Temperature-dependent Raman spectra of Bi2Sn2O7 ceramics

The Bi₂Sn₂O₇ pyrochlore is known to undergo a sequence of structural phase transitions with an increase in temperature. Raman spectroscopy was employed in the investigation of the temperature dependence of the active phonons in the Raman spectrum. We observed 19 broad modes at room temperature, reflecting the low symmetry of the α-phase of Bi₂Sn₂O₇. The modes were discussed in relation to the Raman spectra of other pyrochlore-based oxides. The temperature dependence of the phonons evidences the α → β structural phase transition observed near 127 °C.     

Photocatalytic activity of a novel Bi-based oxychloride catalyst Na0.5Bi1.5O2Cl

A Bi-based oxychloride Na_0.5Bi_1.5O₂Cl with a layered structure as a novel efficient photocatalyst was studied in the present paper. The powder was synthesized by a solid state reaction method. It was characterized by X-ray diffraction, scanning electron microscope and UV–vis diffuse reflectance spectrum. Degradation of methyl orange was used to evaluate the photocatalytic activity. The as-synthesized Na_0.5Bi_1.5O₂Cl has a smaller optical band gap of 3.04 eV than BiOCl (E_g = 3.44 eV). It possesses a fair visible-light-response ability. The UV-induced photocatalytic activity follows the decreasing order of BiOCl > Na_0.5Bi_1.5O₂Cl > TiO₂, different from the order of Na_0.5Bi_1.5O₂Cl > TiO₂ > BiOCl under visible light irradiation. The dispersion of Pt over Na_0.5Bi_1.5O₂Cl leads to an obvious increase in the photocatalytic performance. The internal electric fields between [Na_0.5Bi_1.5O₂] and [Cl] slabs favor the efficient separation of photostimulated electron–hole pairs.     

Bi2O3 deposited on highly ordered mesoporous carbon for supercapacitors

A simple route has been employed to prepare nanosized Bi₂O₃ deposited on highly ordered mesoporous carbon. The electrochemical measurements reveal that, by loading only 10% Bi₂O₃ on the mesoporous carbon, the specific capacitance of the composite is improved by 62%, with the maximum value reaching 232 F g^?1 at a sweep rate of 5 mV s^?1. The specific capacitance of Bi₂O₃ is calculated and reaches 1305 F g^?1 at 1 mV s^?1. It is found that the mass transfer in the framework of the crystalline oxide is still difficult in spite of its nanosize, as evidenced by the decline of the specific capacitance of the Bi₂O₃ with the increase of the sweep rate. The cyclic life of composite materials is also measured and the capacitance only declines 21% after 1000 cycles.     

Diffuse X-ray scattering in the lead-free piezoelectric crystals Na1/2Bi1/2TiO3 and Ba-doped Na1/2Bi1/2TiO3

X-ray diffuse scattering has been measured by single-crystal X-ray diffraction on the lead-free piezoelectric perovskites Na_0.5Bi_0.5TiO₃ (NBT) and (Na_0.5Bi_0.5)_0.89Ba_0.11TiO₃ (NBT–Ba_0.11). Evidence of a pronounced local structure or included phase different from the rhombohedral average symmetry (space group R3c), or of a modulated structure with a characteristic period of 40 Å, is presented for NBT from high-resolution synchrotron X-ray mapping and CCD diffractometer investigations. The additional scattering shows pronounced anisotropy and produces satellite peaks displaced by 0.2 in the h and k indices for reflections such as {640} indexed on the pseudo-cubic unit cell with a = 7.761(2) Å. By contrast, weaker, less anisotropic diffuse scattering is observed for tetragonal NBT–Ba_0.11 (space group P4mm), which is directed along the pseudo-cubic 〈110〉. The lack of satellite peaks and pronounced anisotropic diffuse scattering in the tetragonal phase of phase NBT–Ba₀.₁₁ lends support to the idea that the massive tetragonal to rhombohedral phase transition in NBT at around 503–573 K, which is absent in NBT–Ba₀.₁₁, may be the key to understanding the local structure found repeatedly at room temperature in this complex material.     

The metallic 1T-WS2 as cocatalysts for promoting photocatalytic N2 fixation performance of Bi5O7Br nanosheets

Recently, widespread attention has been devoted to the typical layered BiOCl or BiOBr because of the suitable nanostructure and band structure. However, owing to the fast carrier recombination, the photocatalytic performance of BiOX materials is not so satisfactory. Loading 1T phase WS₂ nanosheets (NSs) onto Bi₅O₇Br NSs can improve the photocatalytic N₂ fixation activity. Among these, the obtained 1T-WS₂@Bi₅O₇Br composites with optimum 5% 1T-WS₂ NSs display a significantly improved photocatalytic N₂ fixation rate (8.43 mmol L⁻¹ h⁻¹ g⁻¹), 2.51 times higher than pure Bi₅O₇Br (3.36 mmol L⁻¹ h⁻¹ g⁻¹). And the outstanding stability of 1T-WS₂@Bi₅O₇Br-5 composites is also achieved. Exactly, the photoexcited electrons from Bi₅O₇Br NSs are quickly transferred to conductive 1T phase WS₂ as electron acceptors, which can promote the separation of carriers. In addition, 1T-WS₂ NSs can provide abundant active sites on the basal and edge planes, which can promote the efficiency of photocatalytic N₂ fixation. This work offers a novel solution to improve the photocatalytic performance of Bi₅O₇Br NSs.     

Thermochemical investigation of the new phase Bi12.5Gd1.5ReO24.5

The preparation of Bi_12.5Gd_1.5ReO_24.5 has been performed by solid-state reaction from Bi₂O₃, Gd₂O₃, and Re₂O₇. The phase has been synthesized for the first time. The X-ray measurements have shown that Bi_12.5Gd_1.5ReO_24.5 has a cubic structure (space group Fm3m). The standard molar enthalpy of formation of Bi_12.5Gd_1.5ReO_24.5 has been determined by solution calorimetry combining the enthalpies of dissolution of Bi_12.5Gd_1.5ReO_24.5 and (6.25Bi₂O₃ + 0.75Gd₂O₃ + 0.5Re₂O₇) mixture in 2 M HCl along with literature data. It has been determined that the above-mentioned phase is thermodynamically stable with respect to their decomposition into binary oxides at room temperatures. It was established that Bi₂O₃, Re₂O₇, R₂O₃, NH₄ReO₄ have not reacted at ambient temperatures. It is estimated that this reaction takes place at a temperature about 600 K.     

Structural characterization,molecular dynamics,dielectric and spectroscopic properties of tetrakis(pyrazolium) bis(μ2-bromo-tetrabromobismuthate(III)) dihydrate, [C3N2H5]4[Bi2Br10]·2H2O

The structure of [C₃N₂H₅]₄[Bi₂Br₁₀]·2H₂O, (PBB) was determined by single crystal X-ray diffraction at 100 K. It crystallizes in the monoclinic space group C2/m, with a = 12.992(4) Å, b = 16.326(5) Å, c = 8.255(3) Å, β = 108.56°(3), V = 1659.9(9) Å³ and Z = 2. The structure consists of discrete binuclear [Bi₂Br₁₀]⁴⁻ anions, ordered pyrazolium cations and water molecules. The crystal packing is governed by strong N–H⋯O and weak O–H⋯Br hydrogen bonds. A sequence of structural phase transitions in PBB was established on the basis of differential scanning calorimetry and dilatometric studies. Two reversible first-order phase transitions were found: (I → II) at 381/371 K (on heating/cooling) and (II → III) at 348/338 K. Dielectric response near both phase transitions is characteristic of crystals with the “plastic-like” phases. Over the phase III a low frequency dielectric relaxator is disclosed. The possible molecular motions in the PBB compound are characterized by the ¹H NMR studies. The infrared spectra of polycrystalline compound in the temperature range 300–380 K are reported for the region 4000–400 cm⁻¹. The observed spectral changes through the structural phase transition III → II are attributed to an onset of motion both of the pyrazolium cations and water molecules.     

Infrared studies of phase transitions in ferroelectric (C5H5NH)5Bi2Br11

Infrared spectra (3500–500 cm⁻¹) of polycrystalline (C₅H₅NH)₅Bi₂Br₁₁ samples were investigated within the temperature range 27–456 K. The assignments of the observed bands in the spectra measured at 27, 310 and 456 K are proposed. A temperature dependence of the wavenumbers and full width at half maximum (FWHM) of the bands arising from some internal vibrations of pyridinium cations are analysed in order to explain the role of cations in the mechanism of the phase transition at 118 (paraelectric–ferroelectric) and 403 K. It was found that numerous bands arising from the internal modes of the cations exhibit the splitting in the vicinity of both phase transitions, that indicates a distinct changes in the motional state of the pyridinium moieties.     

Phase diagram and local environment of Sn and Te: SnTeBi and SnTeBi2Te3 systems

From thermal analyses and X-ray diffraction the phase diagram of the BiSnTe and SnTeBi₂Te₃ sections was determined. The local environment of Sn and Te atoms was studied by ¹¹⁹Sn and ¹²⁵Te Mössbauer spectroscopy. The BiSnTe section showed a eutectic reaction at 267 °C and 20 % mole SnTe–80 % mole Bi. No intermediate compound was detected. The SnTeBi₂Te₃ section is characterized by a eutectic reaction at 585 °C and 40 % mole SnTe–60% mole Bi₂Te₃ and a peritectic reaction at 600 °C and 50 % mole SnTe–50% mole Bi₂Te₃. It corresponds to the compound SnBi₂Te₄, which has a rhombohedral layered structure with unit cell parameters a=4.3954(4) Å and c=41.606(1) Å. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASSnTe / Bi / Bi₂Te₃ / phase diagram / Mössbauer     

Lithium storage in hollow spherical ZnFe2O4 as anode materials for lithium ion batteries

Hollow microspheres composed of phase-pure ZnFe₂O₄ nanoparticles (hierarchically structured) have been prepared by hydrothermal reaction. The unique hollow spherical structure significantly increases the specific capacity and improves capacity retention of this material. The product of each phase transition during initial discharge (ZnFe₂O₄ ? Li_0.5ZnFe₂O₄ ? Li₂ZnFe₂O₄ → Li₂O + Li–Zn + Fe) and their structural reversibility are recognized by X-ray diffraction and electrochemical characterization. The products of the deeply discharged (Li–Zn alloy and Fe) and recharged materials (Fe₂O₃) were clarified based on high resolution transmission electron microscopic technique and first-principle calculations.     

New metastable hybrid phase,Zn2(OH)2(C8H4O4), exhibiting unique oxo-penta-coordinated Zn(II) atoms

The metastable phase (phase 1) Zn(OH)₂(tp)₂ (tp = C₈H₄O₄^2?) was found to be an intermediate forming during the hydrothermal synthesis of Zn₃(OH)₄tp (phase 2). Its structure has been determined ab initio from synchrotron powder diffraction data and refined with the Rietveld method: space group P2₁/c, a = 3.48856(2) Å, b = 5.84645(2) Å, c = 22.1331(1) Å, β = 103.46(1)°, D_x = 2.488 g/cm³, R_p = 0.10, R_B = 0.095 (402 independent reflections). The structures of the two analogues were compared. Whereas a mixed coordination of the zinc atoms was found in phase 2, phase 1 exhibits only penta-coordinated Zn(II). Moreover, different optical properties were observed, Zn₂(OH)₂(tp) showing photoluminescence at 378 nm under λ_ex = 316 nm.     

Built-in piezoelectric field improved photocatalytic performance of nanoflower-like Bi2WO6 using low-power white LEDs

Photocatalysis technology has been proved to be a potential strategy for removal of organic dyes, however high-power light sources are generally necessary to initiate photocatalytic reaction. In this work, we employed an excellent photocatalyst of Bi₂WO₆ with visible light harvest and meanwhile an intrinsic ferroelectricity, which realized the efficient degradation of organic dye via the synergetic photopiezocatalysis. Through coupling the illumination by a low-power (9 W) LED and the ultrasonic vibration (120 W) by an ultrasonic cleaner, the nanoflower-like Bi₂WO₆ composed of ultrathin nanosheets showed a much more enhanced photopiezocatalysis performance for purification of organic dye than the individual photocatalysis and piezocatalysis. Furthermore, the high mineralization efficiency and the good durability of the Bi₂WO₆ catalyst were demonstrated. The possible mechanism of photopiezocatalysis was finally proposed, where the ultrasound-induced piezoelectric field in Bi₂WO₆ drove photo-generated electrons and holes to diffuse along opposite directions, consequently promoting the separation efficiency of charge carriers. This work indicates that the synergetic photopiezocatalysis by coupling irradiation and ultrasonic vibration is a promising strategy to purify organic pollutants in wastewater.     

A facile green approach to the synthesis of Bi2WO6@V2O5 heterostructure and their photocatalytic activity evaluation under visible light irradiation for RhB dye removal

Bismuth tungsten oxide and vanadium pentoxide (Bi₂WO₆/V₂O₅) heterostructures are produced by a green synthesis approach using Azadirachta indica extract for photocatalytic performance. The hydrothermal method at temperatures between 120 °C and 140 °C is used to synthesize Bi₂WO₆. Bi₂WO₆ and V₂O₅ phases are formed in pure orthorhombic wells according to the XRD pattern. The SEM displays V₂O₅ nanorods, Bi₂WO₆ hierarchical microspheres that resemble flowers at 120 °C, and particles with a particle-like character at 140 °C. In V₂O₅, the asymmetric stretching vibrations of the triplely coordinated oxygen (chain oxygen) bonds and the vibration of the doubly coordinated oxygen (bridge oxygen) bonds are responsible for a peak at 611 cm^?1. In FTIR spectra between 600 and 1600 cm^?1, the major absorption bands in Bi₂WO₆ are attributed to the W-O stretching, Bi-O stretching, and W-O-W bridging stretching modes. Bi₂WO₆@V₂O₅ at 120 °C has the lowest bandgap energy (2.32 eV) and optical electronegativity (0.62), as well as the highest refractive index (2.57), extinction coefficient (2.21), and dielectric constant (ε_r = 0.72 and ε_i = 11.4) among all samples, making it a suitable material for photocatalysis. Rhodamine blue (RhB) dye degradation is used to measure the photocatalytic activity (PCA) of certain materials. The results showed that heterostructure V₂O₅@Bi₂WO₆ synthesized at 120 °C is more attractive among all samples due to high degradation of RhB dye under sunlight irradiation in 90 min.     

Thermodynamic evaluation and optimization of the (MgCl2 + CaCl2 + MnCl2 + FeCl2 + CoCl2 + NiCl2) system

A complete critical evaluation of all available phase diagram and thermodynamic data has been performed for all condensed phases of the (MgCl₂ + CaCl₂ + MnCl₂ + FeCl₂ + CoCl₂ + NiCl₂) system, and optimized model parameters have been found. The model parameters obtained for the binary subsystems can be used to predict thermodynamic properties and phase equilibria for the multicomponent system. The Modified Quasichemical Model was used for the molten salt phase, and the (MgCl₂ + MnCl₂ + FeCl₂ + CoCl₂ + NiCl₂) solid solution was modeled using a cationic substitutional model with an ideal entropy and an excess Gibbs free energy expressed as a polynomial in the component mole fractions. This is the first of two articles on the optimization of the (NaCl + KCl + MgCl₂ + CaCl₂ + MnCl₂ + FeCl₂ + CoCl₂ + NiCl₂) system.     

Thermodynamic evaluation and optimization of the (NaCl + KCl + MgCl2 + CaCl2 + MnCl2 + FeCl2 + CoCl2 + NiCl2) system

A complete critical evaluation of all available phase diagram and thermodynamic data has been performed for all condensed phases of the (NaCl + KCl + MgCl₂ + CaCl₂ + MnCl₂ + FeCl₂ + CoCl₂ + NiCl₂) system, and optimized model parameters have been found. The (MgCl₂ + CaCl₂ + MnCl₂ + FeCl₂ + CoCl₂ + NiCl₂) subsystem has been critically evaluated in a previous article. The model parameters obtained for the binary subsystems can be used to predict thermodynamic properties and phase equilibria for the multicomponent system. The Modified Quasichemical Model was used for the molten salt phase, and the (MgCl₂ + MnCl₂ + FeCl₂ + CoCl₂ + NiCl₂) solid solution was modeled using a cationic substitutional model with an ideal entropy and an excess Gibbs free energy expressed as a polynomial in the component mole fractions. Finally, the (Na,K)(Mg,Ca,Mn,Fe,Co,Ni)Cl₃ and the (Na,K)₂(Mg,Mn,Fe,Co,Ni)Cl₄ solid solutions were modeled using the Compound Energy Formalism.     

Synthesis and characterization of hierarchically structured mesoporous MnO2 and Mn2O3

Hierarchically structured mesoporous MnO₂ with high surface area was prepared by a facile precursor route. Well-defined morphological manganese oxalate, synthesized by adding l-lysine via a hydrothermal method, was used as precursor. Mesoporous amorphous MnO₂ with high Brunauer–Emmett–Teller (BET) surface area (340 m²/g) and mesoporous Mn₂O₃ composed of nano-crystals (BET surface area 188 m²/g) were obtained by selective calcination of the oxalate precursor at 330 °C and 400 °C, respectively. Thermogravimetric and differential thermal analyses (TG–DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂-sorption analysis and X-ray photoelectron spectroscopy (XPS) were used to characterize the structure and property of products. Cyclic voltammetry (CV) and charge–discharge measurements were used to preliminarily study the electrochemical performance of the products. The range of pH value (about 5.0–7.0) in the synthesis process is apt to prepare the hierarchical structured manganese dioxide. Other types of amino acids were also employed as the crystallization modifiers and different morphologies of manganese dioxides were obtained.     

Thermodynamic evaluation and optimization of the (LiF + NaF + KF + MgF2 + CaF2 + SrF2) system

A complete critical evaluation of all available phase diagram and thermodynamic data has been performed for all condensed phases of the (LiF + NaF + KF + MgF₂ + CaF₂ + SrF₂) system, and optimized model parameters have been found. The (LiF + NaF + KF + MgF₂ + CaF₂) subsystem has been critically evaluated in a previous article. The model parameters obtained for the binary and ternary subsystems can be used to predict thermodynamic properties and phase equilibria for the multicomponent system. The Modified Quasichemical Model for short-range ordering was used for the molten salt phase, and the low-temperature and high-temperature (CaF₂ + SrF₂) solid solutions were modelled using a cationic substitutional model with an ideal entropy and an excess Gibbs free energy expressed as a polynomial in the component mole fractions. Finally, the (Li, Na, K)(Mg, Ca, Sr)F₃ perovskite phase was modelled using the Compound Energy Formalism.     

Spectroscopic properties of RBiBO4 (R = Ca,Sr) glasses doped with TiO2

Transparent glasses, melt quenching derived, containing 10RO·20Bi₂O₃·(70 ? x)B₂O₃·xTiO₂ [R = Ca, Sr] with x = 0, 0.5, 1.0 wt% were characterized by X-ray powder diffraction. Physical and spectroscopic properties viz., density, absorption, emission, electron paramagnetic resonance (EPR) and FTIR were investigated. The absorption band around 823 nm in pure glass samples is attributed to the electronic transition of ³P₀ to ³P₂ of Bi⁺ radicals. A small absorption hump centered around 609 nm is found in all doped glasses due to ²T_2g to ²E_g transition of octahedral Ti³⁺ ions. The emission results revealed that all the samples exhibit a broad emission band covering entire visible-light range, with λ_ex = 360 nm, centered 470–520 nm corresponds to electronic transition of ³P₁ to ¹S₀ of Bi³⁺ ions, therefore the present materials can be potentially used as tunable or full-color display systems. And a strong emission around 706 nm with λ_ex = 514 nm due to transition of ²P_3/2 to ²P_1/2 of Bi²⁺ ions. In SrO mixed glasses Ti⁴⁺ ions effect the environment of Bi³⁺ ion symmetry units from C₂ to C_3i. A small EPR signal (at room temperature) is observed in titanium doped glasses due to Ti³⁺ ions. In both the series with increase of TiO₂ concentration BO₄ units are gradually converted into BO₃ units and new cross linkages are formed, like B–O–Ti, Bi–O–Ti at the expense of B–O–B bonds.     

The local structure of bismuth germanate glasses and glass ceramics doped with europium ions evidenced by FT-IR spectroscopy

Glasses of the xEu₂O₃·(100 − x)[4Bi₂O₃·GeO₂] system, with 0 ≤ x ≤ 30 mol%, have been characterized by FT-IR spectroscopy measurements in order to obtain information about the influence of Eu₂O₃ on the local structure of the 4Bi₂O₃·GeO₂ glass matrix. FT-IR spectroscopy data suggest that the europium ions play the network modifier role in the studied glasses and both Bi₂O₃ and GeO₂ play the role of network formers. Melting at 1100 °C and the rapid cooling at room temperature permitted to obtain glass samples. In order to improve the local order and to develop crystalline phases the glass samples were kept at 700 °C for 17 h. Both the influences of the europium ions as well as of the heat treatment on the local order of 4Bi₂O₃·GeO₂ glass matrix have been discussed.     

Li0.93[Li0.21Co0.28Mn0.51]O2 nanoparticles for lithium battery cathode material made by cationic exchange from K-birnessite

Li_0.93[Li_0.21Co_0.28Mn _0.51]O₂ nanoparticles with an R-3m space group is hydrothermally prepared from Co_0.35Mn_0.65O₂ obtained from an ion-exchange reaction with K-birnessite K_0.32MnO₂ at 200 °C. Even at a hydrothermal reaction temperature of 150 °C, the spinel (Fd3m) phase is dominant, and a layered phase became dominant by combining an increase in the temperature to 200 °C with an increase in lithium concentration. The as-prepared cathode particle has plate-like hexagonal morphology with a size of 100 nm and thickness of 20 nm. The first discharge capacity of the cathode is 258 mAh/g with an irreversible capacity ratio of 22%, and the capacity retention after 30 cycles is 95% without developing a plateau at ∼3 V. Capacity retention of the cathode discharge is 84% at 4C rate (=1000 mA/g) and shows full capacity recovery when decreasing the C rate to 0.1 C.