Vegard’s law deviation in band gap and bowing parameter of AlxIn1-xN

Numerical simulation based on first-principle calculations is applied to study the wurtzite Al_xIn_1-xN. Simulation results suggest that the Vegard’s law deviation parameter is 0.063 ± 0.014 Å for the a lattice constant, and -0.160 ± 0.015 Å for the c lattice constant. The band gap bowing parameter is 3.668 ± 0.147 eV with the lattice constants by means of the minimized equilibrium energy, and 3.457 ± 0.152 eV with the lattice constants derived from Vegard’s law. PACS 71.15.-m; 71.20.Nr; 81.05.Ea     

First-principles calculation for bowing parameter of wurtzite AlxGa1-xN

Numerical calculation based on first-principles is applied to study the structural characteristics and the band-energy properties of wurtzite Al_xGa_1-xN. The lattice constants obtained from the equilibrium energy are larger than those obtained from the Vegard’s law. The deviation parameter is 0.040±0.005 Å for the a lattice constant and 0.125±0.009 Å for the c lattice constant. The band gap energy is overestimated with the Vegard’s law. The bowing parameter of direct (indirect) band gap energy of 0.752±0.069 eV (0.889±0.057 eV) is obtained with the equilibrium lattice constants, and 0.352±0.046 eV (0.271±0.056 eV) is obtained with the lattice constants derived from the Vegard’s law. PACS 71.15.-m; 71.20.Nr; 81.05.Ea     

Measurement of solid-liquid interfacial energy for solid Zn in the Zn-Cd eutectic system

The equilibrated grain boundary groove shapes for the Zn solid solution in Zn-Cd liquid solutions were directly observed. From the observed grain boundary groove shapes, the Gibbs-Thomson coefficient for solid Zn (Zn-15 wt.% Cd) in Zn-Cd liquid solutions has been determined to be (2.5 ± 0.1) × 10⁻⁸ Km by a direct method. The solid-liquid interfacial energy between solid Zn and Zn-Cd liquid solution has been obtained to be (165.5 ± 19.0) mJ/m² from the Gibbs-Thomson equation. The grain boundary energy for the same alloy has been determined as (317.8 ± 39.9) mJ/m². The thermal conductivities of the solid and liquid phases at the eutectic composition and temperature have also been measured.     

Accurate measurement of lattice parameters of Ag-Au solid solutions

Using the ratio method in the variant effectiveD method the measurement of the lattice parameters of a solid solution of Ag-Au on samples of purity 99·999% was performed. The mean values of the parameters can be expressed by the functiony=4·086 17.x + 4·078 42. (1–x) –0·021 3₉.x(1–x) [Å] wherex is the atomic concentration of Ag. The mean displacement of the mean values of the parameters measured at single concentration is ±0·000 05 Å. A decrease of the lattice parameters of solid solutions to below the values of pure components was confirmed.The author thanks Assoc. Prof. M. Cernohorský CSc. for having introduced him to the problems of the precise measurement of lattice parameters and Mrs. V. Gregorová and L. Adamcová and Mr. M. Jelínek who took part in the experiments.     

Atomic mobility in a ternary liquid Ga–In–Sn alloy of the eutectic composition

The nuclear spin–lattice relaxation and Knight shift of ⁷¹Ga, ⁶⁹Ga, and ¹¹⁵In nuclei in a ternary liquid gallium–indium–tin alloy of the eutectic composition, which was introduced into pores of an opal matrix and porous glasses with pore sizes of 18 and 7 nm, have been investigated and compared with those for the bulk melt. It has been found that longitudinal relaxation is accelerated and the Knight shift is decreased, depending on the size of pores. The correlation time of the atomic motion has been calculated for the nanostructured melt in porous matrices. It has been shown that the atomic mobility in the melt decreases with decreasing size of pores in the glasses.

     

Structure of rutile TiO2 (1 1 0) in water and 1 molal Rb at pH 12: Inter-relationship among surface charge, interfacial hydration structure, and substrate structural displacements

The rutile (1 1 0)-aqueous solution interface structure was measured in deionized water (DIW) and 1 molal (m) RbCl + RbOH solution (pH 12) at 25 °C with the X-ray crystal truncation rod method. The rutile surface in both solutions consists of a stoichiometric (1 × 1) surface unit mesh with the surface terminated by bridging oxygen (BO) and terminal oxygen (TO) sites, with a mixture of water molecules and hydroxyl groups (OH⁻) occupying the TO sites. An additional hydration layer is observed above the TO site, with three distinct water adsorption sites each having well-defined vertical and lateral locations. Rb⁺ specifically adsorbs at the tetradentate site between the TO and BO sites, replacing one of the adsorbed water molecules at the interface. There is no further ordered water structure observed above the hydration layer. Structural displacements of atoms at the oxide surface are sensitive to the solution composition. Ti atom displacements from their bulk lattice positions, as large as 0.05 Å at the rutile (1 1 0)-DIW interface, decay in magnitude into the crystal with significant relaxations that are observable down to the fourth Ti-layer below the surface. A systematic outward shift was observed for Ti atom locations below the BO rows, while a systematic inward displacement was found for Ti atoms below the TO rows. The Ti displacements were mostly reduced in contact with the RbCl solution at pH 12, with no statistically significant relaxations in the fourth layer Ti atoms. The distance between the surface 5-fold Ti atoms and the oxygen atoms of the TO site is 2.13 ± 0.03 Å in DIW and 2.05 ± 0.03 Å in the Rb⁺ solution, suggesting molecular adsorption of water at the TO site to the rutile (1 1 0) surface in DIW, while at pH 12, adsorption at the TO site is primarily in the form of an adsorbed hydroxyl group.     

Neutron spectroscopy study of magnons in the austenite and martensite phases of a Ni-Mn-Ga Heusler alloy

The low energy region of magnon excitations of an off-stoichiometric Ni_49.1Mn_29.4Ga_21.5 single crystal has been investigated by neutron spectroscopy. The lowest magnetic exchange stiffness constant D of 97±2 meV Å² has been found in the cubic austenite phase. In the two martensitic phases the exchange stiffness constants are significantly larger with values of 149±4 meV Å² in the tetragonal phase and 198±7 meV Å² in the low temperature martensite. The large value of D in the low temperature phase compared to the other phases cannot be explained solely by renormalization effects due to magnon-magnon interaction and is attributed to a stronger magnetic coupling. In both the martensitic phases a gap of magnon excitation at the Γ-point of about 0.2 meV was observed.     

X-ray superstructure determination of ordered oxygen and Cu-displacements in a single domain of YBa2Cu3O6.35

The superstructure of ordered oxygen atoms in a single domain of YBa₂Cu₃O_6.35 has been determined by X-ray diffraction. The 45^o rotated superstructure cell is orthorhombic—spacegroup Pbam, lattice constants , witha=3.8652(3) Å,c=11.815(2)Å the dimensions of the fundamental tetragonal cell. The arrangement of the oxygen atoms in the basal plane minimizes their electrostatic repulsion by avoiding near and next-near neighbors. There are displacements of copper atoms of 0.13 Å in the Cu(1) plane and 0.06 Å in the Cu(2) plane. Only 12% of the oxygen atoms in the basal plane contribute to the superstructure.     

Heteroepitaxial thin film of iron phthalocyanine on Ag(1 1 1)

Ordering of submonolayer iron phthalocyanine (FePc) molecules deposited on Ag(1 1 1) was investigated using scanning tunneling microscopy. The room temperature deposition of FePc alone, without any annealing, results in no ordered overlayers. However, posterior annealing the substrate to 475 K leads to the formation of a two-dimensional oblique lattice with the lattice constants of 16.2 ± 0.3 Å and the angle of 78 ± 1° between them. The resulting FePc lattice is commensurate to the substrate lattice. In addition, the nearest neighbor distance in the lattice is significantly increased through a distinctive molecular orientation of the FePc molecules within the unit cell. The commensurate lattice with a large intermolecular distance is in sharp contrast to that observed from a close-packed square lattice that many other metallo-phthalocyanine molecules often self-assemble into. A possible reasoning behind this intriguing structure is discussed.     

Enhanced field emission from Si doped nanocrystalline AlN thin films

Si doped and undoped nanocrystalline aluminum nitride thin films were deposited on various substrates by direct current sputtering technique. X-ray diffraction analysis confirmed the formation of phase pure hexagonal aluminum nitride with a single peak corresponding to (1 0 0) reflection of AlN with lattice constants, a = 0.3114 nm and c = 0.4986 nm. Energy dispersive analysis of X-rays confirmed the presence of Si in the doped AlN films. Atomic force microscopic studies showed that the average particle size of the film prepared at substrate temperature 200 °C was 9.5 nm, but when 5 at.% Si was incorporated the average particle size increased to ∼21 nm. Field emission study indicated that, with increasing Si doping concentration, the emission characteristics have been improved. The turn-on field (E_to) was 15.0 (±0.7) V/μm, 8.0 (±0.4) V/μm and 7.8 (±0.5) V/μm for undoped, 3 at.% and 5 at.% Si doped AlN films respectively and the maximum current density of 0.27 μA/cm² has been observed for 5 at.% Si doped nanocrystalline AlN film. It was also found that the dielectric properties were highly dependent on Si doping.     

La—Ga二元系相图

本文用X射线衍射和差热分析方法对La-Ga二元系相图的富Ga部份进行了研究。发现在La-Ga系相图的富Ga部份存在着一包晶反应,形成一新的金属互化物,其均匀范围很窄。确定了这一新化合物为LaGa₆,属四方晶系,空间群为P4/nbm,点阵常数α=6.1041?,c=7.7052?。另外还发现LaGa₂这一金属互化物存在着一个相区,其均匀范围为66.7A/0 Ga到71.6A/0 Ga。在这一相区内,点阵常数α随Ga含量的递增而增大,c随之减少。论证了这一固溶体系由一对Ga原子无序地替代一个La原子的结果。 关键词：     

Search for superconducting effect on the decay of99m Tc

An experimental investigation of the influence of superconductivity on the decay rate of^99m Tc, _T, has been performed by means of a differential method. The^99m Tc samples containing⁹⁹Tc as a carrier were prepared by electrodeposition on a copper plated tungsten wire. For production of metallic technetium the samples were reduced in pure hydrogen gas at 800~1000°C. X-ray analysis of the samples showed they were surely metallic with anhcp structure. The lattice constants observed area=2.741 Å andc=4.398 Å. The transition temperature was found to be 7.5±0.2K. Comparison of two sources, normal (room temperature) and superconducting (4.2 K), did not show an appreciable effect of superconductivity on the decay rate of^99m Tc exceeding the limit of uncertainty of our experiment:/ _T=(1.1±2.7) × 10^–4.     

Study of the structure and electrical properties of the copper nitride thin films deposited by pulsed laser deposition

Copper nitride thin films were prepared on glass and silicon substrates by ablating a copper target at different pressure of nitrogen. The films were characterized in situ by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and ex situ by X-ray diffraction (XRD). The nitrogen content in the samples, x = [N]/[Cu], changed between 0 and 0.33 for a corresponding variation in nitrogen pressure of 9 × 10⁻² to 1.3 × 10⁻¹ Torr. Using this methodology, it is possible to achieve sub-, over- and stoichiometric films by controlling the nitrogen pressure. The XPS results show that is possible to obtain copper nitride with x = 0.33 (Cu₃N) and x = 0.25 (Cu₄N) when the nitrogen pressure is 1.3 × 10⁻¹ and 5 × 10⁻² Torr, respectively. The lattice constants obtained from XRD results for copper nitride with x = 0.25 is of 3.850 Å and with x = 0.33 have values between 3.810 and 3.830 Å. The electrical properties of the films were studied as a function of the lattice constant. These results show that the electrical resistivity increases when the lattice parameter is decreasing. The electrical resistivity of copper nitride with x = 0.25 was smaller than samples with x = 0.33.     

Field evaporation of impurity ions from a liquid gallium ion source

The charge states of ions emitted from a gallium liquid metal field ion (LMI) source contaminated with tin and copper have been measured. The results for tin show that the proportion of Sn²⁺ to Sn⁺ is much larger than is found for a LMI source of pure tin. A model in which Sn²⁺ is assumed to be produced by post-ionization of Sn⁺ is used to set an upper limit to the electric field at the point of emission. Its value is approximately equal to that predicted by field evaporation theory for a pure gallium source. Consequently the charge states of emitted impurity ions are determined by field strengths imposed by the main component.     

Plasma resonance in granular gallium films deposited on rough NACl and KCl single-crystal surfaces

Granular gallium films deposited in high vacuum on the rough surfaces of NaCl and KCl single crystals maintained at 400°C consist of two layers, in which two resonance bands are excited simultaneously. In isolated grains of the upper layer, a band at the normal electron oscillation frequency ω₀ is excited. As a result of supercooling, the grains are in the liquid state. The gallium plasma frequency calculated from the measured ω₀ and dielectric constants of NaCl and KCl coincides with that obtained by other authors by metallooptic methods. In gallium deposited on room-temperature substrates, only one resonance band is excited, with the interband absorption band superposed on its low-frequency edge.     

Growth, thermal, optical and birefringence studies of semiorganic nonlinear optical thiosemicarbazide cadmium chloride monohydrate (TCCM) single crystals

A semiorganic nonlinear optical material (thiosemicarbazide cadmium chloride monohydrate) (TCCM) was synthesized and single crystals was grown from aqueous solution by slow evaporation method at ambient temperature. Crystal of average size up to 5×4×3 mm³ were harvested and characterized by powder X-ray diffractometry (XRD) and Fourier transfer infrared (FTIR) technique to confirm the identity of the compound formed. The thermal stability was analyzed by TG/DTA. The birefringence values (Δ_n) were determined in the wavelength region 540-6500 Å. The second harmonic generation (SHG) from the materials was confirmed using Nd:YAG laser.     

Synthesis, growth, and characterization of a non-linear optical crystal-glycine lithium chloride

A new non-linear optical material, glycine lithium chloride, was synthesized and single crystals were grown by slow evaporation solution growth technique at constant temperature from its aqueous solution. Transparent and well-crystallized hexagonal prisms were obtained by controlled evaporation at a constant temperature of 45 °C. The grown crystals were characterized by X-ray diffraction methods, Fourier transforms infrared spectroscopy, and optical absorption spectrum. Single crystal X-ray diffraction analysis revealed that the crystal lattice of glycine lithium chloride is hexagonal with unit cell a = b = 7.023 Å, c = 5.478 Å, α = β = 90°, γ = 120°, V = 234 Å³. The dielectric response of the crystal with varying frequencies was studied. The second harmonic generation efficiency of the crystal was studied and is found to be larger than KDP.     

Solid solubility in isolated nanometer-sized alloy particles in the Sn-Pb system

The finite size effect on both the solid solubility and the thermal expansion coefficient in nanometer-sized lead particles was examined by in-situ transmission electron microscopy. The solid solubility of tin in approximately 12-nm-sized particles of lead at room temperature was evaluated be higher than 30 atomic percent, which is almost ten times higher than that in the corresponding bulk lead. The thermal expansion coefficient of lead increased from 29×10^-6 K^-1 for bulk to 38×10^-6 K^-1 when the size of particles decreased from ∞ (bulk) to 16 nanometers. The increment of the thermal expansion coefficient with decreasing size of particles suggests the reduction of the cohesive energy and therefore the reduction of the elastic modulus with decreasing size of particles. It is then considered that the suppression of the strain energy in the solid solution may be responsible for the enhanced solid solubility in nanometer-sized alloy particles.     

The configurational quantum cat map

The first atomically resolved atomic force microscopy images of the LiF (001) surface are presented. A square lattice with a spacing of 2.8±0.1 Å is resolved, which can be attributed to the F^– ions. Li⁺ is not resolved due to its small size. The origin of the image contrast is discussed briefly and the results are compared with those from helium scattering.     

A mild reduction-phosphidation approach to nanocrystalline GaP

Nanocrystalline gallium phosphide (GaP) has been prepared through a reduction-phosphidation by using Ga, PCl₃ as gallium and phosphorus sources and metallic sodium as reductant at 350 °C. The XRD pattern can be indexed as cublic GaP with the lattice constant of a=5.446 Å. The TEM image shows particle-like polycrystals and flake-like single crystals. The PL spectrum exhibits one peak at 330 nm for the as-prepared nanocrystalline GaP.