Within the framework of the model of coupled anharmonic oscillators a ferroelectric solid solution is described with only a few parameters. In high-temperature approximation a linear relation between the square of the order parameter and the transition temperature is obtained. This is illustrated by an application to several modified lead titanate ceramics, the linear relation between transition temperature and tetragonal lattice deformation is verified. For tris-sarcosine calcium chloride (bromide) (TSCC/B) at very low temperatures the expected deviation from the classical behaviour is seen.
Results of thermal annealing of extremely highly doped GaAs:Te on room temperature Hall electron concentration and diffuse X-ray scattering are briefly reported. Reversible decrease/increase in electron concentration vs. temperature of annealing perfectly coincides with a strong increase/decrease in diffuse X-ray scattering intensity. An analysis of X-ray results indicates an arising of correlations in impurity positions in crystal lattice points in GaAs:Te solid solution for very high doping level. We give a sketch of a new formal model of tight bond creation between impurity atoms, which can consistently describe our results. The model is free from difficulties in describing annealing results encountered by a widely spread model of charge compensation by native acceptors.
Phase equilibria in Gd_2O_3-CeO_2-CuO system were studied based on samples obtained from appropriate mixtures of the starting components, prepared by a conventional solid state reaction in air. The phase relations prevalent in this system have been deduced based on X-ray diffraction of well-equilibrated samples. An isothermal cross-section diagram through the system at 980°C is presented and chemical constitution of the Ce-doped Gd_2CuO_4 phase is proposed.
Two samples of nominal composition Gd_{1.80}Ce_{0.15}CuO_{4.0} and Gd_{1.85}Ce_{0.15}CuO_{4.075} were subjected for a long-term treatment in air at 1100°C, comprised with a precise control of their weights vs. time of the treatment applied. The gravimetric analysis and the iodometric titration, both showed a deficiency on Cu-site, steadily increasing vs. time with a slight tendency to a saturation. The X-ray analysis of the resulting products showed that even after 272 h of sintering, the samples remained single phase of the "214" structure type. Refinement of their crystal structures as well as density measurements, both allowed confirming Cu deficient structure constitution of these samples. Results of dc-magnetization measurements, performed prior to reduction of the sample's overall oxygen content, did not show any sign of superconductivity down to 4.2 K. These preliminary results indicate that copper deficiency, developing during synthesis, may occur to be a decisive factor for the reported lack of superconductivity in Ce-doped Gd_2CuO_4 phase. Oxygen content reduction and further tests on the samples obtained are in progress.
The principles of small-angle X-ray scattering are briefly reviewed with emphasis on the particular advantages of the use of synchrotron radiation. The application of the technique is illustrated by several examples concerning precipitates in alloys, internal oxidation, porous materials, as well as the structure of bone.
We consider a crosslinked polymer blend made of two polymers of different chemical nature. We suppose that such a system incorporates small colloidal particles, which prefer to be attracted by one polymer, close to the spinodal temperature. This is the so-called critical adsorption. As assumption, the particle diameter, d_0, is considered to be small enough in comparison with the size of microdomains (mesh size) ξ* ~ an^{1/2}, with a - the monomer size and n - the number of monomers between consecutive crosslinks. The critical fluctuations of the crosslinked polymer mixture induce a pair-potential between particles located in the non-preferred phase. The purpose is the determination of the Casimir pair-potential, U_2 (r), as a function of the interparticle distance r. To achieve calculations, use is made of an extended de Gennes field theory that takes into account the colloid-polymer interactions. Within the framework of this theory, we first show that the pair-particle is attractive. Second, we find for this potential the exact form: U_2(r)/k_{B}T = - A_{H}(d_0/r)^2 exp(-r/ξ*) - B_{H}(d_0/r)^4 exp(-2r/ξ*), with the known universal amplitudes A_{H} > 0 and B_{H} > 0 (the Hamaker constants). This expression clearly shows that the pair-potential differs from its homologue with no crosslinks only by the two exponential factors exp(-r/ξ*) and exp(-2r/ξ*). The main conclusion is that the presence of reticulations reduces substantially the Casimir effect in crosslinked polymer blends.
To understand the combined effect of divalent ion and A-site vacancy (Ø) self-doping, beyond the binary La_{1-x}MnO_3 system, polycrystalline La_{1-x-y}Ca_xØ_yMnO_{3+δ} samples were studied. Samples with Ca substitution and excess Mn in the range x<0.33 and y<0.45, respectively, were prepared by standard solid-state reactions. Structural and phase analysis of the samples were done by X-ray diffraction and transmission electron microscopy. The manganite structure in this composition range includes rhombic and orthorhombic phases. Most samples contain the hausmanite phase (Mn_3O_4) coexisting with the manganite phase. A small amount of perovskite related AMn_7O_{12} (A=La,Ca) structure near the hausmanite-manganite boundary is also found in samples treated in oxygen atmosphere. The calculated vacancy content accommodated in the manganite phase can be higher than 1/8, the reported limit for La_{1-x}MnO_3, which is confirmed in our x=0 sample. For the compositions studied, a ferromagnetic metallic behavior below T_C is found, confirming the stability of the Mn sublattice, and near the optimum stoichiometric ((x=0. 33, y=0) system, a net A-site vacancy content (up to y=0.2) does not degrade the magnetic and electric properties.
Small-angle X-ray scattering method recognised to be very useful in the studies of structural problems of materials inhomogeneous in nanoscale. Studies by small-angle X-ray scattering on the Al-Ag alloys are presented. The size distributions of spherical Guinier-Preston zones were calculated using Vonk's and Glatter's methods. Small-angle X-ray scattering studies were performed on Al-5.0 at.%Ag alloy containing spherical Guinier-Preston zones in theη-state. For this alloy one can assume that scattering particles are of spherical shape, have uniform electron density and scatter independently. Moreover, the size distributions were calculated for Al-1.6 at.%Ag alloy containing Guinier-Preston zones in ε-state (scattering particles with not uniform electron density). Both, Vonk's and Glatter's, methods gave similar size distributions. Profiles of size distributions for Guinier-Preston zones inε-state are more complex in comparison to Guinier-Preston zones inη-state. It is connected with a complex structure of Guinier-Preston zones in ε-state. A modification of the structure model of Guinier-Preston zones inε-state is proposed.
The building units of biological systems, the biomolecules, cannot easily be organized or even classified into defined categories. They can be as simple as water or complex as tintin, a muscle protein extremely large with several thousand atoms. To understand their function, one must know their characteristics, where they occur and what they do. One approach to reach such an ambitious task is to determine their structure, as single molecules or assembled into aggregates. Small angle X-ray scattering is the most important method for this purpose. We present studies carried out on several systems, and aiming at different questions about them. We start with lipids, the main components of the cell membranes. These membranes form the cell boundaries, the moiety required for the so-called membrane proteins, but also influence significantly several aspects of biological activity. More complex systems like a muscle fibre is also presented, showing that changes in the structure are related to the movement mechanism. It becomes easy to conclude that knowing the structures and the changes occurring in them is an important way to understand the function of biomolecules and therefore their role in the life cycle.
The increase in Τ_{c} for high temperature superconductors can be realized, among others, by appropriate substrate/film combinations. SrLaGaO_{4}-SrLaΑlO_{4} solid solutions were grown by the Czochralski method. The already achieved results allow to obtain single crystals of SrLaΑl_{1-x}Ga_{x}O_{4} with lattice constant a in the range from 0.3754 to 0.3775 nm, and SrLaGa_{1-x}Αl_{x}O_{4} crystals with lattice constant a in the range from 0.3843 to 0.3826 nm. Electron-probe microanalysis along obtained single crystals was used for determination of segregation coefficient between aluminum and gallium ions.
The X-ray photoelectron emission microscopy at the Advanced Light Source has a spatial resolution of 0.2 microns at an accelerating voltage of 12 kV. The tunability of the photon energy is used to provide chemical state information using near edge X-ray absorption fine structure spectroscopy on the sub-micrometer scale. The homogeneity of thin films of polymer blends was studied for various film thicknesses. The polystyrene/polyvinylmethylether film of 194 Å showed protrusions of 2-3 μm diameter with an enriched polystyrene content while the polystyrene/polystyreneacrylonitrile 504 Å thick films showed 5-6 μm segregated regions without any topological structure.
The results of comparative studies of the effect of hydrogenation upon structural and optical properties of the commercial soda-lime silicate glasses doped either with monovalent silver or copper have been presented. The samples were characterised by means of the scanning and transmission electron microscopy and the UV-VIS-IR absorption spectroscopy. It has been shown that the annealing of suitably ion-exchanged specimens in gaseous hydrogen atmosphere could be considered as an effective procedure for obtaining composite materials of expected properties correlated with the induced changes of the morphology of both the glassy matrix and the dopant.
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