Polymorphism of Bismuth Sesquioxide. I. Pure Bi2O3

Stability relationships of the four polymorphs of bismuth oxide have been determined by means of DTA and high-temperature x-ray studies. The stable low-temperature monoclinic form transforms to the stable cubic form at 730 ±5 °C, which then melts at 825 ± 5 °C. By controlled cooling, the metastable tetragonal phase and/or the metastable body-centered cubic (b.c.c.) phase appear at about 645 °C. Whereas b.c.c. can be preserved to room temperature, tetragonal will transform to monoclinic between 550 and 500 °C. Tetragonal Bi₂O₃, however, is easily prepared by decomposing bismutite (Bi₂O₃·CO₂) at 400 °C for several hours. The greatest transition expansion occurs at the monoclinic to cubic inversion, and cubic Bi₂O₃ shows the greatest coefficient of volume expansion. With exposure to air, Bi₂O₃ carbonates and partially transforms to bismutite and an unknown phase.     

Effect of impurities on the electrical properties of the defect perovskite Li<Subscript>0.33</Subscript>La<Subscript>0.57</Subscript>TiO<Subscript>3</Subscript>

A perovskite phase with the composition Li_0.33La_0.57TiO₃ modified with up to 7 wt % Bi₂O₃, SiO₂, Li₃PO₄, or Li₃BO₃ has been prepared by solid-state reactions. The samples in the LLTO–Li₃PO₄, LLTO–Bi₂O₃, and LLTO–SiO₂ systems were single-phase over the entire composition range studied. In the LLTO–Li₃BO₃ system, increasing the lithium borate concentration causes a transition from a defect perovskite structure to the layered perovskite-related structure of Li₂La₂Ti₃O₁₀. The addition of Bi₂O₃ and Li₃PO₄ has been shown to increase the total conductivity of the ceramics by almost one order of magnitude. Li₃BO₃, Li₃PO₄, Bi₂O₃, and SiO₂ additives improve the sintering behavior of the Li_0.33La_0.57TiO₃ ceramics.     

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.     

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.     

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.     

Heat conductivity of a beryllia-based composition at low temperatures

The thermal conductivity of beryllium oxide with additions of Y₂O₃, MgO, Fe₂O₃, TiO₂, A1₂O₃, SiO₂, ZrO₂, and glass was measured at 120–260°K. The additions were made in amounts up to 5 wt. %.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 6, pp. 980–985, December, 1980.     

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.     

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.     

Growth of Sillenite-Structure Single Crystals

The main processes for preparing bulk single crystals and films of photorefractive and piezoelectric Bi₁₂M _x O_20±δ (M = Group II–VIII elements) sillenite compounds are considered. Experimental data are summarized on the crystal growth of Bi₁₂M _x O_20±δ from the melt and under hydrothermal conditions, and the key morphological features of sillenites are analyzed. Various types of macroscopic growth defects in sillenite-type crystals are described, and their origin is discussed. The compositions of second-phase inclusions in undoped and doped (Group I–VIII elements) Bi₁₂SiO₂₀, Bi₁₂GeO₂₀, and Bi₁₂TiO₂₀ single crystals are presented, and the main physicochemical properties of various Bi₁₂M _x O_20±δ crystals are summarized.     

Direct atomic emission analysis of bismuth and germanium oxides and bismuth orthogermanate crystals using two-jet arc plasma

Simple, high-speed procedures have been proposed for direct analysis of Bi₂O₃, GeO₂, and crystals of bismuth orthogermanate (BGO) by atomic emission spectroscopy with a high-power two-jet arc plasma as an excitation source. The detection limits for 36 impurities have been shown to lie in the ranges of 0.008–2 (Bi₂O₃), 0.006–4 (GeO₂), and 0.008–3.5 μg/g (BGO).     

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.     

Thermally Stimulated Currents in Sillenite Crystals

Thermally stimulated currents in nominally undoped, doped, and vacuum-annealed Bi₁₂TiO₂₀, Bi₁₂SiO₂₀, and Bi₁₂GeO₂₀ crystals were measured between 80 and 300 K. The results were used to assess the depth and concentration of trapping centers.     

Preparations and photocatalytic degradation of methyl orange in water on magnetically separable Bi12TiO20 supported on nickel ferrite

A magnetically separable photocatalyst Bi₁₂TiO₂₀/SiO₂/NiFe₂O₄ (BSN) with a typical ferromagnetic hysteresis was prepared by a simple process: the magnetic 200 wt% SiO₂/NiFe₂O₄ (SN) dispersion prepared by a liquid catalytic phase transformation method and the visible-light-active photocatalyst Bi₁₂TiO₂₀ prepared by a simple coprecipitation processing were mixed, sonificated, dried, and calcined at 550°C. The prepared photocatalyst showed high photocatalytic activity for the degradation of methyl orange in water under UV irradiation and visible-light irradiation (λ>400 nm), and it was easy to be separated from a slurry-type photoreactor under the application of an external magnetic field, being one of promising photocatalysts for wastewater treatment. Transmission electron microscope (TEM) and X-ray diffractometer (XRD) were used to characterize the structure of the photocatalyst, indicating that the magnetic SN particles adhered to the surface of the Bi₁₂TiO₂₀ congeries. SiO₂ layer round the surface of NiFe₂O₄ nanoparticles prevented effectively the injection of charges from TiO₂ particles to NiFe₂O₄, which gave rise to the increase in photocatalytic activity.     

Preparation of Bi2O3SiO2 films by a rapid quenching technique and their crystallization process

Using a rapid quenching technique, Bi₂O₃SiO₂ films were prepared. The films of δ-Bi₂O₃ solid solution (ss) were obtained from the quenched melt containing 5 to 17% SiO₂. The films were transparent when the SiO₂ content was more than 10%. In the films with the composition from 20 to 35% SiO₂, δ-6Bi₂O₃·SiO₂ ss coexisted with glass phase, while glass films were obtained from the melt containing more than 40% SiO₂. The δ-phase in the films found to show the perfect grain-orientation, as the (111) were parallel to the film plane. The quenched films were annealed at various temperatures to investigate their crystallization process.     

Photoreduction synthesis of silver on Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> nanocomposites and their catalytic activity for the degradation of methyl orange

This paper demonstrates the preparation of pure TiO₂, 40% of Bi₂O₃ in TiO₂ and Ag loaded Bi₂O₃/TiO₂ nanocomposites by the hydrothermal method followed by the photoreduction process. The crystal structure, morphology and composition of the samples were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy respectively. The dispersion of Ag nanoparticles on the surface of Bi₂O₃/TiO₂ nanocomposites are found to bring the conduction band near to the valence band, resulting in the narrow band gap compared to pure TiO₂ and Bi₂O₃/TiO₂ nanocomposites. The XRD analysis demonstrated that silver nanoparticles were dispersed finely on the surface of Bi₂O₃/TiO₂ nanocomposites. All the characterization results revealed that the Ag/Bi₂O₃/TiO₂ nanocomposites were smaller crystallite size, stronger absorbance in the visible region and greater surface area than pure TiO₂ and Bi₂O₃/TiO₂ nanocomposites. The photoluminescence intensity decreases with an increase in the UV-illumination time of Ag loaded Bi₂O₃/TiO₂ revealing a decrease in the recombination rate of electron–hole pairs. In order to test them as a photocatalyst, methyl orange was used as a standard. The photocatalytic degradation of methyl orange shows that the ABT5 sample exhibits the maximum degradation efficiency of 99% within 180 min of irradiation.     

Dielectric properties of Bi4(GeO4)3 and Bi4(SiO4)3

Dielectric constant, dielectric loss and conductivity of Bi₄(GeO₄)₃ and Bi₄(SiO₄)₃ single crystals have been measured as a function of frequency and in the temperature range from liquid nitrogen temperature to 400° C. The values of the static dielectric constant at room temperature are 16·4 and 13·7 for Bi₄(GeO₄)₃ and Bi₄(SiO₄)₃ respectively. The plots of log (σ) against reciprocal temperature at different frequencies of these crystals merge into a straight line beyond 250°C and the activation energies calculated in this region are found to be 0·95 eV and 1·2 eV for Bi₄(GeO₄)₃ and Bi₄(SiO₄)₃ respectively.     

Effects of TiO2 addition on the superconducting properties of Bi-Sr-Ca-Cu-O system

The effects of TiO₂ addition in Bi₂Sr₂CaCu₂O₈Ti_y (Bi-2212) with y = 0, 0.05, 0.10 and 0.15 and (Bi_1.6Pb_0.4)Sr_1.6Ca₂Cu_2.8O₁₀Ti_y (Bi-2223) with y = 0, 0.10,0.20 and 0.40 are studied and compared. The samples have been investigated by powder X-ray diffraction (XRD), dc electrical resistance, critical current density (J_c) and scanning electron microscopy (SEM). The XRD patterns of the Bi₂Sr₂CaCu₂O₈Ti_y materials showed the Bi-2212 as the dominant phase. In the TiO₂ added samples (with x = 0.05 and 0.1), the c lattice parameter decreased slightly from the non-added sample showing the possibility of Ti incorporating into the crystal structure of the Bi-2212 phase. In the undoped (Bi_1.6Pb_0.4)Sr_1.6Ca₂Cu_2.8O₁₀ material, the XRD pattern showed the existence of mixed phases of Bi-2223 and Bi-2212. The TiO₂ added Bi-2223 samples do not show any systematic variation in the c lattice parameter, indicating that Ti may not be incorporated into the Bi-2223 crystal structure. The T_c values in both systems decreased with the addition of TiO₂. The critical current densities, J_c at 40 K in the Bi₂Sr₂CaCu₂O₈ system and at 77 K in the (Bi_1.6Pb_0.4)Sr_1.6Ca₂Cu_2.8O₁₀ system also decreased with the addition of TiO₂. SEM micrographs of both systems showed a slight decrease in average grain size when TiO₂ was added.     

Steady-State Photocurrent Characteristics in Sillenite Crystals

The steady-state photocurrent through Bi₁₂TiO₂₀, Bi₁₂GeO₂₀, and Bi₁₂SiO₂₀ crystals was measured as a function of temperature (77–300 K) and illumination. The four-level energy-band diagram inferred from the experimental data adequately describes the carrier generation, recombination, and photocurrent quenching processes in wide-gap sillenite semiconductors.     

Two-liquid phase structure of photoconducting oxide glasses

An electron microscopic study of a series of photoconducting oxide glasses in the systems CdO/B₂O₃/SiO₂, CdO/B₂O₃/GeO₂ and PbO/Al₂O₃/SiO₂, has shown the essential diphasic nature of these materials. The samples were examined by replica techniques, in transmission and by scanning electron microscopy. Two-liquid features have been identified, as has very fine-scale crystallisation, both on the basis of textures of fracture surfaces and of electron diffraction patterns. Physical models proposed to explain the electronic properties of glasses such as these must take account of this obvious two-phase nature of such materials.     

Ternary novel TiO2/MgBi2O6/Bi2O3 nanocomposites with n-n-p heterojunctions: Impressive visible-light-triggered photocatalytic degradation of tetracycline

A series of novel ternary TiO₂/MgBi₂O₆/Bi₂O₃ nanocomposites were synthesized by a facile hydrothermal method. The ternary nanocomposites were characterized by XRD, FESEM, HRTEM, EDX, PL, EIS, Photocurrent, UV–vis DRS, BET, XPS, Raman, and FT-IR analyses. The photocatalytic performance of TiO₂ for the degradation of tetracycline antibiotic after combining with MgBi₂O₆/Bi₂O₃ was significantly improved, which is 46.1 and 18.5 times higher than pristine TiO₂ and MgBi₂O₆/Bi₂O₃ photocatalysts, respectively. Furthermore, the ternary photocatalyst efficiently degraded MO, RhB, and MB dye pollutants, which is 22.5, 30.4, and 30.0 as high as TiO₂ and 11.2, 14.4, and 17.8 folds larger than MgBi₂O₆/Bi₂O₃ photocatalysts, respectively. The photoluminescence and electrochemical analyses confirmed promoted separation and facile transfer of the charges thanks to construction of n-n-p heterojunctions among n-TiO₂, n-MgBi₂O₆, and p-Bi₂O₃ components and more production of charge carriers due to integration of small band gap MgBi₂O₆ and Bi₂O₃ components with wide band gap TiO₂.