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.     

Growth of lithium triborate (LiB3O5) single crystals in the Li2O-B2O3-MoO3 system

Solubility curves are plotted and optimal solvents for growing lithium triborate (LiB₃O₅) single crystals are determined. The crystallization range of this compound in the Li₂O-B₂O₃-MoO₃ system is refined. The molybdenum content in the crystals grown is analyzed.     

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)     

Single crystal preparation of MnxCr3–xO4 by flux method

Mn_xCr_{3 x}O₄ was prepared by the flux method. Melts of PbO PbF₂, Bi₂O₃ B₂O₃, B₂O₃, Na₂B₄O₇, and Na₂W₂O₇ Na₂WO₄ were used. The best results could be yielded with the PbO PbF₂ flux, from which crystals with 2–4 mm in thickness were grown. The Bi₂O₃ B₂O₃ flux produced crystals with 1–2 mm in thickness. The spinell structure of the chromite was proved by X-ray investigation.     

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)     

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.     

Growth of Tetragonal Hausmannites MnxFe3−xO4 from Bi2O3-V2O5 Fluxes

The growing of tetragonal single crystals of Mn_xFe_3−xO₄ (x = 3.0 and 2.93) with hausmannite type structure is reported. Samples with x = 2.69 were no more monocrystalline due to the cubic-tetragonal transformation. Crystals were grown at temperatures from 1150 to 950°C from Bi₂O₃ + 10 wt.% V₂O₅ melts. The largest crystals reached nearly 10 mm in length and weights till to 0.5 g.     

Morphology of synthetic chrysoberyl and alexandrite crystals: Analysis of experimental data and theoretical modeling

A morphological analysis of chrysoberyl and alexandrite crystals obtained by flux crystallization has been performed. Seven morphological types of crystals are selected. The surface energies of the faces of chrysoberyl and alexandrite crystals and their isostructural analogs, BeCr₂O₄ and BeFe₂O₄, have been calculated by atomistic computer modeling using the Metadise program. A “combined” approach is proposed which takes into account both the structural geometry and the surface energy of the faces and thus provides better agreement between the theoretical and experimentally observed faceting of chrysoberyl and alexandrite crystals.     

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).     

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 Mixed Titanium Sillenites

Top Seeded Solution Growth (TSSG) technique has been used to produce Bi₁₂TiO₂₀ (BTO) and some mixed Bi₁₂Ti_0.8M_0.2O₂₀ single crystals in which Ti was substituted with M=V, Pb or Ga. Pure Bi₄₀Ga₂O₆₃ (BGaO) single crystals have been grown, too. Thermal conditions enabling growth of uniform [110] sillenite single crystals with totally flat (110) interface have been found. Influence of composition on spectral characteristics of the BTO mixed crystals has been investigated.     

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)     

Growth and characterization of Fe1 ? x M x VO4 single crystals (M = Al, Cr, Co, Ga)

Triclinic Fe_{1 ? x} M _x VO₄ single crystals (M = Al, Ga, Co, Cr) have been grown by the flux method from systems based on PbO-V₂O₅. Their crystallographic parameters are determined by powder X-ray diffraction. Fe_{1 ? x} Ga _x VO₄ single crystals (x = 0?C0.3) with a volume more than 1cm³ are grown using the seeding technique. The temperature and field dependences of magnetization and magnetic susceptibility of the grown Fe_{1 ? x} Ga _x VO₄ and Fe_{1 ? x} Al _x VO₄ single crystals (x = 0.3 in the solution-melt) are reported. It is shown that the magnetizations of these crystals exceed that of FeVO₄, and both of their antiferromagnetic phase transitions are shifted to lower temperatures.     

Crystal growth and dislocation arrangement of Czochralski alexandrite (BeAl2O4:Cr) single crystals

The [001] oriented alexandrite (BeAl₂O₄:Cr) single crystals have been grown by the Czochralski pulling technique. The X-ray topographic investigation indicates that the grown-in dislocations mainly originate from the seed and propagate along the path normal or nearly perpendicular to the growth interface. The dislocation density and arrangement is closely related to the quality of the seed crystal.     

Structure of phases of the sillenite family in the Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-V<Subscript>2</Subscript>O<Subscript>5</Subscript> system

X-ray diffraction studies of sillenite Bi₂₄V₂O₄₀ single crystals grown by the hydrothermal method are performed for a separate crystal and powdered crystals. It is found that the composition of the two specimens is described by the (Bi_{24 − x} ▭ _x )[Bi _y ³⁺V_1−y ⁵⁺]₂ O₄₀ general formula with completely populated oxygen sites but differs in the content of vacancies at the bismuth site (this was established for the first time) and the Bi: V ratio at the tetrahedral site. The structural models of all the vanadium-containing sillenites reported in the literature are considered, and the possibility that Bi atoms are located at the centers of BiO₄ tetrahedra is established.     

Growth and optical characterization of cerium and lead‐doped Bi12TiO20 sillenite single crystals

Bi₁₂TiO₂₀ (BTO) single crystals doped with PbO and CeO₂ were grown by the Top Seeded Solution Growth (TSSG) technique from the liquid phase with nominal compositions of 10Bi₂O₃ : (1–x)TiO₂ : x PbO and 10Bi₂O₃ : (1–x)TiO₂ : xCeO₂ with x = 0.25 and 0.10. No growth‐related difficulties were encountered other than those typical of sillenite crystals. Samples with good optical quality were obtained and were characterized by optical absorption, dark current, spectral photocurrent dependence, optical activity and electro‐optic coefficient measurements. A comparison is made of the results of the optical measurements of doped and undoped BTO crystals. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High temperature superconducting thin films on CaGdAlO4 substrates

Single crystals of CaGdAlO₄ that have not been used previously as a substrate material for HTSC films were grown by Czochralski method and applied for the deposition of the Bi₂Sr₂CaCu₂O_8−x and LuBa₂Cu₃O_7−z thin films. The main advantages of the CaGdAlO₄ crystals are sufficiently low dielectric constant, appropriate crystal lattice match with most HTSC, fairly high crystal perfection and reproducibility of the growth process. Bi₂Sr₂CaCu₂O_8−x films were prepared by “off-axis” DC magnetron sputtering and LuBa₂Cu₃O_7−z films were obtained by flash evaporation MOCVD technique on the (0 0 1) oriented CaGdAlO₄ substrates.     

Influence of the melting conditions of heavy metal oxide glasses containing bismuth oxide on their optical absorption

Glasses of the systems Bi₂O₃-SiO₂, Bi₂O₃-PbO-Ga₂O₃, Bi₂O₃-PbO-Ga₂O₃-GeO₂ and Bi₂O₃-GeO₂-Li₂O have been prepared and the interaction of their melts with crucibles of different materials has been analytically determined. Silica and porcelain crucibles were very strongly corroded and the glass composition was noticeably altered. Instead platinum crucibles are not affected if the Bi₂O₃ content is not too high. The color of the glasses changes in all cases from pale yellow to deep brown when the melting temperature reaches approximately 1000 °C. The higher the temperature and the Bi₂O₃ content the darker the brown color, independently of the nature of the employed crucible. The addition of oxidizing ions (Sb⁵⁺, As⁵⁺ or Ce⁴⁺) to the glass batch prevents darkening. Nanoparticles of elementary bismuth Bi⁰ are observed by transmission electron microscopy in the glasses melted above 1000 °C. The partial thermoreduction of the Bi₂O₃ during the heating of the glass melt is proposed as the mechanism responsible for the observed darkening.     

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.