Single‐crystal growth and characterization of mullite‐type orthorhombic Bi2M4O9 (M = Al3+, Ga3+, Fe3+)

Pure and homogeneous single crystals of orthorhombic mullite‐type Bi₂M₄O₉ (M = Al³⁺, Ga³⁺, Fe³⁺), and a mixed Bi₂Fe_1.7Ga_2.3O₉ crystal from an equimolar Ga/Fe composition were grown by the top seeded solution growth (TSSG) method. All these compounds melt incongruently in the range of about 800 and 1100 °C. In case of bismuth gallate and ferrate inclusion‐free crystals with dimensions up to several cubic centimeters can be grown. Limited solubility in Bi₂O₃ and the high steepness of the liquidus curve are the reasons for getting only small imperfect bismuth aluminate crystals. In contrast to ceramic materials preparation reported in literature, divalent calcium and strontium could not be incorporated into the mullite‐type structure during the melt growth process. Several fundamental physical properties like heat capacity, thermal expansion, heat conductivity, elastic constants, high‐pressure behavior and oxygen diffusivity were determined by different research groups using single‐crystalline samples from the as‐grown materials. Furthermore, the refractive indices of Bi₂Ga₄O₉ were measured in the range of 0.430 and 0.700 μm. Such as many other bismuth containing compounds the refractive indices of Bi₂Ga₄O₉ are larger than 2, and Bi₂Ga₄O₉ is an optic biaxial positive crystal. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal Growth and Basic Characterisation of the Bismuth Borate Bi2B8O15

Top‐seeded growth of Bi₂B₈O₁₅ from a stoichiometric melt is reported. Crystals have been grown with dimensions of up to 24 × 12 × 5 mm³ . Bi₂B₈O₁₅ shows twin domains at room temperature; their origin was investigated by methods of thermal analysis. The domains were found to result from a sluggish phase transition that occurs in a broad temperature range (ΔT ≈ 20 K) around ∼390 K. Unit cell parameters at room temperature were determined: a = 4.3140(6) Å, b = 22.148(2) Å, c = 6.4695(6) Å, β = 105.46(1)° , possible space group P2₁ (no.4), the non‐centrosymmetry was proved using powder SHG.     

Improved Single Crystal Growth of the Boron Sillenite “Bi24B2O39” and Investigation of the Crystal Structure

The boron sillenite, up to now known as the 12:1 compound Bi₂₄B₂O₃₉ in the system Bi₂O₃ – B₂O₃ andcrystallizing in the space group I23, melts incongruently at 655 °C only about 25 K above the eutectic tie line and corresponding to a steep liquidus line. Single crystals with dimensions larger then 1 cm ³ have been successfully grown in [100], [110], and [111] direction by an improved Top Seeded Solution Growth (TSSG) technique equipped with crucible weighing, accelerated crystal rotation technique and air‐cooled pulling rod. The structure of the boron sillenite was analyzed by X‐ray diffraction method, which was possible due to the high crystalline quality achieved. A defect‐free sublattice corresponding to a Bi‐O framework is isostructural with all sillenites, but a 2 Å environment around the origin is occupied by different cations with different population coefficients. The best calculation results in the formula Bi_24.5BO_38.25 which is more Bi‐rich than the 12:1 assumption.     

Crystalline bismuth oxide nanorods fabricated on Pt‐coated substrates using a trimethylbismuth and oxygen mixture

The current work reports the fabrication of crystalline Bi₂O₃ nanorods on Pt‐coated Si substrates using trimethylbismuth and O₂ as the bismuth and the oxygen sources, respectively, in the metalorganic chemical vapor deposition process. Their microstructures were characterized by scanning electron microscopy, X‐ray diffraction, and transmission electron microscopy. The obtained nanorods were crystalline, with their diameters in the range of 20–200 nm. The absence of tip‐nanoparticle and the presence of predeposited Bi₂O₃ layer indicated that the growth was dominated by a vapor‐solid process. The photoluminescence measurements of the Bi₂O₃ nanorods at room temperature exhibited an emission band peaked at around 422 nm. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Evaporation induced diameter control in fiber crystal growth by micro‐pulling‐down technique: Bi4Ge3O12

Diameter self‐control was established in Bi₄Ge₃O₁₂ fiber crystal growth by micro‐pulling‐down technique. In accordance with Bi₂O₃‐GeO₂ phase diagram, the diameter was controlled due to compensation of solidification with evaporation of volatile Bi₂O₃ self‐flux charged into the crucible with excess. The crucibles had capillary channels of 310 or 650 μm in outer diameter. The crystals up to 400 mm long and 50‐300 μm in diameter were grown at pulling‐down rates of 0.04‐1.00 mm/min. The melt composition and the pulling rate were generally only two parameters determining solidification rate. As a result, crystals with uniform (± 10%) diameter and aspect ratio up to 10⁴ were produced without automation of the process. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermal and optical properties of glasses of the system Bi2O3 – B2O3

In the binary system Bi₂O₃ – B₂O₃ glasses were prepared in the composition range 57.5 mol% ‐ 80 mol% B₂O₃ by defined slow cooling of large melt samples (about 75 cm³, each). Temperatures of crystallization, of melting and of glass transition were determined and density data of the glasses were derived using the hydrostatic weighting method. Thermal expansion coefficients and high precision refractive indices, together with their dispersion, were measured. The measured physical properties indicate subtle discontinuous structural changes of the glasses with glass composition, that match with the ranges of existence of the crystalline compounds of the binary system Bi₂O₃ – B₂O₃. Thermal investigations together with X‐ray powder diffraction analyses of crystallized glass samples prove the so far doubtful existence of a borate compound named “BiBO₃” in the PDF within the composition range 52.5 – 57.5 mol% B₂O₃.     

A new nonlinear optical crystal: Bi2O2(OH)(NO3)

The nonlinear optical (NLO) properties of Bi₂O₂(OH)(NO₃) crystals have been reported for the first time. Bi₂O₂(OH)(NO₃) crystals with dimensions of 1.3×1.2×0.1 mm³ have been grown by hydrothermal method, and the crystals characterized by X‐ray powder diffraction (XRD), SEM and IR. The measured second harmonic generation (SHG) effect of Bi₂O₂(OH)(NO₃) was about 7 times that of KDP. The mechanism responsible for the large SHG of Bi₂O₂(OH)(NO₃) was explained according to its structure. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal Growth of Boron Sillenite Bi24B2O39

The Sillenite type Bi₂₄B₂O₃₉ is an incongruently melting compound at T_p = 650 °C. Single crystals have been grown from non-stoichiometric melts as well as from high temperature solutions by the Czochralski method and by a top seeded solution growth technique (TSSG), respectively. The main difficulty in the crystal growth of Bi₂₄B₂O₃₉ arises from the very small field of crystallization in the binary system Bi₂O₃–B₂O₃. Further problems are caused by the nearly simultaneous formation of the 2:1 compound Bi₄B²O₉ and the 12:1 compound Bi₂₄B₂O₃₉. Therefore, a precise thermal reinvestigation of the phase diagram was carried out using DTA-technique on the Bi₂O₃-rich side. Additionally, crystal growth runs have been started in the ternary system Bi₂O₃–B₂O₃–Li₂O in order to extend the crystallization field. Homogeneous melts were more difficult to prepare because of the high density difference between Bi₂O₃ (∂ = 9.3 g/cm³) and B₂O₃ (∂ = 2.46 g/cm³). The homogeneity of the melts were improved, using Bi₂O₃ and synthesized Bi₄B₂O₉ (∂ = 8.25 g/cm³) as starting materials. As a result of this procedure, small crystals of Bi₂₄B₂O₃₉ were grown from these starting materials and the lattice parameter were determined.     

Growth and structure of α-Bi2B8O15 crystals

α-Bi₂B₈O₁₅ crystals (5-to 7-mm-thick, 2.7 × 2.7 cm² in cross section) have been grown by the Czochralski method from a melt of stoichiometric (Bi₂O₃: B₂O₃ = 20: 80) and nonstoichiometric (Bi₂O₃: B₂O₃ = 21.9: 78.1) compositions. It is established that there is a solid-solution range from 78.1 to 84.7 mol % B₂O₃ for α-Bi₂B₈O₁₅. The structure of a Bi₂(B₈O₁₅)(Bi₂O₃)_0.06 crystal, which was grown from a melt of nonstoichiometric composition and is an interstitial solid solution, has been refined (sp. gr. P2₁).     

Vibrational spectra of crystals of bismuth borates

The vibrational spectra of crystals of bismuth borates Bi₂₄B₂O₃₉, Bi₄B₂O₉, BiBO₃, and Bi₂B₈O₁₅ were obtained for the first time, and the spectra of Bi₃B₅O₁₂ and BiB₃O₆ crystals measured in the range 30–1600 cm^?1 at room temperature were refined. The lines observed were assigned to the corresponding vibrational transitions on the basis of the theoretical group analysis and comparison of the obtained results with the vibrational spectra of borates of different composition. The complication of the structure of bismuth borates with increasing content of B₂O₃ was traced by the example of vibrational spectra.     

Synthesis and characterization of strontium chloroborate whiskers

Single crystalline strontium chloroborate (Sr₂B₅O₉Cl) whiskers with uniform diameter have been synthesized by a facile route based on the calcination of precursor. The precursor was prepared by the sedimentation reaction between SrCl₂ and Na₂B₄O₇ aqueous solution. The products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transform infrared spectrum (FT‐IR). An optimal synthesis temperature for preparing Sr₂B₅O₉Cl whiskers was obtained, and the possible formation process was also presented.     

Structural investigation of bismuth borate glasses and crystalline phases

Glasses of the system: xBi₂O₃-(100−x)B₂O₃ (x = 20 to 66 mol%) were prepared and characterized by density, DSC, UV-visible absorption and ¹¹B MAS-NMR spectroscopy. Glass molar volume increases while the glass transition temperature decreases with Bi₂O₃ concentration. Densities of some bismuth borate glasses are found to be greater or very close to those of single crystal phases with equal composition. B¹¹ MAS-NMR studies determined that the fraction of tetrahedrally coordinated borons (N₄) is maximum at 42 mol% of Bi₂O₃ and that there is a local maxima in N₄ at Bi₂O₃ concentration of 50 mol%. Glasses containing Bi₂O₃ concentration of 33 mol% and higher show an unusual, intense absorption band just below the optical band gap. Two crystalline phases: Bi₃B₅O₁₂ and Bi₄B₂O₉ were prepared by devitrification of glasses and characterized by X-ray diffraction, FTIR and ¹¹B MAS-NMR studies. Both crystalline phases contained significantly lower N₄ than glasses with equal composition.     

Recrystallization of natural chrysoberyl in multicomponent melts

Chrysoberyl and alexandrite crystals have been grown from solutions in melts based on the Li₂CO₃-MoO₃, Bi₂O₃-MoO₃, PbO-V₂O₅, Na₂B₄O₇, and K₂MoO₄-MoO₃ systems using natural alexandrite and chrysoberyl debris as the initial BeAl₂O₄ compound. An analysis of the morphology and homogeneity of the crystals grown has revealed the Bi₂O₃-MoO₃ solvent to be the most appropriate. The optimal color characteristics (??quality?? of alexandrite effect) manifest themselves when adding about 5 mol % Cr₂O₃. The largest crystals (up to 10 mm in size) were obtained from a solution in melt based on PbO-V₂O₅ at a ratio of the crystal-forming component to the solvent of 9: 91 wt %; These characteristics, along with a relatively low operating temperature (970°C), give grounds to consider this type of solvent promising.     

Real Structure of LiB3O5 (LBO) Crystals Grown in Li2O‐B2O3‐MoO3 System

The liquidus surface structure and field of LiB₃O₅ (LBO) primary crystallization have been revealed in Li₂O‐B₂O₃ ‐MoO₃ ternary system. The optimization of charge composition and growth conditions results in large volume optical quality LBO single crystals yielding. Crystallographic properties and real defect structure of grown LBO single crystals have been investigated by X‐ray powder diffraction method and X‐ray reflection topography. The volume of the crystals is partly free of any structural imperfections.     

Multi‐factors on photocatalytic properties of Y‐doped Bi2WO6 crystallites prepared by microwave‐hydrothermal method

Different contents of Y‐doped Bi₂WO₆ crystallites were synthesized by a microwave‐hydrothermal method. The photocatalytic properties with different contents of Y‐doped Bi₂WO₆ crystallites were studied. The Y‐doped Bi₂WO₆ crystallites were also characterized by XRD, EDX, SEM and UV‐vis DRS and the multi‐factors on photocatalytic properties of Y‐Doped Bi₂WO₆ crystallites were discussed. The results indicate that Y³⁺ replacing Bi³⁺ enters into the Bi₂WO₆ lattice, producing a degree of Bi₂WO₆ lattice distortion. It also has an impact on the crystallinity of Bi₂WO₆ and the band gap is from 2.49 eV to 2.71 eV. The photocatalytic results show that when the content of Y doping becomes 10%, the degradation rate of rhodamine B is above 90% after 40 min irradiation, which shows that doping the proper rare earth ions is conducive to the photocatalytic properties of Bi₂WO₆ crystallites.     

Growth,structure, and superconducting properties of Bi2Sr2Ca2Cu3O10 and (Bi,Pb)2Sr2Ca2Cu3O10‐y crystals

Large and high‐quality single crystals of both Pb‐free and Pb‐doped high temperature superconducting compounds (Bi_1‐xPb_x)₂Sr₂Ca₂Cu₃O_10‐y (x = 0 and 0.3) were grown by means of a newly developed “Vapour‐Assisted Travelling Floating Zone” technique (VA‐TSFZ). This modified zone‐melting technique was realised in an image furnace and allowed for the first time to grow Pb‐doped crystals by compensating for the Pb losses occurring at high temperature. Crystals up to 3×2×0.1 mm³ were successfully grown. Post‐annealing under high pressure of O₂ (up to 10 MPa at T = 500°C) was undertaken to enhance T_c and improve the homogeneity of the crystals. Structural characterisation was performed by single‐crystal X‐ray diffraction (XRD) and the structure of the 3‐layer Bi‐based superconducting compound was refined for the first time. Structure refinement showed an incommensurate superlattice in the Pb‐free crystals. The space group is orthorhombic, A2aa, with cell parameters a = 27.105(4) Å, b = 5.4133(6) Å and c = 37.009(7) Å. Superconducting studies were carried out by A.C. and D.C. magnetic measurements. Very sharp superconducting transitions were obtained in both kinds of crystals (ΔT_c ≤ 1 K). In optimally doped Pb‐free crystals, critical temperatures up to 111 K were measured. Magnetic critical current densities of 2�10⁵ A/cm² were measured at T = 30 K and μ₀H = 0 T. A weak second peak in the magnetisation loops was observed in the temperature range 40‐50 K above which the vortex lattice becomes entangled. We have measured a portion of the irreversibility line (0.1‐5 Tesla) and fitted the expression for the melting of a vortex glass in a 2D fluctuation regime to the experimental data. Measurements of the lower critical field allowed to obtain the dependence of the penetration depth on temperature: the linear dependence of λ(T) for T < 30 K is consistent with d‐wave superconductivity in Bi‐2223. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Czochralski-growth of germanium crystals containing high concentrations of oxygen impurities

Oxygen-containing germanium (Ge) single crystals with low density of grown-in dislocations were grown by the Czochralski (CZ) technique from a Ge melt, both with and without a covering by boron oxide (B₂O₃) liquid. Interstitially dissolved oxygen concentrations in the crystals were determined by the absorption peak at 855 cm⁻¹ in the infrared absorption spectra at room temperature. It was found that oxygen concentration in a Ge crystal grown from melt partially or fully covered with B₂O₃ liquid was about 10¹⁶ cm⁻³ and was almost the same as that in a Ge crystal grown without B₂O₃. Oxygen concentration in a Ge crystal was enhanced to be greater than 10¹⁷ cm⁻³ by growing a crystal from a melt fully covered with B₂O₃; with the addition of germanium oxide powder, the maximum oxygen concentration achieved was 5.5×10¹⁷ cm⁻³. The effective segregation coefficients of oxygen in the present Ge crystal growth were roughly estimated to be between 1.0 and 1.4.     

Investigations of the non-isothermal crystallization of Bi4Ge3O12 (2:3) glasses

Amorphous Bi₄Ge₃O₁₂ glass samples were produced by melt quenching procedure stating with Bi₄Ge₃O₁₂ (BGO) powder, obtained by solid state reaction between oxides. The kinetics of non-isothermal crystallization of BGO nano-crystals has been investigated. Differential Thermal Analysis (DTA) can give the main parameters of crystallization with an exothermic peak from 813 K to 851 K depending on the heating rate, which was assigned to the crystallization of cubic BGO in the amorphous matrix and compared with the X-Ray Diffraction (XRD) patterns. The nano-crystal dimensions were calculated from the XRD patterns by using the Debye–Scherrer method and were compared with Transmission Electron Microscopy (TEM) images. It was shown that the Ozawa model is most suitable for describing the behavior of non-isothermal crystallization of BGO nano-crystals within the glass matrix. Experimental results suggest a disk-shape type growth mechanism for the Bi₄Ge₃O₁₂ nano-crystallites. The Flynn–Wall–Ozawa method has shown that the average activated energy value is 385 ± 14 kJ/mol which was computed within the same model and agrees very well with the activation energy of the crystallization.     

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.     

Infrared and Raman spectroscopy of mullite‐type Bi2Ga4O9

Mullite‐type Bi₂Ga₄O₉ single‐crystals were grown by the top‐seeded solution growth (TSSG) method and investigated by vibrational spectroscopy. Polarised IR specular reflectance and attenuated total reflectance (ATR) spectra, as well as polarised micro‐Raman spectra were acquired at room temperature. Powder IR spectra of sol‐gel‐derived samples were also recorded. Using model calculations and comparison to other mullite‐type compounds, bands at ∼ 850 – 400 cm^‐1 could be assigned to stretching and bending vibrations of the structural GaO₄ and GaO₆ units. Low‐energetic modes were attributed to motions involving Bi atoms. The IR spectra of Bi₂Ga₄O₉ display close similarities to those of the mullite‐type alkali gallates (9Ga₂O₃ · Rb₂O), while their differences to those of mullite sensu stricto (3Al₂O₃ · 2SiO₂ and 2Al₂O₃ · SiO₂, repectively) are assigned to Si‐O stretching vibrations of the corresponding tetrahedral units. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)