Synthesis, structural and electrical properties of La and Nb modified Bi4Ti3O12 ferroelectric ceramics

Polycrystalline ceramic samples of Bi_4−xLa_xTi₃O₁₂ (x=0.0, 0.5 and 1) and Bi_3.5La_0.5Ti_3−yNb_yO₁₂ (y=0.02 and 0.04) have been synthesized by standard high temperature solid state reaction method using high purity oxides and carbonates. The effect of lanthanum doping on Bi-site and Nb doping on Ti-site on the structural and electrical properties of Bi₄Ti₃O₁₂ powders was investigated by X-ray diffraction, scanning electron microscopy, dc conductivity and dielectric studies. A better agreement between the observed and calculated X-ray diffraction pattern was obtained by performing the Rietveld refinement with a structural model using the non-centrosymmetric space group Fmmm in all the cases. A better agreement between observed and calculated d-values also shows that the lattice parameters calculated using the Rietveld refinement analysis are better. The increase in lanthanum and niobium contents does not lead to any secondary phases. It is found that La³⁺ doping reduces the material grain size and changes its morphology from the plate-like form to a spherical staking like form. The substitution of Nb for Ti ions affected the degree of disorder and modified the dielectric properties leading to more resistive ceramic compounds. The shape and size of the grains are strongly influenced by the addition of niobium to the system. The activation energies of all the compounds were calculated by measuring their dc electrical conductivities. The frequency and temperature dependent dielectric behavior of all the compounds have also been studied and the results are discussed in detail. The substitution of La and Nb on the Bi and Ti sites decreased the T_c and improved the dielectric and ferroelectric behavior.     

Conical parametric scattering of a single extraordinary beam in BaTiO3 crystal

In this paper, the parametric scattering of a single extraordinary polarized beam of laser in BaTiO₃ photorefractive crystal has been investigated experimentally and theoretically. The resulting pattern consists of beam fanning, isotropic ring, and anisotropic one. Among all parts of scattering pattern, isotropic ring has not been studied as much as beam fanning and anisotropic ring, and there still are some differences in reports about it. Therefore, the study has mainly focused on this part. In this experimental configuration, isotropic ring is just visible in positive angles although the other parts of parametric scattering pattern can be visible from behind and in front of the crystal. In addition to steady state pattern in forward and backward directions, its transient behavior with the rotation of crystal has been studied. The results of experiments have been analyzed carefully, and their theoretical explanations have been presented based on the standard theory of parametric scattering in photorefractive crystals. It has been shown that this configuration corresponds to the so called parametric B-process scattering.     

Electron magnetic resonance investigation of gadolinium diffusion in zircon powders

The electron magnetic resonance (EMR) technique was used to investigate the diffusion of gadolinium in zircon (ZrSiO₄) powders. The EMR absorption intensity was measured for several annealing times and three different temperatures of isothermal annealing: 1273, 1323 and 1373 K. The activation energy for diffusion, calculated from the experimental data using a theoretical model based on the Fick equation, was found to be E_A=506±5 kJ mol⁻¹. This value is close to the ones for the diffusion of Gd in UO₂ and CeO₂, but much larger than for the diffusion of gadolinium in a compound with the same crystal structure as zircon, YVO₄. This is attributed to a difference in the relative sizes of the ions involved in the diffusion process.     

Novel superconducting phases in copper oxides and iron-oxypnictides: NMR studies

We reexamine the novel phase diagrams of antiferromagnetism (AFM) and high-T_c superconductivity (HTSC) for a disorder-free CuO₂ plane based on an evaluation of local hole density (p) by site-selective Cu-NMR studies on multilayered copper oxides. Multilayered systems provide us with the opportunity to research the characteristics of the disorder-free CuO₂ plane. The site-selective NMR is the best and the only tool used to extract layer-dependent characteristics. Consequently, we have concluded that the uniform mixing of AFM and SC is a general property inherent to a single CuO₂ plane in an underdoped regime of HTSC. The T=0 phase diagram of AFM constructed here is in quantitative agreement with the theories in a strong correlation regime which is unchanged even with mobile holes. This Mott physics plays a vital role for mediating the Cooper pairs to make T_c of HTSC very high. By contrast, we address from extensive NMR studies on electron-doped iron-oxypnictides La1111 compounds that the increase in T_c is not due to the development of AFM spin fluctuations, but because the structural parameters, such as the bond angle α of the FeAs₄ tetrahedron and the a-axis length, approach each optimum value. Based on these results, we propose that a stronger correlation in HTSC than in FeAs-based superconductors may make T_c higher significantly.     

Three-dimensional simulation of quasi-particle structures in nano-scaled superconductors

We have developed numerical simulation method for quasi-particle structures in the three-dimensional nano-sized superconductors, using the three-dimensional finite element method and the Bogoliubov-de Gennes equation. Using this method, we analyzed the superconducting state in the nano-sized cubic superconductors. We found the spatial oscillations of order parameter because of the confinement of superconducting electrons, and also we found the quasi-particle bound states at the corners of the cubic superconductors because of suppression of superconductivity at the corners.     

Effect of polymers doping on flux pinning behavior of MgB2

The effects of polymers doping on irreversibility field (H_irr) and critical current density (J_c) of MgB₂ have been investigated in this work. It is found that both J_c, and H_irr, are improved by doping at relative lower temperature region. The J_c−B curves of all samples studied in this work are well fitted using J_c(B) formula in percolation model. The values of upper critical field anisotropy (γ) are obtained from the fitting result at various temperatures. It is observed that values of γ for polymers doping samples are reduced at these temperatures. This is considered to be responsible for the enhancement of values of J_c for doped samples. Moreover, the percolation threshold, p_c, is found to be enhanced with increasing temperature. It is believed that the grain boundary pinning is still dominating in MgB₂, while the deviation of experimental data from the theoretical values is due to the percolation of suppercurrent in polycrystalline MgB₂.     

Synthesis and characterization of CdS nanoparticles in the presence of oleic acid as solvent and stabilizer

Oleic acid (OA)-capped CdS nanoparticles (NPs) have been successfully synthesized via the direct reaction of Cd(CH₂COO)₂·2H₂O with S powder in OA solvent at 230 °C under nitrogen flow, which was a kind of clean and air-stable solvent. The morphologies and structures of the as-synthesized CdS NPs are examined by transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-ray diffractometer (XRD), and Fourier transform infrared (FTIR) spectroscopy, and the typical Ostwald ripening growth mechanism is concluded. Moreover, the collected ultraviolet–visible (UV–vis) absorption spectroscopy and photoluminescence (PL) spectroscopy demonstrate good optical properties of CdS NPs.     

The quantitative monitoring of mechanochemical reaction between solid l-tartaric acid and sodium carbonate monohydrate by terahertz spectroscopy

The solid-state reaction of chiral tartaric acid and alkali carbonate was studied by terahertz time-domain spectroscopy (THz-TDS). The sodium tartrate dihydrate was synthesized with high efficiency by mechanical grinding in the solid-state without waste that is particularly sustainable and environmentally benign. Distinct THz absorptions were observed for reactants and products. It indicates that THz spectroscopy is sensitive to different materials and crystal structures. The characteristic THz absorption peak at 1.09 THz of l (+)-Tartaric acid was selected for quantitative analysis. The reaction kinetics could be expressed by the Second-order equation and the Jander equation, which is consistent with a three-dimensional diffusion mechanism. The combination of multi-techniques including synchrotron radiation X-ray powder diffraction (SRXRPD), Fourier transform infrared (FT-IR) and scanning electron microscopy (SEM) was used to investigate the grinding process and presented supporting evidences. The results demonstrate that THz spectroscopy technique has great potential applications in process monitoring and analysis in pharmaceutical and chemical synthesis industry.     

Modeling of lattice constant and their relations with ionic radii and electronegativity of constituting ions of A2XY6 cubic crystals (A=K, Cs, Rb, Tl; X=tetravalent cation, Y=F, Cl, Br, I)

In the present paper a new empirical model is proposed to describe and predict the lattice constants for a series of cubic crystals, all of which have the A₂XY₆ composition (A=K, Cs, Rb, Tl; X=tetravalent cation, Y=F, Cl, Br, I). The model is based on a thorough analysis of structural properties of 85 representative crystals from this group. It was shown that the lattice constant is a linear function of the ionic radii and electronegativity of the constituting ions. A simple empirical equation was obtained as a result of the performed analysis. It gives very good agreement between the experimental and modeled values of the lattice parameters, with an average error of 1.05%. The developed approach can be efficiently used for a simple, fast, and reliable prediction of lattice constants and interionic distances in isostructural materials having a similar composition.