Synthesis of a bismuth germanium oxide source material for Bi4Ge3O12 crystal growth

A technique has been developed for the preparation of a modified source material for the crystal growth of bismuth orthogermanate, Bi₄Ge₃O₁₂ (BGO). It includes dispersion of molten bismuth through mixing with germanium oxide (GeO₂) powder in a rotating reactor, followed by oxidation with oxygen. The source material thus prepared contains, in addition to bismuth and germanium oxides, considerable proportions of germanates (Bi₂GeO₅, Bi₄Ge₃O₁₂, and Bi₁₂GeO₂₀), which improve the reactivity of the components of the source material during homogenization before the crystal growth process. After sintering at 880°C, the density of the modified source material (3.9 g/cm³) is a factor of 1.5 higher than that of a source material prepared from Bi₂O₃ and GeO₂ powders. BGO crystals grown using the synthesized source material possess good scintillation characteristics.     

The crystallization behaviour of bismuth germanate glasses

Glassy samples containing Bi₂O₃ and GeO₂ in different ratios were prepared and absorption measurements were performed on the samples in the ultraviolet region in order to determine the optical behaviour. Thermal analysis performed on the vitreous samples gave the glass transition temperature and the crystallization temperature. The main crystalline phases separated during the heat treatment were Bi₄Ge₃O₁₂ (BGO) and Bi₂GeO₅. The activation energy of the crystallization process of the sample BGO (1∶3) was measured using the isothermal method, and the value obtained was 47.1 kcal mol⁻¹; the reaction order was equal to 1.1.     

Non-isothermal crystallization kinetics of niobium-doped BGO70 glasses

Niobium-doped BGO glass of composition 85[3Bi₂O₃–7GeO₂ (BGO70)]–Nb15 was investigated for its kinetic parameters n and activation energies E _i : i = 1, 2, g of transformations via available models. Two crystallization exothermal peaks were observed for all heating histograms exploiting 5, 10, 15, and 20 K/min heating rates following the glass transition in non-isothermal DTA curves. An exothermal shoulder prior to first crystallization peak P₁ was identified as thermally favorable crystallizing metastable phase Bi₂GeO₅. P₁ was assigned to thermally stable crystal phase Bi₄Ge₃O₁₂. T _g lied between 729 and 731 K ± 4 K; T _p1 varied from 835–855 K ± 2 K, and T _p2 had values 965–986 K ± 2 K. Independent of the model exploited, activation energies E _g, E _p1, and E _p2 were 32.89 ± 3.1, 47.33 ± 0.72, and 62.74 ± 0.72 kcal/mol, respectively. P₁ corresponds to crystalline disks of Bi₄Ge₃O₁₂, and P₂ identified stable phase BGO crystal rods growing inward from the surface imbedded in glass matrix. Niobium doping modified the transformation kinetic parameters n and E _a of the composition by providing a control on growth mechanism for BGO platelets or rods.     

From eulytine to the ambidextrous crystal via the amorphous route: Lead & calcium bismuth germanate glasses

Lead and calcium bismuth germanate glasses containing at least 40 mole% GeO₂ were prepared. Physical properties and infrared spectra suggest that for glasses close to the PbO · Bi₂O₃ · 2GeO₂ composition: (a) packing efficiency is directly dependent upon cation size, (b) refraction is dependent upon cation type, (c) calcium can substitute for lead in nearly ideal fashion, (d) the open eulytine arrangement of a 2Bi₂O₃ · 3GeO₂ glass has collapsed, and (e) Ge₂O₇ dimers and small chains of GeO₄ tetrahedra (characteristic of Pb₅Ge₃O₁₁ and PbGeO₃ respectively) may persist. These findings offer a rationale for the limited yields of Ca₃Al₂Ge₃O₁₂ garnets obtained from such melts at high temperatures. A noncubic crystalline phase that may be Pb₃Bi₂Ge₃O₁₂ is also reported.     

Dynamics of energy absorption versus crystallization in Bi4Ge3O12 (BGO) amorphous materials

Theoretical and experimental determinations show that the intrinsic luminescence of BGO materials is strongly influenced by GeO₄⁻ tetrahedra around Bi³⁺ ions. Besides Bi³⁺ transitions, the excitonic spectra mask the BGO bandgap. Theoretical computations give the bandgap at 5.19 eV compared with the experimental one at 5.17 eV in BGO crystals and a significant decrease at 3.26 eV in the amorphous materials. This is due to the high disordering of the GeO₄⁻ tetrahedral which also reduces the refractive index of the amorphous materials. Formation of excitonic states and their overlap with the Bi³⁺ transitions suggests that the energy absorption takes place via the p-electrons of the O₂⁻ ions and is then transferred to the Bi³⁺p-electrons (excited states) close to the conduction band. Light emission appears after de-excitation to the ground state.     

FTIR spectroscopic study of some bismuth germanate glasses containing gadolinium ions

Since infrared spectroscopy and density are advantageous tools for the investigations of glasses, we have used them to obtain information concerning the local structure of xGd₂O₃·(100 − x)[4Bi₂O₃·GeO₂] glass system, with 0 ≤ x ≤ 30 mol%. Gd₂O₃ play the network modifier role in the studied glasses and determines, by progressive addition, the increase of structural volume difference between the hypothetical crystalline compounds and the quenched samples. FTIR spectroscopy data show that the glass structure consists on the BiO₆, GeO₄ and GeO₆ structural units, and the conversion among these units mainly depends on the Gd₂O₃ content.     

Varistor characteristics of vanadium oxide-doped zinc oxide ceramics modified with bismuth oxide

The effect of Bi₂O₃ on microstructure, electrical properties, dielectric characteristics, and aging behavior vanadium oxide-doped zinc oxide varistor ceramics was systematically investigated. Analysis of the phase indicated that the ceramics modified with Bi₂O₃ consisted of ZnO grain as a main phase and a few secondary phases such as Zn₃(VO₄)₂, ZnV₂O₄, BiVO₄, V₂O₅, and Mn-rich phase. The average grain size increased from 5.6 to 7.2 μm and the sintered density decreased in the range of 5.51–5.37 g/cm³ up to 0.05 mol%, whereas a further addition increased it to 5.40 g/cm³ at 0.25 mol%. The breakdown field decreased from 4,874 to 2,205 V/cm with an increase in the amount of Bi₂O₃. The ceramics added with 0.025 mol% Bi₂O₃ were characterized by a surprisingly high nonlinear coefficient (60) and very low leakage current density (20 μA/cm²). Bi atoms in the bulk acted as a donor to increase the electron concentration with an increase in the amount of Bi₂O₃.     

Depolymerization of GeO2 and GeO2 · Sb2O3 glasses by Bi2O3

Glasses that contain at least 60 mol% GeO₂ were prepared in the Bi₂O₃ · GeO₂ and Bi₂O₃ · Sb₂O₃ · GeO₂ systems. Their densities, refractive indices, and infra-red spectra were recorded. Negative molar volume deviations and positive refraction deviations occur for all of the binary glasses. These create deviations for the 60 to 80 mol % GeO₂ ternary glasses that indicate non-ideal mixing when Sb³⁺ substitutes for Bi³⁺. Also, the main Ge-O stretching vibration shifts to as low as 695 cm^?1 for the Bi₂O₃-rich binary and ternary glasses. All of these findings show that Bi₂O₃ more effectively depolymerizes GeO₂ than does Sb₂O₃. The probable structural reasons for this behaviour are discussed.     

Synthesis of bismuth oxide nanoparticles at 100 °C

A simple gel to crystal conversion route has been followed for the preparation of ultrafine Bi₂O₃ particles at 80–100 °C under refluxing conditions. Freshly prepared bismuth hydroxide gel is allowed to crystallize under refluxing and stirring conditions for 6–12 h. Formation of nanocrystallites of Bi₂O₃ is confirmed by X-ray diffraction (XRD) study. The thermal decomposition of bismuth hydroxide yields Bi₂O₃ only at 400 °C. This shows the advantage of the present method. Transmission electron microscope (TEM) investigations revealed that the average particle size is 50 nm for these oven-dried powders.     

Solvothermal synthesis of bismuth selenide nanotubes

Bismuth selenide (Bi₂Se₃) nanotubes have been synthesized for the first time using dimethyl formamide (DMF) by solvothermal method. Ammonium bismuth citrate (C₂₄H₂₀Bi₁₄O₂₈,6NH₃,10H₂O), elemental selenium (Se) and DMF when treated solvothermally in an autoclave at 165 °C formed Bi₂Se₃ nanocrystals of different morphologies. The morphological modifications are found to depend on the duration of heat treatment and the relative proportions of the reactants. Heat treatment for 24 h yields Bi₂Se₃ nanotubes whereas large needle shaped structures are produced on altering the relative proportions of the reactants during the same period. The as synthesized products were characterized by XRD, EDXA and TEM.     

Microhardness and interatomic binding in some cubic crystals

Vicker’s microhardness measurements have been carried out on single crystals of CaF₂, SrF₂, BaF₂, CdF₂, PbF₂, EuF₂, ThO₂, NaClO₃, NaBrO₃, Bi₄ (GeO₄)₃, Bi₄(SiO₄)₃, Bi₁₂GeO₂₀ and Bi₁₂SiO₂₀. The hardness values are discussedvis-a-vis the interatomic binding in these crystals. While most of the fluorite-type crystals are highly ionic, covalency is indicated in the bismuth compounds studied.     

Fabrication of Active Fluoroaluminosilicate Fibers for High-Power Fiber Lasers

An all-vapor phase modified chemical vapor deposition (MCVD) process has been proposed for the fabrication of active-fiber preforms with a fluorine-rich F–Yb₂O₃–GeO₂–Al₂O₃–SiO₂ glass core. The composition of the glass has been shown to differ significantly from that of the vapor–gas mixture because of the formation of GeF₄, AlF₃, and YbF₃, which are volatile at typical MCVD temperatures. We have identified F–Yb₂O₃–GeO₂–Al₂O₃–SiO₂ glass deposition conditions that ensure uniform distributions of the dopants along the length of the preform.     

High-Temperature Heat Capacity and Thermodynamic Properties of HoBiGeO<Subscript>5</Subscript> and ErBiGeO<Subscript>5</Subscript>

Polycrystalline HoBiGeO₅ and ErBiGeO₅ samples have been prepared by solid-state reactions, by firing stoichiometric mixtures of Ho₂O₃ (Er₂O₃), Bi₂O₃, and GeO₂. The effect of temperature on the heat capacity of the synthesized compounds has been investigated by differential scanning calorimetry in the range 350–1000 K. The experimental C_p(T) data have been used to evaluate the thermodynamic functions of bismuth holmium and bismuth erbium germanates: enthalpy increment, entropy change, and reduced Gibbs energy.     

The Czochralski growth of bismuth-germanium oxide single crystals

The conditions necessary for the growth of high quality Bi₁₂GeO₂₀ single crystals by the Czochralski method have been determined. If growth conditions are not optimal cloudy inclusions appear in the crystals. By use of X-ray microprobe analysis no platinum has been found in the cloudy area, but the ratio Bi₂O₃ : GeO₂ is 3 : 2, compared to 6 : 1 in high quality regions. Optical transmission, as well as the dependence of the linear expansion coefficient on temperature, have been measured.     

Ternary bismuth oxides Bi26−xMxO40−y (M = Mg,Al, Co and Ni) related to α - Bi2O3

Reaction of Bi₂O₃ with MgO, NiO, Co₃O₄ and Al₂O₃ gives rise to the corresponding ternary bismuth oxides, Bi₁₈Mg₈O₃₆, Bi₁₈Ni₈O₃₆, Bi₂₀Co₆O₃₉ and Bi₂₄Al₂O₃₉. These oxides have the general formula Bi_26?xM_xO_40?y and exhibit BCC structures related to α - Bi₂O₃. In the first three solids, the metal ions, M, replace bismuth randomly at the octahedral 24r sites (space group 123); in the last case, aluminium ions occupy the tetrahedral 2a sites, the phase being isostructural with Bi₂₄Ge₂O₄₀. Starting from Bi₂O₃ and NiO, orthorhombic Bi₂Ni₂O₅ has also been obtained.     

High-temperature heat capacity and thermodynamic properties of TbBiGeO<Subscript>5</Subscript> and DyBiGeO<Subscript>5</Subscript>

Polycrystalline TbBiGeO₅ and DyBiGeO₅ samples have been prepared by solid-state reactions, by firing stoichiometric mixtures of Tb₂O₃ (Dy₂O₃), Bi₂O₃, and GeO₂ in air at 1003, 1073, 1123, 1143, 1173, and 1223 K. The molar heat capacity of the bismuth terbium and bismuth dysprosium germanates has been determined by differential scanning calorimetry. The experimental C _p (T) data obtained in the range 350–1000 K have been used to evaluate the thermodynamic functions of the synthesized oxide compounds: enthalpy increment, entropy change, and reduced Gibbs energy.     

Polymorphism of Bismuth Sesquioxide. II. Effect of Oxide Additions on the Polymorphism of Bi2O3

The effect of small oxide additions on the polymorphism of Bi₂O₃ was studied by means of high-temperature x-ray diffractometry. Solidus and occasional liquidus temperatures were approximated, so that tentative partial phase diagrams for 33 oxide additions were constructed. Only the monoclinic and the cubic forms of Bi₂O3 were found to be stable. Other phases, frequently reported by previous investigators, such as tetragonal and body-centered cubic (b.c.c.), were shown to form metastably from cooled liquid or cubic. An impure b.c.c. phase of distinct but variable composition appeared in systems of ZnO, PbO, B₂O₃, Al₂O₃, Ga₂O₃, Fe₂O₃, SiO₂, GeO₂, TiO₂, and P₂O₅. The impure b.c.c. phase in the systems with SiO₂, GeO₂, and TiO₂ melted congruently about 100 °C above the m.p. of Bi₂O₃. The impure b.c.c. phase was formed metastably in systems with Rb₂O, NiO, MnO, CdO, V₂O₅, and Nb₂O₅; the conditions of formation were dependent on composition, preparation, and heating schedules. The impure b.c.c. phases, both stable and metastable, had smaller unit cell dimensions than that of pure Bi₂O₃.     

Mechanochemical Synthesis of Crystalline Compounds in the Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>–GeO<Subscript>2</Subscript> System

Crystalline mechanochemical synthesis products in the Bi₂O₃–GeO₂ system are studied by x-ray diffraction. The results indicate the formation of sillenite (Bi₁₂GeO₂₀), eulytite (Bi₄Ge₃O₁₂), and Aurivillius (Bi₂GeO₅) phases. The Aurivillius phase is shown to be in mechanochemical equilibrium with the sillenite phase in the 2Bi₂O₃ + GeO₂ system and with the eulytite phase in the Bi₂O 3 + GeO 2 system. The structural parameters of the synthesized metastable solid solutions are determined. The three phases contain high concen-trations of vacancies. In addition, the sillenite and Aurivillius phases are characterized by compositional disordering. Structural and ESR data point to partial reduction of the oxides, which accounts for the formation of the Aurivillius phase. According to x-ray photoelectron spectroscopy results, mechanical activation of bismuth oxide produces reduced binding energy states of Bi and O, which is tentatively attributed to clustering and the formation of complex radicals.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 6, 2005, pp. 711–719.Original Russian Text Copyright © 2005 by Zyryanov, Smirnov, Ivanovskaya.     

A family of mixed-layer type bismuth compounds

Existence of a family of mixed-layer type bismuth compounds with the general formula Bi₄A_2m?1B_2m+1O_6m+9, where A is Na(1+), Sr(2+), Bi(3+), etc., B is Ti(4+), Nb(5+), W(6+), etc. and m is an integer as 1,2, or 3, was confirmed. Following new compounds, each of which has an orthorhombic cell, were found: Bi₅TiNbWO₁₅ and Na_0.5Bi_4.5Nb₂WO₁₅ for m=1, Bi₇Ti₄TaO₂₁, SrBi₆Ti₃Nb₂O₂₁ and BaBi₆Ti₃Nb₂O₂₁ for m=2 and BaBi₈Ti₇O₂₇ and PbBi₈Ti₇O₂₇ for m=3. An extensive study for thermal phase transitions was made by both DTA method and dielectric measurement.     

Compositional dependence of the structural and dielectric properties of Li<Subscript>2</Subscript>O–GeO<Subscript>2</Subscript>–ZnO–Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>–Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

(10Li₂O–20GeO₂–30ZnO–(40-x)Bi₂O₃–xFe₂O₃ where x = 0.0, 3, 6, and 9 mol%) glasses were prepared. A number of studies, viz. density, differential thermal analysis, FT-IR spectra, DC and AC conductivities, and dielectric properties (constant ε′, loss tan δ, AC conductivity, σ _ac, over a wide range of frequency and temperature) of these glasses were carried out as a function of iron ion concentration. The analysis of the results indicate that, the density and molar volume decrease with an increasing of iron content indicates structural changes of the glass matrix. The glass transition temperature T _g and onset of crystallization temperature T _x increase with the variation of concentration of Fe₂O₃ referred to the growth in the network connectivity in this concentration range, while glass-forming ability parameter ΔT decrease with increase Fe₂O₃ content, indicates an increasing concentration of iron ions that take part in the network-modifying positions. The FT-IR spectra evidenced that the main structural units are BiO₃, BiO₆, ZnO₄, GeO₄, and GeO₆. The structural changes observed by varying the Fe₂O₃ content in these glasses and evidenced by FTIR investigation suggest that the iron ions play a network modifier role in these glasses while Bi₂O₃, GeO₂, and ZnO play the role of network formers. The temperature dependence of DC and AC conductivities at different frequencies was analyzed using Mott’s small polaron hopping model and, the high temperature activation energies have been estimated and discussed. The dielectric constant and dielectric loss increased with increase in temperature and Fe₂O₃ content.