Synthesis, structure, and impedance spectra of iron-containing bismuth titanates

Iron-containing bismuth titanates with a cubic pyrochlore Bi_1.6Fe _x Ti₂O_{7 ? δ} structure were synthesized by the ceramic method. The region in which iron-containing pyrochlores are formed was determined. The pycnometric specific gravity of the samples was measured. Based on XRD and pycnometric specific gravity data, it was concluded that iron atoms lie in the bismuth sublattice. The electric properties of the materials were studied by high-temperature impedance spectroscopy. A strong effect of polarization on the impedance spectra was found.     

Electrophysical properties of bismuth titanates with the pyrochlore structure Bi<Subscript>1.6</Subscript>M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ti<Subscript>2</Subscript>O<Subscript>7–δ</Subscript> (M = In,Li)

Lithium-containing bismuth titanates with the pyrochlore-type structure Bi_1.6Li_xTi₂O_7–δ were obtained for the first time. The formation of the pyrochlore phase was confirmed by X-ray diffraction analysis, scanning electron microscopy and local microanalysis. In Bi_1.6M_xTi₂O_7–δ, the lithium and indium are occupied the bismuth sites, primarily. The electrophysical properties of doped bismuth titanates were studied by impedance spectroscopy in the frequency range 1–10⁶ Hz. In the low-temperature range (of up to ~400°C), electron conductivity predominates; above 400°C, the oxygen-ion type of conductivity is revealed. In the range p(O₂) = 0.21–1 atm, the average value of the sum of ion transport numbers is 0.5 at 500–550°C. The relaxation process was found from the frequency dependences of the dielectric parameters (ε', tan δ, M''), which was of the same type for systems with different dopants (In, Li) probably due to the hopping mechanism of oxygen conductivity.     

Investigation of synthesis and microstructure of bismuth titanates with TiO<Subscript>2</Subscript> rich compositions

Preliminary examinations regarding formation of bismuth titanates in a part of Bi₂O₃—TiO₂ system rich with TiO₂ have been carried out. Bismuth titanates have been synthesized from mixtures of Bi₂O₃ and TiO₂ (anatase) by the conventional solid-state method at the temperatures ranged from 1273 to 1473 K. Differential thermal analysis (DTA), powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) have been used to study the formation of bismuth titanates. The following compounds have been achieved: Bi₄Ti₃O₁₂, Bi₂Ti₂O₇ and Bi₂Ti₄O₁₁. Existence of controversial bismuth titanate of formula Bi₂Ti₃O₉ in the Bi₂O₃—TiO₂ system has not been confirmed.     

Structural, microstructural and surface properties of a specific CeO2-Bi2O3 multiphase system obtained at 600 °C

Polycrystalline samples of (1−x) CeO₂−x/2 Bi₂O₃ phases, where x is the atom fraction of bismuth have been synthesized by the precipitation process and after the thermal treatment at 600 °C, under air. Samples are first characterized by the X-ray diffraction and scanning electron microscopy. To determine the samples specific surface areas, Brunauer-Emmett-Teller (BET) analyses have been performed. In the composition range 0≤x≤0.20, a cubic solid solution with fluorite structure is obtained. For compositions x comprised between 0.30 and 0.90, two types of T′ (or β′) and T (or β) tetragonal phases, similar to the well-known β′ or β Bi₂O₃ metastable structural varieties, are observed. However, the crystal cell volumes of these β′ or β Bi₂O₃ phases increase with the composition x in bismuth: this might be due to the presence of defects or substitution by cerium atoms, in the tetragonal lattices. Using X-ray diffraction profile analyses, correlations between bismuth composition x and crystal sizes or lattice distortions have been established. The solid-gas interactions between these polycrystalline materials and air-CH₄ and air-CO flows have been studied as a function of temperature and composition x, using Fourier transform infrared (FTIR) analyses of the conversions of CH₄ and CO gases into the CO₂ gas. The transformations of CH₄ and CO molecules as a function of time and temperature are determined through the intensities of FTIR CO₂ absorption bands. Using the specific surface areas determined from BET analyses, these FTIR intensities have been normalized and compared. For all bismuth compositions, a low catalytic reactivity is observed with air-CH₄ gas flows, while, for the highest bismuth compositions, a high catalytic reactivity is observed with air-CO gas flows.     

Structural changes of (K,Gd)2Ta2O7 pyrochlore at high pressure

The structure of K-bearing tantalate pyrochlore (K_2-xGd_x)Ta₂O_6+x(x∼0.4) was studied at high pressures using in situ X-ray diffraction and Raman scattering methods. Experimental results indicated that (K_2-xGd_x)Ta₂O_6+x(x∼0.4) retains the pyrochlore structure up to 40 GPa, but partial amorphization occurred at pressures above 23 GPa. The amorphous phase was also confirmed in the quenched sample by means of transmission electron microscopy. The tantalate pyrochlore lattice is more stable than pyrochlore compounds in other systems, such as rare earth titanates, zirconates and stannates. The structural stability of pyrochlore tantalate may be mainly related to the size ratio of cations on the 16d and 16c sites in the lattice.     

Thermal,structural and electrical studies of bismuth zinc borate glasses

Bismuth Zinc Borate glasses with compositions xBi₂O₃–30ZnO–(70 − x)B₂O₃ (where x = 30, 35, 40 and 45 mol %) have been prepared by melt quenching method. These glasses were characterized by X-ray diffraction (XRD), Differential Thermal Analysis (DTA), Fourier Transform Infrared Spectrometer (FTIR) and Broad Band Dielectric Spectrometer (BDS). DTA and FTIR analysis reveals that Non-Bridging Oxygens (NBOs) increase with increase of bismuth content in the glass. Electrical data have been analyzed in the framework of impedance and modulus formalisms. The activation energy for dc conductivity decreases with increase of bismuth concentration. The imaginary part of modulus spectra has been fitted to non-exponential Kohlrausch–Williams–Watts (KWW) function and the value of the stretched exponent (β) is found to be almost independent of temperature but slightly dependent on composition.     

BIFEVOX Composites: Manufacture and Characterization

Design of composites is a way to improve the quality of solid electrolytes. By mechanically mixing and annealing substituted bismuth vanadate with nanosized aluminum, bismuth, and zirconium binary oxides, we obtained heterogeneous materials Bi₄V_1.7Fe_0.3O_{11 – δ}/xAl₂O₃, Bi₄V_1.7Fe_0.3O_{11 – δ}/xBi₂O₃, and Bi₄V_1.7Fe_0.3O_{11 – δ}/xYSZ. The investigation tools were X-ray powder diffraction and electron microscopy with energy-dispersive microanalysis. The composition of materials was studied, the non-interaction of components was elucidated in the aluminum oxide and zirconium oxide composite series, and a nonuniform distribution of nanopowder particles across the surfaces and cleaves of sinters was discovered. The bismuth atoms from bismuth oxide were shown to be capable of incorporating into the Bi₄Fe_0.3V_1.7O_{11 – δ} structure. The charge transport characteristics of the materials were studied by impedance spectroscopy. No changes were observed in logσ–10³/T trends in composites with various binary oxides and various oxide contents. An increase in binary oxide concentration was shown to give rise to an insignificant decay in electrical conductivity.     

Substituted bismuth vanadates and chromates: New aspects

The work is devoted to the synthesis and attestation of a number of substituted vanadates and chromates of bismuth. For bismuth vanadates of the BIMEVOX family, the homogeneity regions of the Bi₄V_{2 – x}Cr _x O_{11 ± d} solid solutions have been refined, the features of the structure change of the compounds with increasing chromium content and changing temperature have been noted, and the powders and ceramics have been studied by electron microscopy. For the first time, as an impurity, an individually substituted bismuth chromate of the composition Substituted bismuth chromate of the Bi₁₃Cr_{5 – y}V _y O_{34.5 – d} (y = 0.95 ± 0.05) composition has been detected for the first time as an impurity and synthesized as an individual compound, which has been characterized by X-ray diffraction, electron microscopy, chemical analysis, and photoelectron spectroscopy data; its homogeneity range has been determined, and electroconductive characteristics have been studied.     

Improvement of lithium-ion conductivity in A-site-disordered lithium lanthanum titanates by V5+/Nb5+ substitution

The V⁵⁺/Nb⁵⁺-substituted lithium lanthaum titanates are synthesized by a conventional solid-state reaction method at high temperature in air. The structural and conductivity studies of the obtained perovskite oxide samples are investigated by x-ray diffraction (XRD), SEM, and impedance spectroscopy. From the powder XRD patterns, it is clearly observed that the synthesized samples exhibit a well-defined cubic structure with the Pm3m (Z = 1) space group. The lattice parameter is decreased with increasing vanadium content in Li_0.5?x La_0.5Ti_1?x V _x O₃, but increased with the increasing niobium content in Li_0.5?x La_0.5Ti_1?x Nb _x O₃. The scanning electron microscope measurements confirmed that these materials consist of fairly ordered grains throughout the surface area. The conductivity variations with the substitution of vanadium/niobium are also reported. The bulk ionic conductivity measured in the temperature range from room temperature to 150 °C is about the same as reported earlier for the related lithium lanthanum titanate. However, the low activation energies for ionic conduction observed for these samples encourage further investigations for better conductors in this system.     

Synthesis,structure, and properties of substituted bismuth niobates Bi<Subscript>3</Subscript>Nb<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Er<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>7–δ</Subscript>

Results are reported of a study of the structural and electrical characteristics of substituted bismuth niobates with composition Bi₃Nb_1–x Er _x O_7–δ, which are promising oxygen-ion conductors. The homogeneity regions of solid solutions were determined by X-ray phase analysis and electron microscopy with X-ray fluorescence microanalysis, and their crystal-chemical parameters were calculated. The electrical conductivity of sintered samples was examined by impedance spectroscopy.     

Li ion dynamics in Li3xLa2/3–x□1/3–2xTiO3 studied by NMR

⁷Li nuclear magnetic resonance relaxation times, T₁ and T_1ρ, versus temperature are reported in the 150–900 K temperature range on the lithium lanthanum titanates, Li_3xLa_2/3–x□_1/3–2xTiO₃, which are fast ionic conductors. Two characteristic frequencies of Li⁺ motions are evidenced in these compounds: the first is in the range of the Larmor frequency when the second one is in the range of the radio-frequency field. These frequencies are respectively attributed to motion of the Li⁺ ion inside the cage formed by the oxygen ions and to jumps between the cages. The T₁ and T_1ρ studies on ⁶Li nuclei confirm the above results and show that the relaxation is not due to quadrupolar interaction at a variance, which is generally accepted.     

New substituted copper titanates with the CaCu3Ti4O12 structure: Li[Cu3−xLix][Ti3−xMV1+x]O12 (MV = Nb: 0.12 ≤ x ≤ 0.33; MV = Ta: x = 0.33)

New substituted copper titanates with the CaCu₃Ti₄O₁₂ perovskite-like structure were synthesized and characterized. Their formula is Li|Cu_3−xLi_x| |Ti_3−xM^V_1+x|O₁₂. There is a homogeneity range for M^V = Nb, 0.12 ≤ x ≤ 0.33, and a unique composition for M^V = Ta, x = 0.33. The unit cell is cubic: a ∼ 2a_p (parameter of the ideal cubic cell) ∼ 7.40 Å. The CaCu₃Ti₄O₁₂-type structure was confirmed from an X-ray powder structural determination. Lithium occupies both square planar sites where it replaces copper and icosahedral sites, with a probable delocalization. Crystal chemistry of the AC₃B₄O₁₂ structure is considered, taking into account the evolution of anion packing and the distortion of polyhedra. A diagrammatic representation is proposed so that precise information on the regularity of the C₃B₄O₁₂ network can be obtained.     

Oxygen-ionic conductors based on substituted bismuth molybdates with column-type structural fragments

The possibility of synthesizing oxygen-ionic conductors from substituted bismuth molybdates containing [Bi₁₂O₁₄] _n ⁸ⁿ⁺ columns, MoO₄ tetrahedra, and isolated Bi ions in their structure was studied. The specifics of their structure and electric conductivity were investigated. The general formula of the solid solutions can be recorded as Bi₁₃Mo_{5 ? x} Me _x O_{34 ? δ}, where Me is the fouror five-valent d metal (Ti, Zr, V, Nb). The electric conductivity of doped bismuth molybdates considerably increased compared with that of the matrix compound. The electric conductivity reached 5.5 × 10^?3 S cm^?1 at 700°C and 1.8 × 10^?4 S cm^?1 at 350°C for the zirconium-doped compound with x = 0.4. The porosity of the ceramics was less than 5%; the thermal expansion coefficient was of the order of 14 × 10^?6 K^?1. Based on the set of their characteristics, these compounds are recommended as materials for membranes of electrochemical devices.     

New lead-free relaxor ceramics derived from BaTiO3 by cationic heterovalent substitutions in the 12 C.N. crystallographic site

Crystallographic and low frequency dielectric studies were performed on new lead-free ceramics derived from BaTiO₃ by cationic substitutions in the 12 C.N. crystallographic site and having the composition Ba_1−xK_x/2La_x/2TiO₃ or Ba_1−xA_2x/3TiO₃ (A=La, Bi). A classical ferroelectric behaviour was shown for compositions close to BaTiO₃, while a relaxor one occured when the composition deviated from BaTiO₃. In any case, the value of T_C (or T_m) and the relaxor characteristics are higher as x increases. Concerning potassium-free ceramics, bismuth compositions compared with lanthanum ones favoured both high ferroelectric and relaxor characteristics due to the lone pair of Bi³⁺. Such new materials are of interest for environmental friendly applications.     

Substitutional solid solutions of bismuth-containing lanthanide dititanates,Ln2−xBixTi2O7

The formation of solid solutions Ln_2?xBi_xTi₂O₇, where Ln = La to Lu and Y, except Ce, Pm, and Eu, has been studied by Raman spectroscopy and to a lesser extent by X-ray diffraction. It has been established that the solubility of bismuth increases with decreasing ionic radius of the lanthanide element. No evidence was experimentally found in this work for the existence of Bi₂Ti₂O₇.     

Photocatalytic degradation of organic dyes on visible-light responsive photocatalyst PbBiO2Br

The layered compound of lead bismuth oxybromide PbBiO₂Br, prepared by conventional solid-state reaction method, has an optical band gap of 2.3 eV, and possesses a good visible-light-response ability. The references, PbBi₂Nb₂O₉, TiO_2−xN_x, BiOBr and BiOI_0.8Cl_0.2, which are excellent visible-light-response photocatalysts, were applied to comparatively understand the activity of PbBiO₂Br. Degradation of methyl orange and methylene blue was used to evaluate photocatalytic activity. The results show that PbBiO₂Br is more photocatalytically active than PbBi₂Nb₂O₉, TiO_2−xN_x and BiOBr under visible light.     

Synthesis and evaluation of lead telluride/bismuth antimony telluride nanocomposites for thermoelectric applications

Heterogeneous nanocomposites of p-type bismuth antimony telluride (Bi_2−xSb_xTe₃) with lead telluride (PbTe) nanoinclusions have been prepared by an incipient wetness impregnation approach. The Seebeck coefficient, electrical resistivity, thermal conductivity and Hall coefficient were measured from 80 to 380 K in order to investigate the influence of PbTe nanoparticles on the thermoelectric performance of nanocomposites. The Seebeck coefficients and electrical resistivities of nanocomposites decrease with increasing PbTe nanoparticle concentration due to an increased hole concentration. The lattice thermal conductivity decreases with the addition of PbTe nanoparticles but the total thermal conductivity increases due to the increased electronic thermal conductivity. We conclude that the presence of nanosized PbTe in the bulk Bi_2−xSb_xTe₃ matrix results in a collateral doping effect, which dominates transport properties. This study underscores the need for immiscible systems to achieve the decreased thermal transport properties possible from nanostructuring without compromising the electronic properties.     

Formation of solid solutions of multiferroics in the Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>–Nd<Subscript>2</Subscript>O<Subscript>3</Subscript>–Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> system

The sequence of phases appearance during the formation of Bi_1–xNd_xFeO₃ solid solutions in powder oxides mixtures of bismuth, neodymium, and iron has been determined. It has been shown that the closeness of the reaction mixture composition to that of the individual compound (BiFeO₃ or NdFeO₃) is essential for the realization of the series of phase transformations yielding solid solutions of multiferroics Bi_1–xNd_xFeO₃ as the final product, due to the prevalence of various interphase contacts in the starting reaction zone.     

Polymeric architectures of bismuth citrate based on dimeric building blocks

Four bismuth complexes, (H₂En)[Bi₂(cit)₂(H₂O)_4/3]·(H₂O) _x (1), (H₂En)₃[Bi₂(cit)₂Cl₄]·(H₂O) _x (2), (HPy)₂[Bi₂(cit)₂(H₂O)_8/5]·(H₂O) _x (3) and (H₂En)[Bi₂(cit)₂](H₂O) _x (4) [cit = citrate^4?; En = ethylenediamine; Py = pyridine] have been synthesized and crystallized. The crystal structures reveal that the basic building blocks in all of these complexes are bismuth citrate dimeric units which combine to form polymeric architectures. The embedded protonated ethylenediamine and pyridine moieties in the polymeric frameworks have been identified by X-ray crystallography and solid-state cross polarization/magic angle spinning (CP/MAS) ¹³C NMR. Based on the framework of complex 1, a structural model of a clinically used antiulcer drug, ranitidine bismuth citrate (RBC) was generated. The behavior of the protonated amine-bismuth citrate complexes in acidic aqueous solution has been studied by electrospray ionization-mass spectrometry (ESI-MS).     

On the structure of evaporated bismuth oxide thin films

The bismuth oxide films evaporated from bulk Bi₂O₃ are shown to vary in stoichiometry. The as-evaporated low rate (1–5 Å/sec) films are microcrystalline and bismuth rich, relative to Bi₂O₃, and their optical absorption edge broadens and shifts to lower energies. High rate (15–25 Å/sec) films are morphous and oxygen-rich with an absorption edge shifted to higher energies. Thermal decomposition of the Bi₂O₃ during evaporation produces the variations in film stoichiometry. The high temperature δ-Bi₂O₃ observed in the as-evaporated low rate films and thermally treated amorphous films indicates the melt and the films are structurally similar. Thermal treatment of the low rate films results in the formation of the β-form. Comparison of X-ray and stoichiometry results suggests that β-Bi₂O₃ be expressed as β-Bi₂O_±3x, where x is the deviation from trioxide stoichiometry.