Transport properties of materials with the scheelite structure and their modification by doping

Materials with the scheelite structure exhibit mixed ionic and electronic conductivity, and are of interest as oxidation catalysts. Scheelite materials, with the general composition ABO₄ are also possible candidates for use as electrolytes or cathode materials in solid oxide fuel cells. Our work on the scheelite system, based on BiVO₄, shows that both ionic and electronic components of the conductivity can be modified by doping this material. Both A and B site doping have been investigated in the range of 5 mol% dopant concentration. The A cation was replaced by Ca²⁺ and Ce⁴⁺, and the B site by Mn⁴⁺ and Mo⁶⁺. The phase purity was verified by XRD methods. The total and partial conductivities of pure and doped BiVO₄ were investigated by use of the emf technique and ac impedance spectroscopy. Measurements were made between 550° C and 700° C and the oxygen gradient in the emf cell was established by oxygen and air gas flows with specified flow rates and oxygen partial pressure. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994.     

Phase transition of BiVO4 nanoparticles in molten salt and the enhancement of visible-light photocatalytic activity

BiVO₄ nanoparticles are prepared by molten salt method. Tetragonal BiVO₄ completely transforms to monoclinic phase after heating in molten LiNO₃ at 270 °C for 18 h. The average particle sizes of monoclinic BiVO₄ varied from 30 to 52 nm while the initial ratio of BiVO₄ to LiNO₃ changes from 1:6 to 1:24. The photocatalytic activity is evaluated by measuring decolorization of N,N,N′,N′-tetraethylated rhodamine dye solution under visible-light irradiation. Both of the de-ethylation and chromophore cleavage are responsible for the decolorization of RB. The sample with an average particle size of 52 nm and a moderate surface area of 10 m²/g exhibit the highest visible-light photocatalytic activity. The shift of Raman peak position indicates that the symmetry distortions in the local structure of the monoclinic BiVO₄. The variations of the local structure result in the modification of the electronic structure, which is responsible for the high visible-light photocatalytic activity.     

Ionic transport in the lithium nitride bromides,Li6NBr3 and Li1 3N4Br

The ionic and electronic conductivities of the lithium nitride bromides Li₆NBr₃ and Li_{1 3}N₄Br have been studied in the temperature range from 50 to 220°C and 120 to 450°C, respectively. Both compounds are practically pure lithium ion conductors with negligible electronic contribution. Li₆NBr₃ has an ionic conductivity Ω of 2 × 10^-6Ω^-1cm^-1 at 100°C and an activation enthalpy for σT of 0.46 eV. Li_{1 3}N₄Br shows a phase transition at about 230°C. The activation enthalpy for σT is 0.73 eV below and 0.47 eV above this temperature. The conductivities at 150 and 300°C were found to be 3.5 × 10^-6 Ω^-1cm^-1 and 1.4 × 10^-3Ω^-1cm^-1, respectively. The crystal structure is hexagonal at room temperature with $\text{a = 7.415 (1)}\text{A}\text{?}$ and $\text{c = 3.865 (1)}\text{A}\text{?}$.     

Additive-free controllable fabrication of bismuth vanadates and their photocatalytic activity toward dye degradation

Bismuth vanadates (BiVO₄) with various crystal structures (tetragonal scheelite, monoclinic scheelite, and tetragonal zircon) and morphologies (sphere-, nanosheet-, dendrite-, and flower-like) were controllably fabricated by using a mild additive-free hydrothermal treatment process under the different preparation conditions. The crystal structures, morphologies, and photophysical properties of the products were well-characterized. Subsequently, their UV- as well as visible-light photocatalytic performance was evaluated via dyes rhodamine B (RB) and methylene blue (MB) degradation. Special attention was paid to evaluate the correlation of the reactivity with crystal structure, morphology, and electronic structure of as-prepared BiVO₄ samples.     

Electrical conductivity of polycrystalline BiVO4 samples having the scheelite structure

Polycrystalline samples of the scheelite oxide BiVO₄ have been prepared with and without 1 m/o additions of CaO. Using impedance spectroscopy the electrical conductivity of these samples was measured in the temperature range 140–550°C and data for the low temperature regime (140–300°C) interpreted in terms of ionic conductivity due to oxygen ion vacancies. This interpretation suggested that the enthalpy of motion of anion vacancies in these materials is relatively low (∼ 0.33 eV).     

α- and β-Mn precipitates in the spinodal Fe30Ni20Mn25Al25 alloy

Precipitates in the spinodal alloy Fe₃₀Ni₂₀Mn₂₅Al₂₅, produced by drop-casting followed by hot extrusion, have been investigated using transmission electron microscopy. The as-extruded microstructure consisted of alternating, coherent body-centred cubic and B2 (ordered body-centred cubic) rods aligned along ?100? directions with a wavelength of ～60?nm. Upon annealing at 550°C, precipitates were formed having two distinct morphologies: relatively spherical precipitates with the α-Mn crystal structure and more elongated precipitates with the β-Mn crystal structure. Both precipitates existed simultaneously after annealing at this temperature for up to at least 99?h. The compositions of the precipitates and the orientation relationships between the precipitates and the spinodal matrix were determined.     

Phase change and pyroelectricity in natural hemimorphite

The phase change of a natural hemimorphite sample from Minas Gerais (Brazil) was investigated by two X-ray diffraction (XRD) methods and by near-infrared reflectance spectroscopy. Applying successive thermal treatments, the crystal structure undergoes two orientation conversions. The first one occurs at about 550 °C, and it was revealed by the Laue method. Below 500 °C, the water molecules were partially expelled without changing the crystal structure. A fact that supports this statement is the sequential disappearance of the water bands at 1400 and 1900 nm by thermal treatment. The second conversion takes place below 939 °C. Moreover, at 972 °C a phase change to the willemite mineral (α-Zn₂SiO₄) has been observed. This last conversion was confirmed by the power XRD. In addition, natural hemimorphite displayed a high pyroelectricity, which is related both to the absence of inverse centre and to the presence of molecular water and hydroxyl groups in the crystal structure.     

Crystal growth and characterization of a new co-ordination complex—barium tetrakis(maleate) dihydrate

Crystals of barium tetrakis(maleate) dihydrate [Ba₄(C₄H₂O₄)₄]^?2H₂O are grown in gelated hydrosilica matrix. Single crystal X-ray diffraction studies show that the crystal system is monoclinic with space group P2₁/c. The unit cell dimensions are a=9.3721(2)  Å, b=20.5880(7)  Å, c=14.0744(4) Å, α=γ=90°, β=90.289(2)°. Powder XRD studies confirmed the single phase nature of the grown crystals. The FTIR data is in conformity with the XRD results. The TG–DTA curves of the material indicate a three-step thermal decomposition. The response of the dielectric properties in the temperature range 30 °C to 500 °C is correlated with the TG–DTA results.     

Orientation dependence of electrical properties of 0.96Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-0.04BaTiO<Subscript>3</Subscript> lead-free piezoelectric single crystal

In this paper, a single crystal of 0.96Na_0.5Bi_0.5TiO₃-0.04BaTiO₃ with dimensions of Φ 30×10 mm was grown by the top-seeded-solution growth method. X-ray powder diffraction results show that the as-grown crystal possesses the rhombohedral perovskite-type structure. The dielectric, piezoelectric and electrical conductivity properties were systematically investigated with 〈001〉, 〈110〉 and 〈111〉 oriented crystal samples. The room-temperature dielectric constants for the 〈001〉, 〈110〉 and 〈111〉 oriented crystal samples are found to be 650, 740 and 400 at 1 kHz. The (T _m, ε _m) values of the dielectric temperature spectra are almost independent of the crystal orientations; they are (306°C, 3718), (305°C, 3613) and (307°C, 3600) at 1 kHz for the 〈001〉, 〈110〉 and 〈111〉 oriented crystal. The optimum poling conditions were obtained by investigating the piezoelectric constants d ₃₃ as a function of poling temperature and poling electric field. For the 〈001〉 and 〈110〉 crystal samples, the maximum d ₃₃ values of 146 and 117 pC/N are obtained when a poling electric field of 3.5 kV/mm and a poling temperature of 80°C were applied during the poling process. The as-grown 0.96Na_0.5Bi_0.5TiO₃-0.04BaTiO₃ crystal possesses a relatively large dc electrical conductivity, especially at higher temperature, having a value of 1.98×10⁻¹¹ Ω⁻¹⋅m⁻¹ and 3.95×10⁻⁹ Ω⁻¹⋅m⁻¹ at 25°C and 150°C for the 〈001〉 oriented crystal sample.     

Preparation and crystallization of Pb(Zr0.95Ti0.05)O3 thin films deposited by radio-frequency magnetron sputtering with a stoichiometric ceramic target

0.95 Ti_0.05)O₃ (PZT 95/5) thin films of perovskite structure were prepared on various substrates by the radio-frequency magnetron sputtering with a stoichiometric ceramic target. The crystal structure of the films showed a strong dependency on the crystal structure of the substrates. After conventional-furnace annealing at 750 °C, crack-free and transparent epitaxial PZT 95/5 films were successfully obtained on SrTiO₃(100) substrates, and highly oriented films on LaAlO₃(012). The films on r-sapphire exhibit slightly preferred orientation along both the (040) and (122) axes of the orthorhombic PZT 95/5 phase. The films on YSZ(100) consist principally of the perovskite PZT 95/5 phase with random orientation with a small portion of unknown phase. However, the formation of the perovskite PZT 95/5 phase was not observed in the films on Si(110) or (111)Pt-coated Si substrates. The crystallization of the perovskite PZT 95/5 on these substrates could be improved by rapid thermal annealing (RTA). Single perovskite phase was obtained in the films on (111)Pt/Si which had RTA at 750 °C for 1 min. No tangible loss of Pb from the films occurred during either the sputtering or annealing. Received: 17 April 1997/Accepted: 18 November 1997     

Defect structure of lanthanum-doped calcium titanate

The defect structure of lanthanum-doped polycrystalline calcium titanate was investigated by measuring the oxygen partial pressure (10⁰–10^?18 atm.) dependence of the electrical conductivity at 1000° C and 1050°C. Two types of charge compensation were observed, namely electronic and ionic. For P₀₂ < 10^?15 atm. the carrier concentration was fixed by the amount of lanthanum (donor) added and the conductivity was found to be independent of oxygen partial pressure (electronic compensation). For higher oxygen partial pressure conditions (P₂₂ > 10^?13 atm.) the extra charge of the lanthanum was compensated by doubly ionized calcium vacancies (ionic compensation). In the ionic compensation region, a model involving a shear structure is discussed.     

Structure of bis(4-Methyl Pyridine) Cadmium Tetracyanonickelate

Abstract

The structure, C₁₆H₁₄CdN₆Ni, consist of corrugated polymeric networks made up of tetracyanonickelate ions coordinated to Cd. The 4-methyl pyridine molecules bound to Cd in trans positions are located on both sides of the network. The bonding in the networks occurs because of a departure of the Ni-C-N-Cd sequence of atoms from linearity at the C and N positions. The crystal structure of the title compound was determined as monoclinic by single crystal X-Ray diffraction technique. The crystal parameters of this compound are as follows: monoclinic C2/m, a=18.156(2) Å, b=7.581(2) Å. c= 6.983(2) Å, β = 110.09(2)°, V = 902.6(5) ų Z=2, Dx = 1.698 g/cm³, F(000) - 456, λ (MoKα) = 0.71070 Å, μ = 2.121 mm^?1. The structure was solved by SHELXS-86 and refined by SHELXL-93. R = 0.02 for 1074 observed reflections with I > 2[sgrave] (I).     

Study of electrical conductivity and phase transition in Bi2O3–V2O5 system

The solid solutions of bismuth–vanadate were prepared by the conventional solid-state reaction. The sample characterization and the study of phase transition were done by using FT-IR, X-ray diffraction (XRD) and DSC measurements. AC impedance measurements proved that the oxide ion conductivity predominantly arises from the grain and grain boundary contributions as two well-defined semicircles are clearly seen along with an inclined spike. The electrical conductivity of Bi₂O₃–V₂O₅ has been studied at different temperatures for various molar ratios. The isothermal conductivity increases with an increase in the concentration of V₂O₅ due to the vacancy migration phenomenon. It has been found that the conductivity of different compositions of Bi₂O₃–V₂O₅ increases and shows a jump in the temperature range 230–260°C due to the phase transition of BiVO₄ from monoclinic scheelite type to that of tetragonal scheelite type. The endothermic peak in DSC at around 260°C reveals the phase transition, which is also confirmed by the XRD and FT-IR analysis. The XRD patterns confirmed the monoclinic structure of BiVO₄.     

Synthesis of the new oxocuprate Cu5Bi2B4O14 and investigation of its structural, magnetic, and resonant properties

Single crystals of the new compound Cu₅Bi₂B₄O₁₄ are grown and its structural, magnetic, and resonant properties are investigated for the first time. It is found that the Cu₅Bi₂B₄O₁₄ crystal synthesized has a triclinic symmetry with space group $P\bar 1$ and the unit cell parameters a=10.132 Å, b=9.385 Å, c=3.458 Å, α=105.443°, β=97.405°, γ=107.784°, and Z=1. At a temperature of 24.5 K, the crystal undergoes a magnetic phase transition to the magnetically ordered state. The assumption is made that the ferrimagnetic structure of the Cu₅Bi₂B₄O₁₄ crystal consists of two ferromagnetic sublattices coupled through the antiferromagnetic exchange interaction. The unit cell of the crystal contains five copper ions, of which one ion belongs to the first sublattice and the other four ions form the second sublattice. Analysis of the resonant and magnetic static properties demonstrates that the Cu₅Bi₂B₄O₁₄ crystal exhibits an easy-axis magnetic anisotropy. The direction of the easy axis coincides with the c axis of the crystal, whereas the a and b axes are the hard magnetic axes with saturation fields approximately equal to 25 and 10 kOe, respectively.     

Photoelectrochemical properties of FTO/m-BiVO4 electrode in different electrolytes solutions under visible light irradiation

Photoelectrochemical properties of FTO/BiVO₄ electrode were investigated in different electrolytic solutions, potassium chloride (KCl) and sodium sulphate (Na₂SO₄), and under visible light irradiation condition. In order to accomplish that, an FTO/BiVO₄ electrode was built by combining the solution combustion synthesis technique with the dip-coating deposition process. The morphology and structure of the BiVO₄ electrode were investigated through X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy. Photoelectrochemical properties were analyzed through chronoamperometry measurements. Results have shown that the FTO/BiVO₄ electrode presents higher electroactivity in the electrolyte Na₂SO₄, leading to better current stabilization, response time, and photoinduced current density, when compared to KCl electrolyte. Besides, this electrode shows excellent performance for methylene blue degradation under visible light irradiation condition. In Na₂SO₄, the electrode has shown higher degradation rate, 51 %, in contrast to 44 % in KCl, plus higher rate constant, 174?×?10^?4 min^?1 compared to 150?×?10^?4 min^?1 in KCl. Results presented in this communication leads to the indication of BiVO₄ thin films as alternate materials to use in heterogeneous photoelectrocatalysis, more specifically in decontamination of surface water.     

A neutron diffraction study of structural changes in one-dimensional K2Pt(CN)4Br0.3 · 3H2O from 77–323°K

Single crystal neutron diffraction measurements of K₂Pt(CN)₄Br_0.3 · 3H₂O (KCP) above room temperature, to the point of irreversible crystal breakdown (318–323°K), and at liquid nitrogen temperature (77°K), give no indication of a crystallographic phase change. Full three-dimensional data collected at 77°K ($\text{a = 9.848(5)}\text{A}\text{?}\text{, c = 5.714(3)}\text{A}\text{?}\text{,}\text{and space group}\text{P4mm}$) indicate that the structure is essentially unchanged from that at room temperature except for increased hydrogen bonding association between H₂O(2) and Br(1). The possible relationship between the hydrogen bonding changes and Br(1) site ordering is discussed.     

Sol-gel synthesis of the lithium-ion conducting perovskite La0.57Li0.3TiO3 effect of synthesis and thermal treatments on the structure and conducting properties

The lithium ionic conducting perovskite La_0.57Li_0.3TiO₃ has been synthesised via a sol-gel method at a temperature of 700 °C. The crystallinity of the product can be greatly increased by further heat treatment at 1000 °C. In this paper the product of the sol-gel synthesis is compared with the product of conventional solid-state synthesis, and the influence of the synthesis method as well as of quenching on the crystal structure and ionic conductivity has been studied. AC-impedance measurements show two contributions to the ionic conductivity, which can be adscribed to intergranular and intragranular effects, respectively. A lower intergranular resistivity is observed for sol-gel samples, while quenching mainly affects the materials prepared by solid-state reaction. The crystal structure of the material prepared by the sol-gel method is identical to that of the material prepared by the solid state reaction, although the synthesis temperature is nearly 600 °C lower. A tetragonal superstructure is observed when either type of material is slowly cooled from 1300 °C. Quenching from the same temperature results in the suppression of that superstructure. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Bromide ion conductivities of eutectic mixtures of quaternary ammonium bromides possessing long alkyl chains

Several quaternary ammonium bromides possessing long alkyl chains and their mixtures were found to be bromide ion conductors. The ionic conductivities of quaternary ammonium bromides themselves were lower than 10^?9 S cm^?1 at a room temperature. On the other hand, the eutectic mixtures of the quaternary ammonium bromides showed large increase of ionic conductivity. The best bromide ion conductors were found for the eutectic of Q₅, Q₇, Q₈, and Q₁₂: 4×10^?8 S cm^?1 at 30°C, and 6.3×10^?6 S cm^?1 at 50°C. Addition of asymmetric quarternary ammonium bromides had a negative effect on the ionic conductivity. These results were explained by a space filling factor in the solid.     

Structural phase transition,ionic conductivity,and dielectric investigations in K3H(SO4)2 single crystals

Optical observation, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) measurements have been carried out on K₃H(SO₄)₂ crystal in the temperature range between 25 and 300 °C. Domain structures of K₃H(SO₄)₂ were observed at room temperature which are the same as those observed in other member of A₃H(XO₄)₂ which show ferroelasticity. Two endothermic peaks of DSC were found at around 206 and 269 °C. In this temperature range, the result of TGA indicate the loss of weight. It supports that partial dehydration is most probable. The first peak can be accounted for the dehydration reaction on the surface of crystals, and the second peak corresponds to the melting of the sample crystal. The impedance measurements were performed as a function of both temperature and frequency. The electrical conductivity increases with increasing temperature and the sample crystal becomes a fast ionic conductor at the temperatures above 206 °C. The high conductivity of the crystal is caused by the increases of the defects due to the dehydration. The dielectric properties of the sample crystal were studied by the impedance spectroscopy.     

Ferroelasticity in BiVO4

A transition in BiVO₄ crystals has been observed at 255°C. Optical and X-ray studies indicate that this is a ferroelastic transition of the type 4/m F2/m.