High frequency dynamics of liquid bismuth

The dynamic structure factor S(Q, ω) of liquid Bi was measured at 580 K in the Q range from 0.15 to 0.6 Å⁻¹ using inelastic neutron scattering. The obtained spectra clearly demonstrate the existence of well defined longitudinal propagating modes. A positive dispersion is found in the low Q region, where the mode velocity undergoes a transition between the hydrodynamic value and a high frequency value 20% larger. The damping of the excitations does not follow the hydrodynamic Q² trend and is stronger than in any metallic liquid investigated so far. The quasielastic lineshape contains a broad Q-independent Lorentzian contribution, other than a small sharp peak, which has yet been observed in liquid Hg and Ga.     

About diffusion mechanism in silica liquid under pressure

We perform a molecular dynamic simulation to study the diffusion mechanism in silica liquid under pressure up to 25 GPa and at temperature of 3000 K. We find that total O―Si―O angle distribution can be expressed by a simple relation between partial O―Si―O angle distribution and fractions of units SiO_x. Specifically, we demonstrate that these liquids consist of identical units SiO₄, SiO₅ and SiO₆ and have common partial O―Si―O angle distribution. We also show that each particle undergoes a series of stages where the particle locates in unchanged unit SiO_x, x = 3, 4, … 7 or OSi_y, y = 1, 2, 3, 4. The diffusivity strongly depends on the rate of transitions Si^ξ → Si^ξ ± 1 and O^ζ → O^ζ ± 1 which is significantly different between low- and high-pressure samples. For low-pressure sample the transitions Si⁴ → Si⁵, Si⁵ → Si⁴, O² → O³ and O³ → O² are dominant, meanwhile for high-pressure sample there are transitions Si^ξ → Si^ξ ± 1 with ξ = 4, 5, 6 and O^ζ → O^ζ ± 1 with ζ = 2, 3, 4. This finding may be common for diffusion in all network-forming liquids. The simulation also reveals the spatially heterogeneous dynamics in low-pressure liquid where a large cluster of immobile particle exists for the time that a number of particles move over several inter-particle distances.     

The structure of liquid Sb---Se alloys

The structure of liquid Sb_1-xSe_x alloys with x = 0.5, 0.6 and 0.7 was investigated in the temperature range of 600–800°C. The neutron diffraction measurements were carried out by using the high intensity total (HIT) scattering spectrometer of the booster synchrotron utilization facility (BSF) at the National Laboratory for High Energy Physics. A prepeak is observed in the structure factor for these three alloys. It suggests that the medium-range order stillo remains melting for liquid Sb_1-xSe_x alloys, with x = 0.5, 0.6 and 0.7. The structure factor liquid Sb₂Se₃ alloys shows no appreciable temperature variation. For liquid Sb_0.5Se_0.5 alloy, the intensity of the prepeak exhibits an apparent decrease in the temperature range in which the non-metal-metal transition occurs. The non-metal-metal transition in liquid Sb---Se alloys is due to the change in the medium range structure.     

The structure of liquid copper telluride

Data are presented for the total neutron scattering patterns for liquid CuTe. The technique of isotopic enrichment was used and the scattering power of the copper was changed by preparing samples out of ⁶³Cu, ⁶⁵Cu and natural Cu. The data are discussed in terms of possible structural models.     

Study of the structure of PyHReO4 under high pressure

The structure of deuterated pyridinium perrhenate (d₅PyH)ReO₄ (C₅D₅NHReO₄) is studied by X-ray diffraction at room temperature and pressures up to 3.5 GPa and by neutron diffraction in the temperature range 10–293 K and at pressures up to 2.0 GPa. Under normal conditions, this compound belongs to the orthorhombic space group Cmc2₁ (ferroelectric phase II). At room temperature and pressures above P > 0.7 GPa, a transition to an orthorhombic phase (paraelectric phase II) is observed. This paraelectric phase is described by the space group Cmcm. At a pressure as high as P = 2.0 GPa, phase I remains stable at temperatures down to 10 K. This fact indicates that the high pressure suppresses the ferroelectric state in deuterated pyridinium perrhenate (d₅PyH)ReO₄.     

Precise sound velocity measurement for liquid Se50Te50 under high pressure

We have measured the sound velocity in liquid Se₅₀Te₅₀ at 19.5, 32.6, 45.6, and 58.2 MHz simultaneously by means of an ultrasonic pulse transmission/echo method. By using a phase-sensitive-detection technique the relative error was reduced to less than 0.1%. The temperature dependence of sound velocity at 100 MPa exhibits a minimum at 600 ^oC and a maximum at 1035 ^oC, which are related to the large structural change accompanied by the semiconductor–metal (S–M) transition. In addition, sound dispersion has been observed at temperatures from 500 °C to 900 °C for the first time, where anomalous sound attenuation was previously reported. This result implies that a structural relaxation on nanosecond timescale takes place in the S–M transition region.     

Low-temperature luminescence and phase structure of 5CB liquid crystal under different cooling regimes

Abstract

The effects of the 5CB liquid crystal (LC) cooling regimes on its luminescence spectra at the low temperature (T?=?4.2 K) have been found out and investigated. Experiments have been carried out at two cooling rates: 2 and 150?deg/min. Differential scanning calorimetry (DSC) data allow us to establish that the 5CB glass-like state (GLS) is formed during the rapid cooling of the sample. Peculiarities of the LC dimer formation under the different cooling conditions are discussed.     

Surface analysis of different oriented GaAs substrates annealed under bismuth flow

Several orientations of GaAs substrates, including (1 0 0), (4 1 1), (1 1 1) and (5 1 1) have been annealed in a metalorganic vapour phase epitaxy (MOVPE) horizontal reactor at different annealing temperatures and under different trimethyl-bismuth (TMBi) flux. Surface morphology of the annealed GaAs substrates was investigated by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results show islands formation on all the studied samples. The density and size of Bi islands vary greatly with annealing temperature and TMBi flow. For different substrate orientations, the activation energies were deduced from Arrhenius plot of island density. Except for (5 1 1) oriented GaAs, all the studied orientations show the same activation energy of 1.8 eV. For low annealing temperature 420 °C, and under different Bi flux, each oriented substrate shows a specific behaviour. For higher temperatures 700 °C and above Bi islands are totally removed and the substrates are smooth. Surface change of (1 0 0) oriented GaAs substrate was in situ monitored by laser reflectometry.     

The structure of liquid lead: EXAFS and MD studies

Accurate L-edge EXAFS (Extended X-ray Absorption Fine Structure) measurements of liquid Pb have been collected and analyzed using an advanced data-analysis technique (GNXAS) based on exact spherical wave multiple-scattering calculation of the absorption cross-section. Structural models obtained using molecular-dynamics (MD) simulations with an empirical and an embedded-atom potential, as well as structural data on liquid lead obtained from neutron and X-ray diffraction by other authors, have been put to a test using our new experimental results. The empirical two body potential, originally designed for the liquid state, reproduces quite accurately our experimental results. At the same time, the embedded atom potential, originally designed for the solid state, has turned out to inappropriately reproduce the first-neighbor g(r) peak. A comparison with previous XD and ND studies has shown that EXAFS analysis can improve our knowledge of the first-neighbor distribution. Previous observations of an apparent reduction in the coordination number in liquid Pb detected by EXAFS are briefly discussed.     

Chain structure of liquid Se at high temperature and pressure investigated by ab initio molecular dynamics simulations

Using ab initio molecular dynamics simulations, the local atomic structural, dynamic and electronic properties of liquid selenium were studied under different temperatures and pressures. Compared with experimental data, the calculated structure factors and viscosities are acceptable on the whole. Our results indicate that the chain structure of crystalline selenium still exists in liquid state even at high temperature and pressure. The fraction of twofold-coordinated atoms decreases obviously under high pressure while it remains nearly invariable with the increase of temperature. The Peierls-type distorted structures in trigonal Se still reserve in liquid state even under high temperature and pressure. The calculated DOS displays an obvious dip at E_F, and the dip becomes shallower with rising temperature.     

Crystallographic structure and defects in epitaxial bismuth films grown on mica

Atomic force microscopy is used to image the surface relief of epitaxial Bi films grown on mica substrates. Monatomic terrace steps are readily resolved, which provide detailed information on crystal growth and defect structures. The films grow from the coalescence of isolated three-dimensional islands, however, each island grows layer-by-layer. The islands are triangular in shape and when they coalesce to form a continuous film, their structures are either in-phase, and consequently large crystallites are formed, or they are out-of-phase and defect structures are formed. The most common defects observed are twinning and stacking faults. Images showing the microscopic details of stacking faults, twinning, and grain boundaries are observed as a result of islands coalescing to form a continuous film.     

The synthesis of aligned silicon nanowires under ambient atmospheric pressure

Highly ordered amorphous silicon nanowires were successfully synthesized from single crystalline silicon wafer at the pyrolysis temperature of 1050 °C under ambient atmospheric pressure. Both poly (phenylcarbyne) and nickel nitrate played important roles in the growth of silicon nanowires. The fabrication of ordered silicon nanowires was controllable and repeatable, confirmed by the experimental results. The morphology and microstructure analysis of the as-obtained samples showed the highly ordered amorphous silicon nanowires were obtained, determined by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and FT-infrared spectroscopy. A solid-liquid-solid growing process was proposed.     

Crystal structure of anhydrous bismuth iodate, Bi(IO3)3

Bismuth iodate was synthesized by precipitation from bismuth nitrate and iodic acid. The powder was dissolved in dilute nitric acid and single crystals were obtained by slow evaporation at 70°C. Bi(IO₃)₃ crystallizes in the monoclinic system, space group P2₁/n (N° 14), a = 8.8882(2) Å, b = 5.9445(2) Å, c = 15.2445(5) Å, = 97.064(1) Å. BiO9 polyhedra are edge-connected forming chains parallel to the b axis; these chains are linked through (IO₃) groups forming layers parallel to (101). The 3D framework is designed by linking these layers through long I–O bonds. DSC analysis shows a remarkable thermal stability.     

Isothermal liquid phase epitaxy of A 3 B 5 compounds under excessive vapor pressure of the B 5 component

A mathematical model has been developed of isothermal epitaxial growth of A³B⁵ films on the A³B⁵ substrates from flux by creating the excessive pressure of diatomic vapors of the B ⁵ component. The model is based on the diffusion equation with the nonlinear boundary condition at the evaporation surface and the thirdkind condition at the crystallization surface. The problem is solved numerically on a computer.     

The structure and effective pair potential of liquid manganese near the melting point

A simple-metal theory can describe, in some favorable cases, the structures of liquid 3d transition metals when the pseudopotential parameters are chosen appropriately. This is shown, using the Percus-Yevick integral equation, for liquid manganese which has a half-filled d band and is claimed to be difficult to handle theoretically. The Ashcroft empty-core form for the s-electron pseudopotential is employed. The value of core radius is determined from the density of liquid manganese at 1533 K and the effective valence, Z_s, is determined by the value of the plasma parameter with which the liquid structures and thermodynamic properties are well described by the one-component plasma (OCP) model. The validity of the value of Z_s is supported by the Gibbs-Bogoliubov method using the OCP reference system.     

Influence of europium ions on structure and crystallization properties of bismuth borate glasses and glass ceramics

Glasses of the xEu₂O₃ · (100?x)[2Bi₂O₃ · B₂O₃] system with 0 ? x ? 25 mol% have been characterized by X-ray diffraction and FTIR spectroscopy measurements. Melting at 1100 °C and the rapid cooling at room temperature permitted us to obtain glass samples. In order to improve the local order and to develop crystalline phases, the glass samples were kept at 625 °C for 24 h. After heat treatment two crystalline phases were put into evidence. One of the crystalline phases was observed for the host glass matrix, the x = 0 mol% sample, and belongs to the cubic system. The second one was observed for the x = 25 mol% sample and was find to be orthorhombic with two unit cell parameters very close to each other. For the samples with 0 < x < 25 mol% there is a mixture of the two mentioned phases. FTIR spectroscopy data suggest that both Bi₂O₃ and B₂O₃ play the glass network former role while the europium ions play the network modifier role in the studied glasses.     

Properties of some perovskites synthesized under pressure

Interstitial solid solutions with the perovskite – like structure were synthesized by solid phase reactions at high pressure and temperature. The synthesis conditions of single phased samples, their stability on heating in air were determined. The magnetic and electrical properties were measured.     

The influence of indium on the growth of GaN from solution under high pressure

The influence of significant fraction (10–50 mole%) indium in liquid gallium on GaN crystallization from a ternary Ga–In–N solution was analyzed. Crystallization experiments of GaN on GaN-sapphire templates from Ga–In solutions, at 1350–1450 °C, with prior to the growth seed wetting at 1500 °C, and 1.0 GPa N₂ pressure, without solid GaN source showed faster growth of GaN on the seed (by a factor of 1.5–2) than using pure gallium solvent. Nevertheless the new grown crystals were morphologically unstable. The instability was reduced by decrease of the wetting temperature down to 1100 °C or by omitting the wetting procedure entirely, which indicated that GaN dissolves much faster in Ga–In melt than in pure Ga and that the unstable growth was caused most likely by complete dissolution of GaN template before the growth. It was observed that the crystals grown on bulk GaN substrates did not show morphological instability observed for GaN-sapphire templates. The influence of indium on thermodynamic and thermal properties of the investigated system is discussed.