Theoretical calculation of dense random packings of equal and non-equal sized hard spheres applications to amorphous metallic alloys

A model describing the structure of amorphous metallic alloys is proposed using a packing of non-equal-sized hard spheres. Models containing up to 1000 spheres were generated by computation. Hypotheses guiding the calculation are chosen in order to obtain a model as compact as possible and to take into account affinity between metal and metalloids: therefore two small spheres are not allowed to be in hard contact. This method of calculation is first tested by building up heaps of equal-sized spheres. Both the radial distribution function and the interference function are calculated for each model and the variations of these curves with the number of spheres are studied. In the case of two sizes of spheres heaps calculations are devoted to the study of the so-called ‘shoulder interference function’ which characterizes many amorphous alloys such as NiP, PdSi, etc. It is shown that the existence of such a shoulder depends on two parameters: the ratio of sphere diameters and the relative concentration of small and large spheres. These results are in good agreement with some recent experimental observations. This shoulder is well-marked when the diameter ratio reaches 10% and when the concentration in small spheres lies in the range 10–15%. In such a structure (e.g. amorphous NiP and PdSi alloys) it is observed that small spheres are always surrounded by nine large spheres such as P or Si atoms in crystalline Ni₃P or Pd₃Si. Large spheres, i.e. metallic atoms, form distorted icosahedra, the distortion of which varies from one to the other. The short-range order has a five-fold symmetry which is not compatible with any long-range order.     

Optical Absorption Spectrum of Co2+ Ion Impurities in Lithium Sodium Potassium Sulphate Single Crystal

Optical absorption spectrum of Co²⁺ ions doped in lithium sodium potassium sulphate single crystal has been studied at room and liquid nitrogen temperatures. The observed bands are assigned as transitions from the ground ⁴T_1g(F) to various excited quartet and doublet levels of Co²⁺ ion in octahedral symmetry. The splitting in one of the bands at liquid nitrogen temperature has been explained as due to spin-orbit interaction. All the observed band positions have been fitted with the parameters B, C, D_q and ζ.     

Topology and symmetry analysis of rare earth borocarbides structural family,analogy to hexaferrites and relation to properties

The topology and symmetry analysis was applied to a series of rare earth borocarbide compounds, which have been gaining increasing interest due to their magnetic and thermoelectric properties. Using principles of OD theory, the crystal structures were deconvoluted into L(1) (B₁₂ icosahedra and C‐B‐C chain) layers and L(2) (rare earth and B₆ octahedral) layers. The arrangement of B₁₂ icosahedra in the L(1) layer is equal to close packed spheres, however, symmetry of the B₁₂ block lowers symmetry of the resulting layer from P 6/mmm to P 3m1. Both layers, L(1) and L(2) possess symmetry P 3m1 and the conjugation of L(1) with L(2) layers occurs in accordance with the symmetry elements. No disorder may appear here because of equal symmetry of single layers and layer pairs and it is not a classical OD family. Only the increasing of the amount of one type of layers, namely L(1), provides the structural variations. Close analogy to the hexagonal ferrites family has been found. Topology and symmetry analysis reveals principles in the building up of the structural family, gives an insight into the particular order‐disorder formation mechanism/criteria of these homologous borocarbide compounds and as the result relation to the properties (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theoretical and experimental studies on the silica hollow spheres

The structures and properties of the silica hollow spheres have been studied by experimental and quantum chemical calculations. The structure of the silica hollow spheres was characterized by TEM, BET and XPS. According to the experimental results, a model of porous silica clusters has been proposed to represent the structure of the silica hollow spheres. The geometry of model was optimized by the semi-empirical PM3 method and the density of states for the model was calculated by ab initio methods at the level of 3-21G and 6-31G. Both the influences of model size and the effect of basis set are discussed. It is found that the calculated binding energies of O1s, Si2s and Si2p are in fairly good agreement with the experimental results. The studies indicate that the small cluster model can be used to reasonably analyzed the structure and properties of silica hollow spheres.     

Identification of crystal symmetry in Kramers and non‐Kramers ions by optical absorption: Divalent copper and nickel ions in diamagnetic lattices

The optical absorption spectra of Ni(II) doped hexaimidazole zinc(II) dichloride tetrahydrate (HZDT) and Cu(II) doped magnesium potassium phosphate hexahydrate (MPPH) have been studied at room temperature. Ni(II)/HZDT spectrum consists of three strong and one weak band. The calculated value of Dq is 1051 cm^‐1 and the interelectron repulsion parameters B and C are 854 and 3626 cm^‐1 respectively. The correlation of optical work with EPR has yielded the spin‐orbit interaction parameters λ and ξ as –225 and –450 cm^‐1 respectively. The symmetry around the Ni(II) ion is distorted octahedral, as suggested by EPR results. The estimated percentage covalency of the nickel‐nitrogen bond is around 30%. The optical absorption studies of Cu(II)/MPPH show three bands, which are identified as ²B_1g → ²A_1g, ²B_1g → ²B_2g and ²B_1g → ²E_g transitions. The octahedral field parameter Dq and the tetragonal field parameters have been evaluated from the observed adsorption bands. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of gamma-rays on some mixed alkali borate glasses containing nickel

The change of optical absorption of some irradiated mixed alkali borate glasses containing nickel has been studied by varying the gamma-ray dose or the mixed alkali oxide content. The induced bands increase as the radiation process proceeds until a certain limit after which a constancy is assumed to be reached.The investigation of the observed spectra shows that there is an induced absorption band with its maximum at 550 nm, in addition to the characteristic absorption bands of the divalent nickel ions with peaks in the visible region at 410, 640, 680 and 750 nm.     

Structural study of sol-gel silicate glasses by IR and Raman spectroscopies

A study of the structure and bonding configuration of sol-gel silicate glasses by Raman and infrared spectroscopies is presented. Moreover, a review of the Raman lines and infrared bands assignment, the identification of the non-bridging silicon-oxygen groups and the ring structures are also demonstrated. The evolution of the changes of the bonding configuration in the composition and the stabilization temperature of the bioactive glasses is discussed in terms of the structural and textural characteristics of the glasses. Raman and infrared analyses contribute to the improvement in understanding of the local symmetry for sol-gel silicate glasses. infrared spectroscopy has allowed to identify the vibration bands of the hydroxyl groups associated with various configurations of the terminal silanol bonds on the glass surface and the free molecular water in the glass matrix. Raman analysis has provided an alternative method of quantifying the network connectivity grade and predicting the textural properties of the sol-gel silicate glasses.     

基于群表示论的微波等离激元谐振器模式响应研究

作为光频段局域表面等离激元的低频对应物,人工局域表面等离激元因其深亚波长局域场增强和高Q值谐振的特点而受到广泛关注。微波等离激元谐振器是产生人工局域表面等离激元的典型器件,其特点是具有多重离散旋转对称性和镜面反射对称性。以往的研究提出了等效媒质法和等效色散法分析微波等离激元谐振器的模式响应,但这两种方法都未能充分考虑谐振器的几何对称性从而未能全面揭示其模式特性。本文针对谐振器的几何对称性提出了群表示论方法分析其模式响应。通过对称性分析,发现谐振器的几何对称性所构成的群的不可约表示数等于谐振器所能支持的人工局域表面等离激元模式的数目。以对称性构成C_7v群的谐振器为例,C_7v群的5个不可约表示数对应了5种人工局域表面等离激元模式,分别为零阶模式(也即磁偶极子)、偶极子、四极子、六极子和十四极子。受限于几何对称性,谐振器将不能支持更多阶的模式。为验证群表示论方法,设计了对称性构成C_7v群的微波等离激元谐振器,全波仿真结果很好地证明了上述理论。本文提出的群表示论方法也可推广到其他频段如光频,因而具有广泛的适用性。     

The origin of electrooptical sensitivity of glassy materials: crystal motifs in glasses

The results of studying electrooptical Kerr sensitivity in heavy metal silicate and phosphate glasses and glass-ceramics are presented. A niobium-lithium-silicate glass demonstrating a record Kerr coefficient (266×10⁻¹⁶ m/V²) has been formed. Formation of the transparent glass-ceramics containing electrooptical sodium niobate microcrystals has been studied, and glass-ceramics demonstrating Kerr coefficients higher than 6000×10⁻¹⁶ m/V² have been elaborated. On the base of the effective medium approximation, it is shown that the Kerr coefficient of these glass-ceramics depends on the volume fraction of sodium niobate microcrystals, v_c as a linear function of v_c(1−v_c)⁻² A conception of the origin of electrooptical sensitivity of glasses is proposed. This conception is based on the hypothesis that in glasses there exist regions with exactly crystalline ordering within 2-3 coordination spheres, with these regions having no phase boundaries. These regions are named the crystal motifs (CM). Due to the highly effective mechanism of nuclear polarizability of the electrooptical crystals, the motifs with the symmetry of such crystals are responsible for high permittivity and Kerr sensitivity of the glasses, and they play a role of pre-nuclei while electrooptical glass-ceramics are forming under glass heat treatment. It has been found that synthesized barium-titanate-silicate and niobium-lithium-phosphate glasses demonstrate extremely low Kerr coefficients, and they do not form transparent glass-ceramics with any electrooptical precipitates. This contradicts literature data and is explained by the difference in the conditions of glass synthesis, which are supposed to be responsible for the formation of proper CMs.     

Layer-by-Layer Growth of Ammann-Beenker Graph

Crystallography Reports - The layer-by-layer growth of Ammann-Beenker graph (a quasi-periodic graph with eightfold symmetry) has been experimentally and theoretically studied. The limiting form for...     

Microwave assisted diffusion of Au-nanoclusters in vitreous silica

Gold nanocluster composite glasses were prepared via the evaporation of a thin gold layer, followed by microwave-assisted diffusion. Increased diffusion using a microwave (MW) oven as compared to the traditional diffusion in a muffle furnace is observed and measured. The influence of diffusion temperature and time on diffusion depth and surface plasmon resonance (SPR) absorption bands of the so-obtained gold nanocluster have been studied. The diffusion parameters have been shown to influence the size of the metal nanoclusters and hence, the position of the SPR absorption bands.     

Optical Absorption Spectrum of Cu2+ Ions in Lithium Sodium Potassium Sulphate Single Crystal

Single crystals of Copper-doped Lithium sodium potassium sulphate were grown at room temperature. The optical absorption spectra were recorded at room and liquid-nitrogen temperatures. The observed absorption bands have been attributed to an ion of Cu²⁺ in tetragonal symmetry with ²B₁ as the ground state. A successful interpretation of all the observed bands have been made by taking into consideration spin-orbit coupling associated with a tetragonal field. The crystal field parameters derived are D_q = 1045 cm⁻¹, Ds = 1352 cm⁻¹ and Dt = 457 cm⁻¹.     

SiO2双尺寸光子晶体的自组装及稳定性探索

采用St?ber法制备不同尺寸SiO2胶体球,利用多次垂直沉积法制备SiO2双尺寸光子晶体.通过场发射扫描电子显微镜(FE-SEM)对所得双尺寸光子晶体的形貌和排列方式进行表征.对双尺寸光子晶体的生长过程进行探讨,研究界面作用力及胶体球间相互作用力对制备稳定结构的影响.研究结果表明,SiO2双尺寸光子晶体具有更加有序稳定的结构.胶体球间的相互作用力随胶体球粒径增大而增大,大粒径胶体球更有利于形成稳定的结构.     

Porosity and structural parameters of Karelian shungites according to the data of small-angle synchrotron radiation scattering and microscopy

The nanoporosity and structure of natural carbons has been investigated on the example of Karelian carbon-rich shungites by comparing the data of small-angle synchrotron radiation scattering and highresolution microscopy. The analysis of small-angle scattering data is based on the model of scattering spheres with lognormal size distribution. It is found that the structure of samples from the Maksovo and Zazhogino deposits subjected to high temperatures in the geological medium and (also to a lesser extent) a sample from the Shunga deposit can be described as an aggregation of polydisperse scattering spheres with lognormal size distribution; the characteristic scatterer size is determined for them. A comparison with microscopy data shows that these scatterers are mainly associated with pores, and the character of their size distribution is similar to that previously established for nanoglobules in schungites.     

Hydrodynamics of layered and cubic quasicrystals

The dissipative processes associated with sound absorption in layered and cubic quasicrystals have been studied within the framework of the hydrodynamic approach. The decagonal, octagonal, and dodecagonal quasicrystals are considered as layered quasicrystals. Proceeding from the general theory, the complex law of dispersion is calculated for various directions of the propagation and polarization of sonic waves in layered and cubic quasicrystals. The effect of the phason modes on the process of sound absorption and the role of symmetry constraints are analyzed. The validity of the macroscopic consideration is discussed in terms of the theory of the generalized color symmetry.     

Three‐dimensional study of the pressure field and advantages of hemispherical crucible in silicon Czochralski crystal growth

The effects of several growth parameters in cylindrical and spherical Czochralski crystal process are studied numerically and particularly, we focus on the influence of the pressure field. We present a set of three‐dimensional computational simulations using the finite volume package Fluent in two different geometries, a new geometry as cylindro‐spherical and the traditional configuration as cylindro‐cylindrical. We found that the evolution of pressure which is has not been studied before; this important function is strongly related to the vorticity in the bulk flow, the free surface and the growth interface. It seems that the pressure is more sensitive to the breaking of symmetry than the other properties that characterize the crystal growth as temperature or velocity fields. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Desymmetrization of OD Structures

It has been shown that the symmetry and structure of individual OD layers may depend on their actual stacking in a (polytypic) OD structure, leading to significant deviations from the ideal OD symmetry. This phenomenon has been called “desymmetrization of OD structures”; its significance for polytypism and for crystallochemistry is discussed.     

Icosahedral and dense random cluster packing in metallic glass structures

It has recently been shown that metallic glass structures can be idealized as inter-penetrating solute-centered atomic clusters that are packed with essentially periodic symmetry. The present work applies the same methodology to explore whether experimental observations can be matched by inter-connected solute-centered clusters that are organized in space via dense random cluster packing, Bergman icosahedral cluster packing or Mackay icosahedral cluster packing. Idealized partial pair distribution functions are developed where the symmetry of the solute positions in the structure is derived from the cluster-packing symmetry and the solute concentration, which establishes occupation of inter-cluster sites, especially β structural sites enclosed by an octahedron of solute-centered clusters. While each of the three models matches major features of the measured solute-solute partial pair distribution functions, the arrangement of clusters with Mackay icosahedral ordering provides the best fit. However, this model is not able to match an essential feature in solute-lean glasses and does not provide the same overall agreement as does periodic cluster packing for solute-rich glasses. Strong similarities between the structure factors in the Mackay icosahedral and periodic cluster-packing models, along with expected deviations from the idealized solute positions studied here, are likely to hinder an unambiguous distinction between these two models.     

The Investigation of Heat,Release in Slip Bands Consisting of Edge or Screw Dislocations

The temperature rise in screw and edge dislocation slip bands of the NaCl crystals by deformation by compression has been studied with the help of cholesteric liquid crystals CLC.     

Circular Dichroism in Cholesteric Liquid Crystals

Crystallography Reports - The circular dichroism in cholesteric liquid crystals (CLCs) has been studied in the absorption bands of dissolved dichroic dyes possessing both the positive and negative...