Local structure and oxide-ion conduction mechanism in apatite-type lanthanum silicates

Abstract

The local structure of apatite-type lanthanum silicates of general formula La_9.33+x(SiO₄)₆O_2+3x/2 has been investigated by combining the atomic pair distribution function (PDF) method, conventional X-ray and neutron powder diffraction (NPD) data and density functional theory (DFT) calculations. DFT was used to build structure models with stable positions of excess oxide ions within the conduction channel. Two stable interstitial positions were obtained in accordance with literature, the first one located at the very periphery of the conduction channel, neighbouring the SiO₄ tetrahedral units, and the second one closer to the channel axis. The corresponding PDFs and average structures were then calculated and tested against experimental PDFs obtained by X-ray total scattering and NPD Rietveld refinements results gathered from literature. It was shown that of the two stable interstitial positions obtained with DFT only the second one located within the channel is consistent with experimental data. This result consolidates one of the two main conduction mechanisms along the c-axis reported in the literature, namely the one involving cooperative movement of O4 and Oi ions.     

Synthesis and ionic conductivities of M (Mg,Ba, Zr) and Al co-doped apatite-type lanthanum germanate electrolytes for IT-SOFC

In this paper, the structure and ionic conductivities have been investigated for M (Mg, Ba, Zr) and Al co-doped apatite-type lanthanum germanates. La_9.0.5Ge_5.5Al_0.5O_26±δ (M?=?Mg, Ba and Zr) were prepared by solid state reaction. The results of XRD analysis showed that the space group is P6₃/m. M (Mg, Ba, Zr) substituting La site and Al substituting Ge site increased the cell volume and improved the lattice distortion of the apatite structure. In addition, M (Mg, Ba, Zr) and Al doping facilitated sintering and improved the relative density. Analysis of AC impedance spectroscopy showed that M (Mg, Ba, Zr) and Al co-doping in lanthanum germanates introduced local distortion and improved the ionic conductivities. Moreover, the thermal expansion coefficient (TEC) of La_9.5M_0.5Ge_5.5Al_0.5O_26±δ (M?=?Mg, Ba and Zr) was improved after doping with M (Mg, Ba, Zr) and Al ions. La_9.5Mg_0.5Ge_5.5Al_0.5O_26.5 sintered at 1400 °C exhibited the highest conductivity (1.26?×?10^?2 S/cm, 800 °C) and improving TEC, which suggests that the (M (Mg, Ba, Zr), Al) doped apatite-type lanthanum germanate can be a potential material for the IT-SOFC electrolyte.     

Electrical properties of the system lanthanum lead cobalt titanium oxide

Measurements of DC electrical resistivity and Seebeck coefficient on the perovskite system La_1−x Pb _x−δ Co_1−x Ti _x O_3−δ for 0·2≤x≤0·9 have been made in the temperature range 300–800 K. AC conductivity,σ _a.c, of all the samples were measured as a function of temperature (300–573 K) and frequency (1 kHz-1 MHz). DC resistivity behaviour of all the samples is similar. However, the resistivity value, which varies over 5–6 orders of magnitude, depends on both the compositionx and the structure of the samples. All the samples exhibitp-type electronic conduction. The value of Seebeck coefficient,α, for samples withx≤0·5 initially increases with temperature up to a particular temperature. Above this temperature, the behaviour ofα for samples withx≤0·5 and for samples withx>0·5 over the entire temperature range is similar to that of La CoO₃. All the samples exhibit frequency-dependent a.c. conductivity at low temperatures.     

Thermal expansion of doped lanthanum gallates

Thermal expansion of several compositions of Sr and Mg-doped LaGaO₃ including an A-site deficient composition (La_0.9Sr_0.1)_0.98(Ga_0.8Mg_0.2)O_2.821 were measured in the temperature range from 298 to 1273 K. The effect of doping on thermal expansion was studied by varying the composition at one site of the perovskite structure (either A or B), while keeping the composition at the other site invariant. Thermal expansion varied nonlinearly with temperature and exhibited an inflexion between 550 and 620 K, probably related to the change in crystal structure from orthorhombic to rhombohedral. The dependence of average thermal expansion coefficient (α _av) on the dopant concentration on either A or B site of the perovskite structure was found to be linear, when the composition at the other site was kept constant. Mg doping on the B-site had a greater effect on the average thermal expansion coefficient than Sr doping on the A-site. Cation deficiency at the A-site decreases thermal expansion when compositions at both sites are held constant.     

钴铁氧体纳米粒子制备及其镧掺杂的磁性能研究

为了制备性能良好的钴铁氧体及改善其磁性能,通过改进的溶胶凝胶自蔓延燃烧法成功地制备了钴铁氧体(CoFe2O4)及掺镧(La)钴铁氧体纳米粒子.采用X射线衍射(XRD),透射电镜(TEM)、能谱分析(EDS)、振动样品磁场计(VSM)对所得粒子进行了结构、形貌、成分及磁性能表征.测试结果表明,利用改进的溶胶凝胶法制得钴铁氧体粒度均匀,且成相温度较低,500℃煅烧1h时平均粒径12nm左右;通过掺杂稀土镧元素对所得铁氧体的相结构有较强的影响,所得掺镧钴铁氧体与目标产物一致;所得钴铁氧体具有较高的矫顽力(737.33Oe),并且通过稀土元素镧的掺杂提高了钴铁氧体的矫顽力.     

Sol-gel synthesis and properties of doped lanthanum chromite nanopowders

La_{1 ? x} Sr _x CrO₃ (0.2 ≤ x ≤ 0.3) powders have been prepared by a sol-gel process and dried in a microwave oven. Synthesis conditions were optimized using differential scanning calorimetry and mass spectrometry data. The phase composition of the synthesis products obtained at temperatures of 800, 850, 900, and 950°C was determined by X-ray diffraction. Scanning electron microscopy was used to determine the particle size of the powders and examine their morphology.     

On the use of photoacoustic technique for monitoring the thermal properties of lanthanum strontium cobalt ferrite-yttria stabilized zirconia two-layer systems

In this work, lanthanum strontium cobalt ferrite (La_0.6Sr_0.4Co_0.2Fe_0.8O_3 − δ) films deposited by spray-pyrolysis onto commercial yttria stabilized zirconia substrates were investigated by photothermal spectroscopy. It is shown that by using the thermal-electrical analogy model it is possible to obtain the thermal properties of two-layer composite systems simultaneously, without the need to spread them, and thus to evaluate the thermal mismatch between the substrate and the deposited film. The thermal diffusivity of the 8YSZ substrate was found to be 6.6 × 10⁻³ cm²s⁻¹, whereas for the La_0.6Sr_0.4Co_0.2Fe_0.8O_3 − δ films it ranged between 0.47 and 9.26 × 10⁻⁴ cm²s⁻¹. We have found that for film thickness beyond 3.06 μm the thermal expansion coefficient becomes relevant, indicating that the optimum film deposition time lies between 10 and 20 min.     

Dielectric properties of cobalt doped cadmium oxalate crystals

Co _x Cd_1−x C₂O₄4H₂O crystals grown by gel technique are characterized for dielectric properties by optical absorption measurements. Loss curve shows a relaxation peak at 500 KHz corresponding to a relaxation time of 0·3 μs. Cole-Cole diagrams give exponent of universal power law to be equal to 0·22. Optical absorption shows peaks due to Co⁺² ion and water of hydration. An attempt is made to understand the results.     

Influence of lanthanum addition on preparation and powder properties of cobalt phosphates

Transition metal phosphates are used as inorganic pigments; however, these materials had a weak point for acid and base resistance. Because lanthanum phosphate is insoluble in acidic and basic solution, the addition of lanthanum cation was tried to improve the acid and base resistance of cobalt phosphate pigment. The lanthanum doped cobalt phosphates were prepared from phosphoric acid, cobalt nitrate, and lanthanum nitrate solution. The additional effects of lanthanum cation were studied on the chemical composition, particle shape and size distribution, specific surface area, color, acid and base resistance of the precipitates, and their thermal products.     

Enhancement of ferromagnetic and dielectric properties of lanthanum doped bismuth ferrite nanostructures

Cylindrical-shaped multiferroic Bi_1?xLa_xFeO₃ (x = 0.0, 0.05, 0.1 and 0.15) were synthesized successfully by hydrothermal method. All samples were found to be rhombohedrally distorted perovskite structure. Diameter of the cylindrical particles reduces from ～450 nm for x = 0.0 to ～100 nm for x = 0.1 prepared under the same conditions. The Neél temperature as well as the dielectric constant was also found to increase with the increase in lanthanum content. Lanthanum doping also enhanced the magnetic properties. Magnetization measurements above room temperature show a significant increase in magnetization at around 400 °C. Enhanced magnetic properties due to lanthanum doping are caused by the breakage of spin cycloid as observed by electron spin resonance study.     

Electrical properties of BaSnO3 in substitution of antimony for tin and lanthanum for barium

Polycrystalline materials of BaSn_1–x Sb _x O_3– and Ba_1–y La _y SnO_3– were prepared. Substitutional solubilities of antimony for tin and lanthanum for barium, respectively, in BaSnO₃ were obtained to be x=0.18 for BaSn_1–x Sb _x O_3– and y<0.052 for Ba_1-y La _y SnO_3–. The X-ray photoemission spectroscopy measurements showed the valence of antimony and tin is mixed in our samples of BaSn_1–x Sb _x O_3–. At lower temperature, magnetic susceptibilities of BaSn_1–x Sb _x O_3– and Ba_1–y La _y SnO_3– satisfy the Curie law, indicating the existence of non-interacting localized electrons at the Sn⁴⁺ site, and forming a Sn⁴⁺+e^– state in these systems. By substitution of antimony and lanthanum in BaSnO₃, the conductive properties are semiconductor-like. To explain this conductive behaviour, three types of mechanism were taken into consideration.     

Electrical properties of In doped CdS thin films

Indium doped polycrystalline cadmium sulphide CdS:In thin films have been prepared by the spray pyrolysis technique on glass substrates in an enclosed dome. The different scattering mechanisms such as lattice, impurity and grain boundary scattering for CdS:In films are observed at low temperature, in the range of 303 to 120 K. The experimentally determined mobilities due to these scatterings are well interpreted with those of theoretically calculated mobilities. The d.c. conductivity for CdS:In films has also been studied in the same temperature region. The Mott variable range hopping conduction process followed below the temperature of 150 K. The Mott parameters such as N(E _F ), R, W and are found to be 1.26 × 10¹⁹ eV^–1cm^–3, 9.8 × 10^–-7cm, 0.02 eV^–1 and 2.38 × 10⁶cm^–1, respectively from the conductivity data.     

Electrical properties of niobium doped barium bismuth-titanate ceramics

BaBi₄Ti_4–5/4xNb_xO₁₅ (BBNTx, x = 0, 0.05, 0.15, 0.30) ceramics have been prepared by solid state method. XRD data indicate the formation of single-phase-layered perovskites for all compositions. SEM micrographs suggest that the grain size decreases with Nb doping. The effect of niobium doping on the dielectric and relaxor behavior of BaBi₄Ti₄O₁₅ ceramics was investigated in a wide range of temperatures (20–777 °C) and frequencies (1.21 kHz to 1 MHz). Nb doping influences T_c decrease as well as the decrease of dielectric permittivity at Curie temperature. At room temperature, undoped BaBi₄Ti₄O₁₅ exhibits dielectric constant of ～204 at 100 kHz, that slightly increases with Nb doping. The conductivity of BBNT5 ceramics is found to be lower than that of other investigated compositions. The value of activation energy of σ_DC was found to be 0.89 eV, 1.01 eV, 0.93 eV and 0.71 eV for BBT, BBNT5, BBNT15 and BBNT30, respectively.     

Electrical and thermoelectric properties of surfactant-assisted calcium cobalt oxide nanoparticles

Journal of Materials Science: Materials in Electronics - We are in need of clean, cost-effective and renewable energy sources because of the energy crisis and global warming. Thermoelectric...     

Preparation and properties of exfoliated graphite doped with nickel and cobalt oxides

We report the first synthesis of doped exfoliated graphite through the electrochemical oxidation of natural graphite in aqueous nickel nitrate and cobalt nitrate solutions, followed by heat treatment. The solutions are found to differ in electrochemical behavior during the anodic oxidation of graphite. The thermal properties of the oxidized graphite prepared by this procedure and utilized as a precursor to exfoliated graphite are described. The exfoliated graphite obtained under optimal conditions contains 2–60 wt % metal oxide and has a specific surface of 60–160 m²/g and loose density of 1.5–5 g/l.     

Electrical conductivity of lanthanum oxide based composites containing carbon nanofibers

We have measured the electrical conductivity of lanthanum oxide based composite materials containing different concentrations of carbon nanofibers as additives. The conductivity has been shown to increase sharply (by two orders of magnitude) at carbon nanofiber contents from 2 to 3 wt % owing to the formation of a three-dimensional network of nanofibers in the bulk of the composite. Particular attention has been paid to the morphology of the particles of the constituent components of the composites and to the chemical and phase compositions of the matrix material.     

Electrical and structural properties of ortho-chloranil doped zinc phthalocyanine films

Electrical conductivity of evaporated zinc phthalocyanine (ZnPc) films, doped with orthochloranil (o-CA) molecules, is investigated. Benzoico-CA solution is introduced in the film and benzene is removed by vapourization afterwards. The film conductivity at first increased and then decreased with incremental doping. The maximum conductivity of the sample appeared at doping level × (=o-CA/ZnPc) 0.2 and is about 10³ times as large as that of the as-evaporated one. About a ten-fold further increase in the conductivity is obtained by the heat-treatment of 150 °C for 12 h. The conductivity change by doping is explained by taking into account thato-CA molecules behave not only as acceptors in the film but destructively for the film structure. The phase change of ZnPc by benzoic solution is also discussed.