Synthesis and properties of layered perovskite compounds

Bi₂A _{n ? 1} B _n O_{3n + 3} ceramics are fabricated where A and B are ions of Ti, W, Pb, Nb, and Mo. The process of synthesis is studied, and the changes in the charge mass and volume during sintering are determined. The ceramic production process is optimized. The dielectric properties of the ceramics are investigated, and the factors affecting the diffuseness of phase transitions are revealed.     

Dielectric,impedance and modulus spectroscopy of Ta-based layered perovskite

PbBi₂Ta₂O₉ ceramic samples were fabricated by high-temperature mixed oxide method. X-ray diffraction determines the structure as orthorhombic. The scanning electron microscopy confirms the formation of densely packed grains in the sample. The dielectric measurements, complex impedance and complex modulus study were carried out in a frequency range of 1?kHz–1?MHz and a temperature rangeof 25–500°C. The conduction mechanism of the material is discussed in details using variable range hoping, nearest neighbor hopping relaxation model and dc activation energy at two different temperature regimes. From the J-E characteristics studies, the occurrence of non-linear curves endorses the non-Ohmic nature of the material.     

Ferromagnetism of metallic compounds with the perovskite structure

On the basis of the concept of strong interaction in one unit cell, it is stated that ferromagnetic instability can occur in a system with hops between oxygen anions and transition metal (Me) cations in A_1?xB_xMeO₃ compounds. A phase diagram is constructed to describe the occurrence of ferromagnetic ordering as a function of the average number of holes (h_{t, d}) and (h_p) in the low-spin or high-spin t _2g ⁶ or high-spin 3d¹⁰ shell of the transition metal and in the 2p⁶ shells of O^2?.     

Effect of iron doping at Mn-site on complex impedance spectroscopy properties of Nd0.67Ba0.33MnO3 perovskite

The effect of Fe-doping at Mn-site on the structural and electrical properties of Nd_0.67Ba_0.33Mn_1?xFe_xO₃ (0 ≤ x ≤ 0.05) perovskites has been investigated. X-ray diffraction patterns show that the structural parameters change slightly due to the fact that the Fe³⁺ ions replacing the Mn³⁺ have similar ionic radius. The electrical properties of these samples have been investigated using complex impedance spectroscopy technique. a function of the frequency at different temperatures. When increasing the Fe-content, a decrease of dc conductivity was observed throughout the whole explored temperature range and the deduced activation energy values are found to increase from 128 meV for x = 0 to 166 meV for x = 0.05. The curves of the imaginary part of impedance (Z″) show the presence of relaxation phenomenon in our samples. The complex impedance spectra show semicircle arcs at different temperatures and an equivalent circuit of the type of R_g + (R_gb//C_gb) has been proposed to explain the impedance results.     

Electrical conductivity and complex impedance analysis of 20% Ti-doped La0.7Sr0.3MnO3 perovskite

The electrical conductance of 20% Ti-doped La_0.7Sr_0.3MnO₃ (LSMO) was measured using admittance spectroscopy over a wide temperature and frequency ranges. The impedance plane plot shows semicircle arcs at different temperatures and an electrical equivalent circuit has been proposed to explain the impedance results. Activation energy inferred from conductance spectrum matches very well with the value estimated from relaxation time indicating that relaxation process and conductivity have the same origin. The electrical conductance of La_0.7Sr_0.3Mn_0.8Ti_0.2O₃ is found to be dependent on temperature and frequency. Also, the electronic conduction appears to be dominated by thermally activated hopping of small polaron (SPH) at high temperatures and by variable range hopping (VRH) at low temperatures.     

XPS study of some selected selenium compounds

X-ray photoelectron spectroscopy (XPS) of the Se 3d peak for Se in Se, SeO₂, As₂Se₃, ZnSe, CdSe, CoSe, FeSe, CrSe, VSe₂ and TiSe were studied. It was found that, firstly, in the case of As₂Se₃, ZnSe, CdSe, CoSe and CrSe, as the change in electronegativity of the anion increases, the change in the binding energy value of the Se 3d peak also increases and, secondly, in all of the Se compounds studied, the Se-Se bond in pure Se is stronger than the Se-x bond, where X = As, Zn, Cd, Co, Fe, Cr, V and Ti.     

Structural,elastic and optoelectronic properties of the hydrogen based perovskite compounds: Ab-initio study

The structural, elastic, electronic and optical properties of the cubic CaMH₃ (M=Ni and Pd) compounds are investigated using first principle calculations based on the density functional theory (DFT) as implemented in the Wien2k code. The calculated lattice constants are found to be in good agreement with the values reported in the previous literature. The analysis of standard enthalpy of formation shows that both CaNiH₃ and CaPdH₃ are thermodynamically stable. The bulk modulus and cohesive energy values indicate that CaNiH₃ is less compressible and more rigid than CaPdH₃. The bonding forces between atoms of the studied compounds are mainly ionic and partially covalent. The real and imaginary parts of the dielectric function are used to calculate the optical properties. The partial density of states for the present compounds are used to explain the main peaks of the optical spectra. Beneficial optoelectronic applications are predicted from the analysis of the optical spectra of CaNiH₃ and CaPdH₃ compounds.     

Predominant factors affecting the dielectric and piezoelectric properties of bismuth-containing complex perovskite solid solution

To study the factors affecting the dielectric and piezoelectric properties of bismuth-containing complex perovskites, the solid solution (1−x)Pb(Mg_1/3Nb_2/3)O₃-xBi(Mg_2/3Nb_1/3)O₃ was prepared by the solid state reaction method and its dielectric and piezoelectric properties were investigated. It is found that (1) at room temperature, the nonlinearity of the DE-loop for Pb(Mg_1/3Nb_2/3)O₃ is completely suppressed at a rather low x (<5%); (2) dielectric constant versus temperature curves deviate from the Curie-Weiss law at a temperature T_d much higher than the dielectric constant peak temperature T_m and T_m−T_d decreases considerably with increasing x; and (3) frequency dispersion ΔT_m=T_m (1 MHz)−T_m (10 kHz) increases with increasing x. Possible factors responsible for the variation of the dielectric and piezoelectric properties with x are discussed.     

部分有机物蝌蚪麻醉活性的预测

基于定量结构-活性相关性（QSAR）原理,研究了49种有机化合物结构与其蝌蚪麻醉活性的内在定量关系。首先应用分子电性作用矢量（molecular electronegativity interaction vector,MEIV）表征49种有机化合物的结构,再采用多元线性回归（MLR）方法建立了相应的QSAR预测模型,最后对所建模型分别进行了内部验证和外部验证。所建模型的复相关系数(Rcum)、留一法(LOO)交互校验复相关系数(RCV)和外部样本校验复相关系数(Qext)分别为0.9415、0.9127和0.9253,证明该模型均具有较高的稳定性和预测能力。     

Structure and optical properties of lead iodide based two-dimensional perovskite compounds containing fluorophenethylamines

Phenethylammonium-based perovskites, which can be regarded as a semiconductor/insulator multiple quantum well consisting of lead halide semiconductor layers sandwiched between phenethylammonium insulator layers were prepared. To investigate the effects of the electronic state and the orientation of organic insulator layers on the optical properties of layered perovskites, fluorine substituted analogues were also prepared. The structure and optical properties were investigated by the XRD, UV–Vis absorption, and fluorescence measurements. The exciton absorption peak was shifted by the substitution of fluorine atoms in organic ammonium compounds. It became clear that the optical properties of two-dimensional perovskite compounds were controlled by the substitution of fluorine atoms.     

Automated multifrequency measurements of the complex impedance of fast ion conductors

An automated apparatus based on phase synchronous detection has been assembled for measurements of the complex impedance of fast ion conductors. Procedures for calibration and for data acquisition and analysis are described. The relative advantages and disadvantages of this method are compared with ac bridge and pulse methods. The method is illustrated with measurements of the sodium beta-aluminas.     

Wideband characterization of the complex wave number and characteristic impedance of sound absorbers

Several methods for measuring the complex wave number and the characteristic impedance of sound absorbers have been proposed in the literature. These methods can be classified into single frequency and wideband methods. In this paper, the main existing methods are revisited and discussed. An alternative method which is not well known or discussed in the literature while exhibiting great potential is also discussed. This method is essentially an improvement of the wideband method described by Iwase et al., rewritten so that the setup is more ISO 10534-2 standard-compliant. Glass wool, melamine foam and acoustical/thermal insulator wool are used to compare the main existing wideband non-iterative methods with this alternative method. It is found that, in the middle and high frequency ranges the alternative method yields results that are comparable in accuracy to the classical two-cavity method and the four-microphone transfer-matrix method. However, in the low frequency range, the alternative method appears to be more accurate than the other methods, especially when measuring the complex wave number.     

Influence of valence band states on the core hole screening in lanthanide perovskite compounds

Investigation on core level and valence band photoelectron spectra of lanthanide perovskite samples (Ag _x La_1–x CoO₃;x<0,3) and reinvestigations of metallic lanthanum and La-oxide layers are reported. Differences in preparation method during perovskite formation are leading to different quality of sample purity and different electronic properties. This could be followed by changes in the density of states near the Fermi energy. On the other hand the screening mechanism after ionization of a La 3d core hole is affected by the relative energy position of occupied valence band states with respect to the empty La 4f ^* level in the core ionized state. Changes in La 3d line profile are discussed as changes in multi electron processes (e.g. shake up or energy gain) during photoionization. We attribute the two electron process during photoemission of La 3d electrons to the well screened energy gain transition in LaCoO₃, whereas the two electron process results in a shake up transition in La₂O₃ and La(OH)₃.     

Algebraic method of determining bond length of perovskite compounds doped with Mn

An algebraic method is proposed for calculating the exponent n and constant K in the power law 10Dq=KR⁻ⁿ. By using this method, the values of n and K for the cubic perovskites KMnF₃ and RbMnF₃ are derived and then used to determine the true Mn²⁺–F⁻ bond length R in crystals of KMgF₃, KZnF₃, RbCdF₃, and CsCaF₃ doped with Mn²⁺ from the reported optical spectra. The results obtained are in good agreement with those derived from the electron paramagnetic resonance data and the extended X-ray absorption fine structure techniques. Thus, as a new theoretical method, it is valuable and convenient for the determination of the bond lengths.     

Probing microstructure of polymer electrolyte-salt complex through impedance spectroscopy

In study of polymer-salt complexes usually one is interested in the dc conductivity, which can be extracted from impedance spectroscopy. However, impedance spectroscopy yields a host of additional information, for example, frequency variation of the measured admittance or impedance, nature of the Cole-Cole plot and dielectric function. Proper analysis of this information reveals details of the microstructure and transport processes in the sample. This in turn, will lead to development of better solid electrolytes with tailor-made properties. The polymer electrolyte PEO-NH₄ClO₄ shows an interesting variation in its properties in two different regimes, (1) where salt concentration is below ∼ 19 wt.% and (2) above this concentration. In the present paper we suggest an equivalent circuit incorporating a constant phase element (CPE), which reproduces the complex admittance behavior at different salt fractions. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Thermal and complex impedance analysis of amorphous and crystalline lithium salt mixtures

Amorphous electrolytes consisting of the lithium salts, Li[R-NSO₂CF₃] were prepared and the attendant low ionic conductivities of the lithium salt mixtures (1×10⁻⁶ S cm⁻¹ at room temperature) are attributed to high glass transition temperatures. An example is the novel amorphous salt, Li[18-C-6NSO₂CF₃] which produces an amorphous salt mixture with Li[N(SO₂CF₃)₂] (LiTFSI).