Narrow Results

The single-phase samples of Y_0.9Ca_0.1Ba_2-yLa_yCu₃O_z and Y_1-xCa_xBa_1.5La_0.5Cu₃O_z were synthesized with both x and y from 0 to 0.5, respectively. Structural and superconducting properties have been investigated by X-ray diffraction, Rietveld refinement, and DC magnetization measurement, respectively. Different from the self-compensating compound of Y_1-xCa_xBa_2-xLa_xCu₃O_z, carriers in these two systems of Y_1-xCa_xBa_2-yLa_yCu₃O_z are not compensated as x ≠ y. By careful study of the chemical bond lengths related to Cu(2)–O plane and Cu(1)–O chain, it is demonstrated that the critical temperature (T_c) changes monotonously with the bond lengths of Cu(1)–O(4) and Cu(2)–O(4). The analysis demonstrates that crystalline structure is one of the important factors to high-T_c superconductivity, and its influence is independent of the carrier concentration.

In the current study, ABO₃ (A = Pb, Ba, Sr and B = Ti) perovskite structures are produced by the auto-combustion route by using citric acid (CA) and nitric acid (NA) as fuel and oxidizer. The X-ray diffraction (XRD) patterns confirmed the perovskite nanostructure with cubic, tetragonal, and rhombohedral for SrTiO₃, PbTiO₃, and BaTiO₃, respectively. Using Scherrer’s equation and XRD pattern, the average crystallite size of the samples were acquired. The effect of Ti–O bond length on the structure of the samples was evaluated. The type of structures obtained depends on Ti–O bond length which is in turn influenced by A${}^{2+}$ substitutions. Microstructural studies of nanostructures calcined at 850${}^{\circ }$C confirmed the formation of polyhedral particles with a narrow size distribution. The values of optical band gaps were measured and the impact of A${}^{2+}$ was discussed. The optical properties such as the complex refractive index and dielectric function were calculated by IR spectroscopy and Kramers–Kronig (K–K) relations. Lead, as the element with the highest density as compared to other elements, changes the optical constants, remarkably due to altering titanium and oxygen distance in TO₆ groups.

Energy stored in a bond depends on the bond length and bond angle. We use computer simulation software to measure the bond lengths and bond angles, diameters and other parameters of single-walled carbon nanotubes (SWCNTs), and employ the results to determine thermal expansion of SWCNTs in the radial and axial directions. It is found in this study that at low temperatures and even at room temperature, SWCNTs show negative thermal expansion in both the radial and axial directions (the diameter and length both decrease with the increase in temperature). At high temperatures the expansion becomes positive in the radial and axial directions (the diameter and length both increase with increasing temperature). Thermal expansion in the radial direction does not depend on the tube helicity. SWCNTs of large diameter show higher thermal expansion in the radial direction than that for small diameter tubes up to 700 K, but beyond 700 K the opposite occurs.

In the present work, nanocrystalline hausmannite Mn₃O₄:Ba thin films have been deposited on glass substrates by chemical spray pyrolysis (CSP). Then, we investigated the impact of Ba doping concentrations on the structural, morphological and optical properties. The structural characteristics were investigated by X-ray diffraction technique and clearly show the films have a spinel Mn₃O₄ polycrystalline structure, the degree of crystallinity was improved by increasing Ba concentrations in Mn₃O₄ matrix with crystallite size range of 15–33nm. The lattice parameters, the unit cell volume and the (Mn-O) bond length of tetrahedral and octahedral sites, were varied by increasing Ba concentrations. SEM micrographs show that the films are homogeneous with nanoparticles dispersed on the surface with sizes range 30–132nm. The optical properties were estimated by UV-Vis-NIR spectrophotometer and exhibited that the optical transmittance and band gap were improved by increasing Ba doping concentration. Empirical equations were suggested to estimate some correlated variables with excellent agreement with the experimental data. The optimum condition was recorded in films doped with 3% of Ba where a better crystallinity, a preferable surface morphology and outstanding optical properties have been achieved.

For the first time, a general viewpoint of electronegativity and chemical bond in alloy semiconductors, e.g., Mg_xZn_1-xO(x = 0.0-1.0) was proposed. The variation of bulk modulus and bond length, as well as their dependence on Mg concentration x were quantitatively simulated. The bulk moduli of Mg_xZn_1-xO alloys decrease with increasing Mg concentration x. The detailed variation of bond lengths of both Mg–O and Zn–O in Mg_xZn_1-xO alloys in the whole composition range was determined, which is less than 0.007 Å. The valence state of Mg is larger than that of Zn when x = 0.0-1.0, which leads to the increase of valence state of O with increasing Mg concentration x. The current results clearly indicate that Mg_xZn_1-xO condenses in an alloy state.