demonstrates the presence of cubic Bi₂O₃ nano-crystallites in the SiPbBi₂O₆ glass matrix. Nano-crystallites mean size has been determined from XRD line width analysis using Scherrer's equation as well as from TEM; and the sizes obtained from both analyses are in good agreement. These sizes varied from about 15 to 170 nm depending on the chemical compositions of parent glasses and, consequently, their structure. Interestingly, replacement of the Bi₂O₃ by PbO in the glass compositions has pronounced effect on the nature, morphology and size of the formed nano-crystallites. Decrease of the Bi₂O₃ content increases the size of the nano-crystallites, and at the lowest Bi₂O₃ extreme, namely 20 mol%, introduces minority of the monoclinic Bi₂O₄ in addition to the cubic Bi₂O₃. The crystallization mechanism is suggested to involve a diffusion controlled growth of the bismuth oxide nano-crystallites in the SiPbBi₂O₆ glass matrix with the zero nucleation rate.     

Periodic modulation on the refractive index in a Bi12TiO20 crystal

It is, well know that some, optical materials to change his refractive index when these exposed to light, depending on particular conditions, they can generate several distributions of refractive index. In this work, an experimental periodic arrangement of refractive index generated on the Bi₁₂TiO₂₀ photorrefractive crystal is presented, the initial condition used was generated by superposing two He‐Ne laser linearly polarized beams in a perpendicular direction to the external field applied to the crystal at 632.8 nm. The diagrams of bands generated with this arrangement is studied, based on the similarities presented by the periodic refractive index with the photonic bandgap (PBG) diagrams one‐dimensional superlattices composed of alternating layers of two distinct materials. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and crystal growth of sillenite phases in the Bi2O3 ‐ TiO2 ‐ Nb2O5 system

The synthesis of Bi₂O₃‐Nb₂O₅ sillenite phase (BNbO) and the solubility of this phase with Bi₁₂TiO₂₀ was investigated by solid‐state reaction synthesis and niobium doped Bi₁₂TiO₂₀ (BTO:Nb) crystals were grown by the Top Seeded Solution Growth (TSSG) technique. The structures of polycrystalline compounds were checked by X‐ray powder diffraction method at room temperature. The correct composition of the sillenite phase stabilized with niobium was determined as Bi₁₂[Nb_0.17Bi_0.83]O_19.7 (BNbO) with unit cell parameter a = 10.261(2) Å. The system BTO‐BNbO is poorly soluble, but niobium doped BTO crystals were grown from the liquid composition 10Bi₂O₃ : xTiO₂ : (1‐x)/2 Nb₂O₅, with x = 0.95 and 0.90. A niobium concentration limit in the liquid phase is established in order to grow BTO:Nb with good crystalline quality. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Brillouin spectroscopy of Nd-Ge co-doped silica fibers

The effect of Nd-doping on both the longitudinal acoustic velocity and material damping coefficient (Brillouin spectral width) of silica fiber are measured. We find that Nd-doping results in anomalously large changes in the acoustic properties of SiO₂, such as that material damping is greatly exacerbated by the incorporation of Nd. We find that the spectral width increases by about 151% per wt% of Nd₂O₃ (for small [Nd]). This is very large compared to the effect of Ge-doping, as observed in this work. This may result from poor coordination in the SiO₂/GeO₂ matrix, and may be considerably different with the use of co-dopants such as Al that tend to improve the solubility of the rare earths. The acoustic velocity is found to decrease at a rate of about 6.1% per wt% of Nd₂O₃.     

Contribution to the study of elastic properties of KGd(WO4)2:Er3+ single crystals by Brillouin spectroscopy

In this paper we present the study of the acoustic phonons propagating in Er³⁺‐doped KGd(WO₄)₂ single crystals by Brillouin spectroscopy. For the investigated crystals the velocities of the longitudinal and transverse acoustic phonons [100], [010], [001], [101] and [110] have been determined. Moreover, the values of the elastic constants: C₂₂, C₄₄ and C₆₆ of Er³⁺‐doped KGd(WO₄)₂ single crystals have been estimated. It was revealed that the presence of the Er³⁺‐ions in KGd(WO₄)₂ crystals, for the used doping concentration 1 at% does not influence their elastic properties. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of TiO2 addition on the chemical durability of Bi2O3–SiO2–ZnO–B2O3 glass system

The effect of the substitution of ZnO for TiO₂ on the chemical durability of Bi₂O₃–SiO₂–ZnO–B₂O₃ glass coatings in hot acidic medium (0.1 N H₂SO₄ at 80 °C) for different times was studied. The thick films produced by a screen-printing method and heat treated at 700 °C/5 min were analyzed by X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. The glass from the Bi₂O₃–SiO₂–ZnO–B₂O₃ system developed Zn₂SiO₄ and a glassy phase that were readily attacked by hot 0.1 N sulfuric acid, whereas the heat treated coating from the Bi₂O₃–SiO₂–TiO₂–ZnO–B₂O₃ system presented a finer microstructure with thin interconnected Bi₄Ti₃O₁₂ crystals and a glassy phase more resistant to hot 0.1 N sulfuric acid attack etching.     

Elastic properties of KGd(WO4)2:Ho3+ single crystals studied by Brillouin spectroscopy

The analysis some of the acoustic phonons propagating in pure and Ho³⁺‐doped KGd(WO₄)₂ single crystals in the GHz frequency range by Brillouin scattering method has been presented. For investigated crystals the velocities of the longitudinal and transverse acoustic phonons [100], [010], [001], [101], [101] and [110] have been determined. Moreover, the value of elastic constants C₂₂, C₄₄ and C₆₆ of pure and Ho³⁺‐doped KGd(WO₄)₂ single crystals have been estimated. It was revealed that the presence of the Ho³⁺‐ions in KGd(WO₄)₂ crystals, for the used doping concentration 1 at% and 8 at%, does not influence their elastic properties. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)