晶体生长时的固液相结构变化分析

晶体的溶解、熔化以及结晶成核生长时的固液相原子结构是怎样变化的,晶体生长时的生长基元是原子还是原子团.本文根据各种材料液相结构的最新研究结果,提出不饱和配位结构转换模式,并以此模式对各种常见晶体材料从溶解、熔化到结晶生长时的液态母相结构变化以及晶体成核过程进行了描述和分析,认为晶体生长时的界面结构和液相结构十分接近,溶解、熔化主要是晶体表面的不饱和配位原子(离子)转换到液相结构的过程,晶体生长主要是液相中的不饱和配位原子(离子)转换到固液生长界面的位错位置,使配位结构更饱和的过程.随着液相过饱和度的增大,液相结构单元的原子数越来越多,吸附到晶体生长界面若来不及转换回液相,就形成新的位错生长中心,形成晶体生长缺陷.     

Electronic structure and properties of liquid caesium up to critical point by LMTO calculations

It is well-known that liquid caesium shows some unusual properties at low densities. We used supercell technique within linear muffin-tin orbital method to investigate this phenomenon. Electronic structure of liquid caesium for different temperatures from the melting point up to critical point was obtained. The atomic structure was simulated for a cluster of 2000 atoms by the Reatto method on the base of experimental structure factors of Cs obtained by Winter and co-workers for different temperatures (from 323 K up to 1923 K). The Kubo–Greenwood formula was applied for the calculations of melts conductivity. The received results indicate that metal–nonmetal transition in liquid cesium is connected not to the gap at Fermi energy in density of electronic states, but more likely with electrons localization on some kind of atomic clusters.     

Functionalisation of cholesteric liquid crystals by direct laser writing

Cholesteric liquid crystals selectively reflect circularly polarised light with the same handedness as the helix. Because of their sensitivity to external stimuli, such as heat and electrical fields, various applications utilising their tunability have been proposed. Tuning is usually performed in the bulk, meaning that cholesteric liquid crystals usually possess a single pitch throughout the medium. However, when the helical structure is locally modulated, different optical properties arise, such as tunable photonic defect modes and multiple reflection bands. Here we show a technique to locally modulate the helical structure of cholesteric liquid crystals on a submicron scale, based on two-photon excitation direct laser lithography. Two examples of cholesteric liquid crystal structures with modulated helical structures will be presented.     

Local structure of liquid Rb–Se mixtures near the miscibility gap

In the liquid Rb–Se mixture there exists a liquid–liquid phase separation (miscibility gap) in the Se-rich concentration region. In order to study how the local structure is associated with the occurrence of the miscibility gap, EXAFS and neutron diffraction measurements for liquid Se, Rb₁₀Se₉₀, Rb₁₅Se₈₅ and Rb₂₀Se₈₀ mixtures have been carried out. The results reveal that the interaction between Rb and Se atoms is very weak and the liquid mixtures are composed of short Se chain-molecules of which chain ends are terminated by Rb atoms. We have carried out detailed structural modeling studies using reverse Monte Carlo simulation. It is found that the chain-like or ring-like Se segments are separated by voids. The Rb atoms adsorbed to Se chain ends segregate in the void and the shortened Se chain-molecules link up with each other when one approaches the miscibility gap.     

The radial distribution function and structure factor of liquid and amorphous gallium as described by the quasi-crystalline model

The far RDF oscillations and the main peak of the structure factor for liquid and amorphous gallium are described in terms of the quasi-crystalline model representing spatial correlations in liquids on the basis of a smeared crystal lattice. The initial lattice is a modified BCC lattice for liquid gallium and β-Ga structure for the amorphous state. The liquid gallium structure is inferred to be closely packed atoms complicated by their covalent coupling. Amorphous gallium has a microcrystalline structure with islets formed in a similar way as for β-Ga crystals.     

The influence of doping with tin on the structure of Cu0.70Si0.30 eutectic melt

The structural changes in liquid Cu_0.70Si_0.30 eutectic melt upon doping with tin (5, 10, 15 and 20 at.%) have been studied by means of the X-ray diffraction method and the obtained structure factors and pair correlation functions have been analyzed. Experimental total structure factors have also been used to calculate partial factors by means of the reverse Monte-Carlo method. It has been shown that Sn atoms exhibit a tendency to form clusters in the matrix of the liquid eutectic melt.     

Supercooled Lennard-Jones liquids and glasses: a kinetic Monte Carlo approach

A kinetic Monte Carlo (KMC) method is used to study the structural properties and dynamics of a supercooled binary Lennard-Jones liquid around the glass transition temperature. This technique permits us to explore the potential energy surface without suffering an exponential slowing down at low temperature. We find a transition temperature that separates two distinct regimes around the dynamical transition temperature of mode-coupling theory. Below this temperature the number of different local minima visited by the system for the same number of KMC steps decreases by more than an order of magnitude. The mean number of atoms involved in each jump between local minima and the average distance they move also decreases significantly, and new features appear in the partial structure factor. At higher temperature the probability distribution for the magnitude of the atomic displacement per KMC step exhibits an exponential decay, which is only weakly temperature dependent.     

Temperature-dependent structural changes in liquid Ge15Te85

The structure of liquid Ge₁₅Te₈₅ has been studied with neutron diffraction in the liquid state up to 740 °C and in the supercooled liquid state down to 345 °C. The temperature dependences of the diffraction patterns are analyzed. It is shown that the liquid Ge₁₅Te₈₅ can be described by the model of heterogeneous structure, which assumes that the melt consists of atoms joined in clusters and a proportion of atoms with higher mobility that fill the space in between clusters. The number and the size of clusters decrease while the volume fraction of ‘free’ atoms increases under heating.     

X-ray diffraction study on amorphous gallium

Amorphous films of some μm in thickness, prepared by low temperature condensation in an ultra-high vacuum onto liquid helium cooled substrates, have been studied in situ by using an X-ray diffractometer operating in a symmetrical reflection mode. The structure factor of gallium has been obtained over the wavevector range 1.3 to 16.1 Å⁻ by means of two wavelengths, Cr_K and Mo_K, monochromatized by balanced filters. The average number of nearest neighbours deduced from the well-resolved first maximum in the radial distribution function is equal to 9.3 atoms. The results are compared to those previously found by electron diffraction measurements on thin films and also to the structure of supercooled liquid.     

The neutron diffraction study of liquid GeTe and As2Te3

The radial distribution analyses for GeTe and As₂Te₃ are made at temperatures above the melting point in the range of momentum transfer between 0.7 and 10.0 Å^?1 by the neutron diffraction technique. Furthermore, the local order in amorphous GeTe is determined by analyzing the intensity data of the electron diffraction of its thin film. The result for the amorphous film indicates that the local distribution of atoms in amorphous GeTe is not characteristic of the structure of its crystalline state. The shape of the peaks of the intensity curve for liquid GeTe differs from that for the amorphous and crystalline states. However, the short bond length and the small coordination number determined from liquid RDF suggest that the covalent-like bonding between nearest-neighbor atoms remains unbroken when melting. The general form of the structure factor for liquid As₂Te₃ is similar to that for the amorphous material reported previously. The position of the first peak of RDF in the liquid state is observed to be shifted to a shorter distance than the average of nearest-neighbor atoms in crystalline As₂Te₃. The structure of GeTe differs considerably between the crystalline, amorphous and liquid states, whereas the local order in the liquid As₂Te₃ is similar to that in the amorphous state but not in the crystalline state.     

Two-order-parameter model of the liquid–glass transition. I. Relation between glass transition and crystallization

Recently we proposed a two-order-parameter model of liquid to understand the liquid–glass transition, liquid–liquid transition, and anomalies of water-type liquids in a unified manner. Here we discuss the relation between our model and previous models of the liquid–glass transition, focusing on the difference in the basic standpoint among the models. We argue that (i) actual liquids universally have a tendency of spontaneous formation of locally favored structures and (ii) liquid–glass transition is controlled by the competition between long-range density ordering toward crystallization and short-range bond ordering toward the formation of locally favored structures due to the incompatibility in their symmetry. Thus, we regard vitrification as phenomena that are intrinsically related to crystallization. Previous models, on the other hand, regard vitrification as a result of (a) an homogeneous increase in the density and the resulting cooperativity in molecular motion or (b) the frustration intrinsic to a liquid state itself. Accordingly, they presuppose the kinetic avoidance of crystallization and thus do not put focus on ‘crystallization’. This leads to the essential difference in the physical picture between our model and previous models. By comparing models, we aim at gaining further insight into what is the physical origin of the liquid–glass transition and its possible connection with crystallization.     

Effect of temperature on the structure of liquid In20Sn80

X-ray diffraction, density and dynamic viscosity measurements on liquid In₂₀Sn₈₀ have been carried out between 300 and 800 °C. An effect of temperature on the atomic structure of molten In₂₀Sn₈₀ is discussed in terms of the model of inhomogeneous structure. It is shown that the structure of liquid In₂₀Sn₈₀ can be presented by atomic clusters with stable composition and atomic packing and a portion of free atoms with higher mobility. The number and the size of clusters decrease and the volume fraction of free atoms increases under heating.     

Stability of amorphous structures with voids

We consider the stability of an amorphous solid state allowing the presence of holes in the structure. We use the density functional method for computing the free energy of the inhomogeneous metastable liquid. For a one component Lennard-Jones liquid we consider here the nature of the amorphous structure which are thermodynamically more favourable compared to the homogeneous liquid state. We explore here the nature of the heterogeneous state described by overlapping density profiles as compared to the strongly localized crystalline structure.     

A tiling approach to counting inherent structures in hard potential systems

The number of distinguishable inherent structures of a liquid is the key component to understanding the thermodynamics of glass formers. In the case of hard potential systems such as hard discs, spheres and ellipsoids, an inherent structure corresponds to a collectively jammed configuration. This work develops a tiling based approach to counting inherent structures that constructs packings by combining sets of elementary locally jammed structures but eliminates those final packings that either, do not tile space, or are not collectively jammed, through the use of tile incompatibility rules. The resulting theory contains a number of geometric quantities, such as the number of growth sites on a tile and the number of tile compatibilities that provide insight into the number of inherent structures in certain limits. We also show that these geometric quantities become quite simple in a system of highly confined hard discs.     

EXAFS spectroscopy of quasicrystals

The results of the investigation of the features of the local structure of quasicrystalline materials by extended X-ray absorption fine structure (EXAFS) spectroscopy with the use of synchrotron radiation are analyzed. The advantages of this method from the point of view of deriving information about the local shifts of the atoms forming an icosahedral structure are demonstrated. The rearrangement of the local environment of copper and iron in Al-Fe-Cu ternary alloys at a transition from the crystalline to the quasicrystalline phase has been investigated. It is established that the nearest copper coordination retains the symmetry characteristic of the crystal; however, rotation and small displacements of copper matrix atoms lead to significant rearrangement of aluminum atoms around iron atoms. As a result, icosahedral clusters with pentagonal symmetry are formed around iron atoms and violation of the translational symmetry is accompanied by the transition of Al-Fe-Cu to the quasicrystalline state.     

X-ray diffraction measurements for liquid Ge–Si alloys using synchrotron radiation

Energy dispersive X-ray diffraction measurements have been carried out for liquid Ge_1−xSi_x alloys (x = 0.0, 0.3, 0.5, 1.0) using synchrotron radiation at SPring-8. We measured the X-ray diffraction spectra of liquid Ge and Si up to a high temperature range, (liquid Ge from 1270 to1870 K and liquid Si from 1680 to 2020 K), liquid Ge_0.7Si_0.3 at 1620 K, and liquid Ge_0.5Si_0.5 at 1540, 1590, 1670 and 1720 K. The total structure factors of the liquid Ge–Si alloys have a characteristic shoulder on the high-wave-vector side of the first peak. We deduced a pair distribution function from the Fourier transform of the observed structure factor, which was weakly dependent on the temperature. The nearest-neighbor coordination number of liquid Ge–Si alloys is close to that of pure liquid Ge and Si. The first peak of the pair distribution function moved to a shorter distance with increasing Si concentration. These results may indicate that the atomic radii of the Si and Ge atoms in the pure liquid are preserved in the liquid alloys.     

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.     

Formation and description of nano-clusters formed during rapid solidification processes in liquid metals

In order to understand the formation mechanism of nano-clusters during rapid solidification process in liquid metals, a molecular dynamics (MD) simulation study for a large-scale system consisting of 400 000 atoms of liquid metal Al has been performed. To describe the clusters, especially the nano-clusters formed in the system during rapid solidification processes, a cluster-type index method (CTIM) has been proposed. It is demonstrated that the nano-clusters (such as containing 150 atoms) are formed by combining some middle clusters and they sharply differ from those obtained by gaseous deposition, ionic spray methods and so on. Though the nano-clusters have different shapes, they all have the corners that can be starting points of dendrite growth in solidification processes in liquid metals.     

The liquid state of large clusters with pairwise atomic interactions

Formation of the liquid state for clusters with a pair interaction between atoms is examined within the framework of the void model, in which configurational excitation of atoms results from formation of voids. Void parameters are found from computer simulation by molecular dynamics methods for Lennard-Jones clusters and from real thermodynamic parameters of the liquid states of condensed inert gases. Phase transitions are analyzed in terms of two aggregate states. This information allows us to divide the entropy jump during the solid-liquid phase transitions in two parts, so that one corresponds to configuration excitation at zero temperature, and the other is a contribution from thermal vibrations of atoms. The latter part contributes from approximately 40% for Lennard-Jones clusters consisting of 13 and 55 atoms up to 56% for bulk inert gases. These magnitudes explain the validity of melting criteria based on thermal motion of atoms, even though this phase transition results from configurational excitation of ensembles of bound atoms. It is shown that the void concept allows us to analyze various aspects of the liquid state of clusters including the existence of the freezing temperature below which no metastable liquid state exists, and the properties of glassy states which may exist below the freezing point.     

The structure of liquid As2Se3 and GeSe2 by neutron diffraction

The liquid structures of As₂Se₃ and GeSe₂ have been investigated using the neutron diffraction patterns. In both cases the structure factor showed a low first peak maximum which follows a weak but apparent pre-peak at very low momentum transfer. It was also observed that the radial distribution function of both materials are characterized by the well-defined first neighbor shell because of the deep minimum on its right-hand side although in the liquid state. These results indicate that strong covalent bondings between unlike atoms in the solid state still remain when melting. Both the structure factor and the distribution curves of these alloys are, on the whole, similar to those in the amorphous phase which have already been examined. A slight difference in the coordination number, however, is found between amorphous and liquid phases of these materials.