Structural study of levitated liquid Y2O3 using neutron scattering

The local structure of liquid Y₂O₃ at 2800 K has been investigated by combining aerodynamic levitation and laser heating with the neutron scattering technique. Despite relatively low counting statistics, we have been able to determine the short-range order in the liquid. In particular, we found Y–O and Y–Y coordination numbers around 6 and 12, confirming our previous study by anomalous X-ray scattering. The unusually sharp main diffraction peak observed with X-ray measurements, and characteristic of a high degree of chemical ordering, is strongly reduced in the neutron structure factor and thus indicates that it is mostly due to Y–Y correlations.     

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.     

The role of argon in yttrium aluminates

Spectral properties of YAG and YAP grown in vacuum or under Ar H₂ protective atmosphere were compared. The latter contained small amounts of argon which was detected by a laser mass spectral analysis. Ar impurity evokes brown colour of unannealed crystals and strenghtens anomalous red colour of annealed (Particularly in O₂) YAP crystals doped with Nd + Cr or Nd + Mo. The same red coloration of the doped YAGs is less stable.     

Microscopic collective dynamics in liquid tellurium

The spectra of collective excitations of liquid tellurium have been studied by means of inelastic neutron scattering. Here we report on the dynamics of liquid Te as measured at two different temperatures, just above melting (T_m = 723 K) and at ∼1000 K. Estimates for the velocity of propagating excitations for both temperatures have been obtained from the experimental data and a contrasting behavior is found with respect to anomalies shown by the adiabatic sound velocity measured by ultrasound methods.     

Simulation of electrooptical effects and dynamics of ferroelectric liquid crystals

The behavior of ferroelectric liquid crystals in an external electric field is simulated numerically. The equations that describe the dynamics of the director of a liquid crystal are derived within the continuum theory of elasticity with due regard for compressibility of smectic layers, finite anchoring energy, and dielectric properties of orienting coatings and external elements of a real electric circuit. These equations make the basis for simulation of the electrooptics of ferroelectric liquid crystals. The specific features and mechanisms of the surface-stabilized bistability and hysteresis-free electrooptical switching (the V-shape effect) are discussed.     

High frequency collective dynamics in liquid potassium

We investigated by inelastic X-ray scattering the dynamical properties of molten potassium in a wide range of momentum transfer, Q, from 1 nm⁻¹ up to the main peak of the structure factor Q ≈ 17 nm⁻¹. The observed increase of sound velocity in the low Q region (1 < Q < 3 nm⁻¹), has been described within a model characterized by two distinct relaxation processes for the collective dynamics. The obtained results are discussed and compared with those from previous neutron scattering experiments. In particular, we associate the speed-up of the sound velocity to the ‘instantaneous’ disorder of the liquid as opposed to the argument, supported by some neutron scattering studies, of a transition from a liquid to solid like response of the system.     

The temperature dependent collective dynamics of liquid Rubidium

Liquids alkali metals show near the melting point at small momentum transfer values an upward bending of the dispersion relation. This so-called positive dispersion can be described within generalised hydrodynamics as a visco-elastic reaction of the liquid. There is a speculation that long-living clusters could be the physical reason behind this phenomenon. To shed light on this question a coherent inelastic neutron scattering experiment on liquid Rubidium was performed at four temperatures starting at the melting point. Distinct deviations from molecular dynamics simulation results occur at the lowest accessible momentum transfer values. With rising temperature the excitation frequencies soften at a momentum transfer value which is correlated with the dimensions of the supposed clusters.     

Structure and dynamics of liquid Al1−xSix alloys by ab initio molecular dynamics simulations

First-principles molecular dynamics (MD) simulations are performed to study the structure and dynamics of liquid Al_1−xSi_x (x = 0.0, 0.12, 0.2, 0.4, 0.6, 0.8) at the temperature of 1573 K. The composition dependence of static structure factors, pair correlation functions, and diffusion constants are investigated. We found that the structure of the liquid Al_1−xSi_x alloys is strongly dependent on the composition. From our simulation and analysis, we can see that although liquid Al_1−xSi_x is metallic, there are some degrees of covalent tetrahedral short-range order in the liquid. The degree of tetrahedral short-range order increases linearly as the Si concentration in the liquid increased. The diffusion coefficients of both Al and Si atoms in liquid Al_1−xSi_x alloys at 1573 K are not very sensitive to the composition.     

Structure of fluorochlorozirconate glasses using molecular dynamics

The structure of fluorochlorozirconate glass with composition Zr_0.55Ba_0.20La_0.05Na_0.20F_2.95−xCl_x with 0 ? x ? 0.9, i.e., up to ∼30% chlorine anions, has been simulated using molecular dynamics methods applied to a unit cell comprising 790 ions. The structure comprises a network of (mainly) corner and edge-linked ZrF_8−pCl_p, ZrF_7−qCl_q and LaF_7−rCl_r polyhedra with sodium, barium and additional chlorine ions occupying irregular positions in space between the polyhedra. For chlorine concentrations above about 15%, the number of chlorine ions occupying corner bridging positions increases markedly which is expected to reduce the glass temperature. For chlorine concentrations greater than about 20%, the spatial distribution of ions is not uniform and clustering into zirconium rich and sodium, barium and chlorine rich regions occurs. The possibility that this phenomenon may be a precursor to the observed precipitation of barium chloride crystals just above the glass temperature is discussed.     

Structure formation in liquid and amorphous metallic alloys

Bulk metallic glasses developed in last 15 years represent a new class of amorphous metallic alloys. These multi-component metallic alloys can be obtained at relatively low cooling rates, which allow the production of large-scale materials by conventional casting processes. Furthermore, bulk metallic glasses show a glass transition well below the crystallization temperature enabling hot deformation, but also to investigate the glass transition phenomenon in a metallic system. The thermal behavior of Zr- and Pd-based bulk metallic glasses was studied by in situ X-ray diffraction at elevated temperatures. The temperature dependence of the X-ray structure factor of the glassy state can be well described by the Debye theory. At the caloric glass transition the temperature dependence of the structure alters, pointing to a continuous development of structural changes in the liquid state. The short-range order of the glass, of the super-cooled liquid, and of the equilibrium melt is found to be very similar. The existence of complex chemically ordered clusters in the melt is supposed to be related to the high glass-forming ability of the alloys. The microstructure of metallic glasses consisting of elements with negative enthalpy of mixing is homogeneous at dimensions above 1 nm. Phase separation in the liquid state appears in metallic systems with large positive enthalpy of mixing of the elements like Nb–Y. Thermodynamic calculations of the Ni–Nb–Y phase diagram show that the miscibility gap of the monotectic binary Nb–Y system extends into the ternary up to large Ni content. Experimental evidence of the phase separation in ternary Ni–Nb–Y melts is obtained by in situ X-ray diffraction at elevated temperatures and differential scanning calorimetry. The phase separated melt can be frozen into a two-phase amorphous metallic alloy by rapid quenching from the liquid. The microstructure depends on the chemical composition and consists of two amorphous regions, one Nb-enriched and the other Y-enriched, with a size distribution from several nanometers up to micrometer dimension. The experimental results confirm the close relationship between the structure of metallic glasses and the corresponding under-cooled liquids.     

Molecular dynamics simulation of viscosity in supercooled liquid and glassy AgCu alloy

The viscosity of the model AgCu alloy is simulated by several methods using (i) correlation functions through the Green–Kubo formalism, (ii) a non-equilibrium molecular dynamics approach and (iii) creep tests under constant stress. Temperature dependences of the shear viscosity and the diffusion coefficient show the breakdown of the Stokes–Einstein relation well above the glass-transition temperature T_g. This observation is interpreted as a manifestation of the development of heterogeneities in the supercooled liquid approaching T_g. Based on a generalized Einstein formula for the viscosity of liquid, a temperature dependence of heterogeneity degree of supercooled liquid is estimated. Using the dependence of the deformation rate on external stress, the activation volume is evaluated to be four atomic volumes in liquid state. However, below the mode-coupling temperature T_c the activation volume increases by several times.     

Atomic dynamics in liquid KxSb1−x alloys

The atomic dynamics of liquid K_xSb_1−x alloys are studied using cold neutron inelastic scattering. Dynamical properties extracted from the experimental spectra, like the dispersions obtained from the maxima of the longitudinal current correlation function, J_l(Q, ω), and the vibrational spectra, show a strong dependence on concentration. With increasing content of Sb, higher frequency modes are observed in the vibrational spectrum. For the Sb-rich compositions the dispersion is split into two branches. This indicates the de-coupling of the dynamics of the two components, due to the difference in atomic mass, already around the position of the principal peak of the static structure factor, S(Q). The observations are consistent with results from earlier experiments on similar systems and from ab initio molecular dynamics simulations.     

Structure of liquid gold from tight-binding driven molecular-dynamics

A tight-binding (TB) driven molecular-dynamics (MD) simulation of liquid gold in a periodic supercell was performed using our recently developed computer code. The structure of the obtained liquid is discussed and compared with the available empirical data and results of other simulations, in terms of pair correlation functions, angular distribution functions and electronic density of states.     

Physico-chemical processes in single crystal growing of calcium aluminates by zone melting

The growing of single crystals of calcium aluminates of compositions 12 CaO · 7 Al₂O₂, CaO · Al₂O₃, and CaO · 2 Al₂O₃ by zone melting under vacuum of 10⁻⁵ mm Hg permitted to establish that some of the Al³⁺ ions in octahedral coordination have been driven back by the moving crystallization front. Energetically, this process can be represented on the basis of the viscous flow model with the activation energy of −45 kcal/mole. The possible mathematical models have been considered for the processes of preparation of single phase crystals of the aluminates mentioned above which take account of incongruent vaporization of the component oxides and refining of the melt from structural impurities by the moving crystallization front.     

Structure and dynamics of non-crystalline materials at the Institut Laue Langevin

The determination of the structural and dynamical properties of non-crystalline materials is usually a difficult task because any scientific insight must be achieved without the mathematical amenities useful in the crystalline state. Special experimental techniques need to be employed and among them, neutron scattering is one of the most powerful. The Institut Laue Langevin (ILL) operates a nuclear reactor, which is the finest steady-state source of neutrons in the world. In this work we introduce a large suite of instruments available at the ILL for structural and dynamical studies on non-crystalline solids or, more generally, amorphous systems, including liquids.     

Molecular dynamics simulation of the crystallization of a liquid gold nanoparticle

Molecular dynamics simulation of the crystallization behavior of a liquid gold (Au) nanoparticle, about 4 nm in diameter, on cooling has been carried out based on the modified embedded-atom-method potential. With decreasing cooling rate, the final structure of the particle changes from amorphous to crystalline via icosahedron-like structure. While the outer shell of the icosahedron-like particle shows crystalline feature with {1 1 1}-like facets, the inner core remains amorphous. It is found that the structure of the fully crystallized particle is polycrystalline face-centered cubic (fcc). The fcc structure of the gold nanoparticle is proved energetically the most stable form.     

Structure and thermal fluctuations in thin films of smectic liquid crystals

Thermal fluctuations of the smectic layers in freely suspended thin liquid crystalline films have been studied by the methods of X-ray diffraction and reflectometry. The fluctuation dynamics is studied by the methods of photon correlation spectroscopy with the use of a source of coherent synchrotron radiation. In freely suspended smectic films, the typical relaxation times are of the order of several microseconds. In thin liquid crystalline films, the simultaneous damped and oscillatory behavior was observed for the layer undulations.     

Slow dynamics of liquid Se studied by Infrared Photon Correlation Spectroscopy

We report on the study of the slow dynamics of supercooled and molten Selenium by means of dynamic light scattering. To achieve this in the present case, where the opaqueness of Se prevents visible light scattering measurements, we have developed a novel experimental setup using infrared radiation as the light source. By measuring the scattered intensity autocorrelation function we have been able to extract the average size of the Se clusters in the liquid state and determine the temperature dependence of the related diffusion coefficient which was found to exhibit Arrhenius behavior.