The crystal structure of Zr2Se reinvestigated by electron crystallography and X-ray powder diffraction

The metal-rich compound Zr₂Se is of particular interest for electron crystallography, since it was one of the first examples that proved that heavy-atom structures can be solved via quasi-automatic direct methods from selected area electron diffraction intensities [1]. For this reason, Zr₂Se has been chosen as a model to discuss the possibilities and the limits of the quasi-kinematical approach that has been successfully used to determine this and related structures from high-resolution electron microscopy (HREM) images and selected area electron diffraction. In order to quantify the achievable accuracy of the electron crystallography techniques used, the corresponding structures are compared with results from structural analysis with X-ray powder data and with a model received from first-principles calculations. The latter structure was chosen in this study as a reference, since the calculations do not depend on experimental parameters. Analysis of the obtained result from electron diffraction structural analysis (EDSA) shows that the structural model is, on average, only off by 0.08 Å, despite the investigated crystal having an effective thickness of 286 Å. The corresponding result from Rietveld refinement with X-ray powder data agrees to within 0.04 Å with the structure from calculation and within 0.03 Å with the result from an earlier single crystal X-ray study [2].     

双折射YVO4晶体原料的合成及最佳制备工艺的探讨

本文介绍了液相法合成用于双折射单晶生长的钒酸钇原料的工艺。着重讨论了制备条件对原料纯度的影响,确定了最佳合成方案。为了适应工业化生产的需要,探讨了原料合成的规范化(以保证保纯性,均匀性,重复性),合成出的原料已成功生长出φ42×42mm的大尺寸优质钒酸钇单晶,且剩料可重复使用10次左右。     

Atomistic examinations of the solid-phase epitaxial growth of silicon

The low-temperature vapor deposition of silicon thin films and the ion implantation of silicon can result in the formation of amorphous silicon layers on a crystalline silicon substrate. These amorphous layers can be crystallized by a thermally activated solid-phase epitaxial (SPE) growth process. The transformations are rapid and initiate at the buried amorphous to crystalline interface within the film. The initial stages of the transformation are investigated here using a molecular dynamics simulation approach based upon a recently proposed bond order potential for silicon. The method is used first to predict an amorphous structure for a rapidly cooled silicon melt. The radial distribution function of this structure is shown to be similar to that observed experimentally. Molecular dynamics simulations of its subsequent crystallization indicate that the early stage, rate limiting mechanism appears to be removal of tetrahedrally coordinated interstitial defects in the nominally crystalline region just behind the advancing amorphous to crystalline transition front. The activation barriers for this interstitials migration within the bulk crystal lattice are calculated and are found to be comparable to the activation energy of the overall solid-phase epitaxial growth process simulated here.     

Outer space formation of a laser host glass

Classical crystallization calculations were performed to determine the possibility of forming a particular type of laser glass with the avoidance of devitrification in an outer space laboratory. Although the laser glass in question readily crystallizes in an earth environment, it is demonstrated that under the homogeneous nucleating conditions obtainable in a zero gravity laboratory this laser glass may be easily quenched to a virtually crystal-free product. Use of this material as a host in a neodymium glass laser would result in a more than 10% increase in efficiency when compared to laser glass rods of similar composition currently commercially available.     

An Investigation of Molecular Reorientation in Crystalline Pyrazine by Pulse NMR and The Atom-Atom Approximation for Intermolecular Forces

Proton NMR relaxation times (T ₁, at 9.2 MHz and 18.5 MHz, and T ₁ at 18.5 MHz) have been measured for crystalline pyrazine over the temperature range 293 K to the melting point at 327 K. No evidence of a discontinuity in these data is evident at either 29°C or 35°C, where previous workers have found evidence of a phase transition using infrared, Raman, ¹⁴N NQR, calorimetric and other techniques. This result was valid for both unpurified samples and for samples obtained as the result of fractional sublimation. With the assistance of potential energy calculations using the atom-atom approximation for intermolecular forces, the nature of the motional process responsible for the spin-lattice relaxation has been proposed to be an in-plane flip by 180 degrees; this proposal appears to be consistent with all experimental information available for the low-temperature phase stable below 29°C. In addition, the nature of the first phase transition at 29°C has been similarly investigated; it is suggested that this involves an inplane rotation by 90 degrees of just one of the two molecules in the unit cell. Again, this suggestion seems capable of rationalising all available information concerning this transition, although it is possibly not unique in this regard.     

Synthesis of high aspect ratio ZnO nanowires with an inexpensive handcrafted electrochemical setup

In this work, high aspect ratio zinc oxide nanowires are synthesized using templated one-step electrodeposition technique. Electrodeposition of the nanowires is done using a handcrafted electronic system. Nuclear track-etched polycarbonate membrane is used as a template to form the high aspect ratio nanowires. The result of X-ray diffraction and scanning electron microscopy shows that nanowires with a good crystallinity and an aspect ratio of more than 30 can be achieved in a suitable condition. The height of electrodeposited nanowires reaches to about 11 μm. Based on the obtained results, high aspect ratio ZnO nanowires can be formed using inexpensive electrodeposition setup with an acceptable quality.     

生长气氛和速度在金红石(TiO2)单晶体生长中的作用研究

本文研究了生长气氛和生长速度在焰熔法金红石单晶体生长中的作用,对比了晶体在空气中与在氧气中退火的结果,测定了晶体试样的摇摆曲线和透过率.研究表明:金红石单晶体的生长受炉膛气氛、生长界面温度和生长速度的影响;炉膛气氛决定晶体能否形成,是关键因素;炉膛气氛中的氧分压大于液固界面(即生长界面)处熔体的氧离解压是生长完整晶体的前提条件;晶体在退火过程中消除热应力,但更重要的是通过氧化反应消除氧空位,在氧气氛中退火,可明显缩短退火时间.在所优化的实验条件下制备的晶体,完整性较好,透过率为70～72;,与商用晶体的透过率基本一致.     

Homogeneous SiGe crystals grown by using the traveling liquidus-zone method

It is indispensable to estimate a diffusion coefficient in a solution zone in order to grow a homogeneous crystal by using the traveling liquidus-zone (TLZ) method. To estimate the diffusion coefficient of Ge in the SiGe solution zone, result of a two-dimensional numerical simulation is compared with an experimental result. From the comparison, the diffusion coefficient is estimated to be 9.5×10⁻⁵ cm²/s. By using this coefficient, a sample translation rate for obtaining a homogeneous SiGe crystal is determined. By translating samples with appropriate rates, homogeneous Si_0.5Ge_0.5 crystals are successfully grown. The typical Ge composition is 0.496±0.006 for more than 13 mm long. The experimental result shows the homogeneity of ±1.2% in the mole fraction. This deviation corresponds to the variation of less than ±0.03% in the lattice constant. Since this variation is negligibly small, the homogeneity is excellent. Thus it is found that the TLZ method is the universal growth technique, which is applicable to the crystal growth of not only the III–V compounds but also the IV–IV compounds.     

Measurement of Refractive Index of GaP Crystal over a Large Temperature Range Using Interferometry

The refractive index of GaP single crystal was measured through room temperature (300K) to 1200K at a wavelength of 780 nm by using an interferometry with a laser diode. To get a more accurate result, the thermal expansion coefficient of GaP crystal, which would be one parameter for the measurement of the refractive index, was measured by a diratometer equipped with laser interferometry against temperature in the range from room temperature to 973K. It was confirmed that the linear thermal expansion coefficient was a function of temperature. In this report, an empirical function was obtained to calculate the refractive index at any temperature for GaP crystal. The result shows that the refractive index of GaP varies from 3.1907 to 3.3354 in the temperature range from 300K to 1200K at the wavelength of 780nm.     

VERTICALLY SUSPENDED SMECTIC FILMS WITH IN-PLANE TEMPERATURE GRADIENTS

Abstract

We present a study of vertically suspended smectic films under the influence of temperature gradients in the film plane. It is shown that such gradients lead to the transport of smectic material, even against the action of the gravitation forces, from the hot to the cold film edge. In addition, we observe thermally driven convection in these films, and it is demonstrated that the surrounding air plays an essential role for this instability. We compare this result with convective patterns in soap films reported by Martin and Wu in Phys. Rev. Lett. 80 1892 (98) and give some evidence that their interpretation has to be modified, the contact with surrounding air has to be taken into account.     

Metallic phase formation in oxide films

Metallic particles embedded in the oxide film play an important role in film’s optical property. In this paper it is demonstrated that metallic phases of Ag, Ti, and Pb, can be formed in different oxide films under heating, X-ray photon, and electron radiation. The metallic phase separated from metal oxide film was investigated by X-ray photoelectron spectroscopy technique. Metal oxygen bond breaking and total energy reduction in the film result in the formation of metal in the film. It is necessary to fully understand the formation mechanism of metallic particles so their shapes and distributions can be tailored to achieve the desired film’s properties.     

Mechanical properties of a TAS fiber: a preliminary study

The mechanical properties, including elastic moduli, hardness, fracture toughness and tensile strength of a glass fiber in the Te-As-Se system (TAS) were studied. The values for the hardness (1.4 GPa) and the fracture toughness (0.18 MPa √m) show that this glass is both soft and brittle in comparison to glasses from other systems. However, indentation measurements should be interpreted with caution due to an indentation creep phenomenon and to a delayed fracture process. In addition, the effect of treatments in air (relative humidity about 60%) at different temperatures below T_g were investigated. The main result of this study is that the studied TAS glass is sensitive to the presence of humidity, and aging treatments have a pronounced detrimental effect on the strength of the uncoated fibers.     

Topological approach to investigation of acoustic axes in crystals

The concept of topological degree of a map is generalized to the case of discontinuous maps. The numerical value of such a degree may be a rational number. The representations developed are used for topological interpretation of the characteristics of special directions of propagation of acoustic waves in crystals (specifically, acoustic axes). The Euler theorem is generalized to the case of singularities with rational indices, and this result is applied to the set of acoustic axes in crystals.     

紫外、深紫外非线性光学晶体探索十年回顾

本文讨论了近10年来我们研究组在探索紫外、深紫外非线性光学晶体方面的经历。这些经历中有成功的一面,也有不成功的一面。成功的一面包括KBBF晶体的发现及最短倍频波的输出(184.7nm),以及最近使用的一种特殊的器件设计,使KBBF晶体能够产生有效的深紫外谐波光输出;KABO晶体的发现及厘米级晶体的获得,有可能使此晶体在Nd∶YAG激光的4、5倍频器件中得到应用。而我们没有想到的是线性和非线性光学性能均很优秀的SBBO晶体,却发现结构的完整性有问题,目前还不能得到实际的应用,在今后仍需花很大的精力去研究它。最后,我们虽然已花了10年时间,但深紫外非线性光学晶体仍旧没有得到理想的解决,还需我们继续努力。由此可见,一个科学问题的认真解决,是需要花非常长的时间。本文希望把我们研究组的近10年来的经验写出来,供大家参考,以求共勉。     

Nondestructive concentration profiling of fiber optic preforms by analysis of Raman spectra

We have demonstrated that Raman scattering may be used to determine P and Ge concentration profiles of optical fiber preforms. For Ge, the technique has an absolute accuracy of about 0.5 mol.% Ge, and spatial resolution to 40 μm was achieved. When used in a nondestructive mode, spatial resolution of 100 μm is possible. The results clearly show that phosphorus as well as germanium burn out as the tube is collapsed, with the spatial extent of the phosphorus burnout region being about twice as great as that for germanium. Our measurements on a graded-P preform provide a determination of the Ge incorporation ratio across the preform with the surprising result that it increases toward the edge, where the temperature is higher and P content is lower.The spectra also provide clear evidence for two types of Ge sites in silica-germanium glasses, where one type of site, possibly associated with network defects, is predominantly occupied at low concentration. The 710 cm^?1 line associated with these defect sites is not detected when either P or B is present, and we speculate that this may be related to the dependence of the Ge incorporation ratio on the phosphorous concentration.     

Promotion of crystal growth rate in aqueous solution by direct contact with gas corona discharge

A new technique to promote crystal growth in aqueous solution using gas plasma is proposed. In this method, short‐lived radical species produced in solution which is contacted with gas corona discharge play a role to increase chemical potential of inorganic solute. In an experimental examination, single crystal of KDP was grown in a supersaturated solution which receives oxygen ions and radicals from adjacent corona discharge in air. KDP crystal has two unique growth faces (100) and (101), and the growth rates of both faces were increased considerably by generating the corona discharge. The both growth rates with and without corona discharge were well converged by one function based on chemical potential supersaturation. This result revealed that the solution in contact with gas corona discharge has a larger capacity of chemical potential than that without the discharge. Short‐lived species induced by gas corona discharge are considered to be anti‐solvents to cause this effect. The crystal growth process proposed here is considered to be an excellent method in terms of low impurity inclusion because such short‐lived species do not remain in the final crystal products and solution. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Multinuclear NMR study of aluminosilicate sol-gel synthesis using the prehydrolysis method

Tetraethoxysilane (TEOS), water and aluminum sec-butoxide (Al(OBu^s)₃) are combined in several stages of prehydrolysis technique, and reaction intermediates at each stage were examined by means of both liquid and solid nuclear magnetic resonance of ¹³C, ²⁹Si, ¹⁷O, and ²⁷Al nuclei. The spectra indicate that when Al(OBu^s)₃ is added to a prehydrolyzed TEOS solution an aluminosilicate precursor is formed in which the aluminum is tetrahedrally coordinated by four silicate ligands. When further water is added, gelation is accompanied by the expansion of aluminum coordination; the spectra indicate that this occurs by the nucleophilic attack of silanol groups. At water contents for which gels of low transparency result, this coordination expansion is accompanied by the formation of new siloxane linkages, but at water contents so high that opaque gels result, the coordination expansion proceeds much more quickly and no new siloxane linkages are observed.     

Electrical and Optical Properties of Oriented (CH)x

We have measured the optical and electrical properties of both graphoepitaxially grown and shear-flow polymerized polyacetylene filmzs. The former show a pronounced optical anisotrop;y whereas the latter do not. This result is is correlated with the orientation of the chain axes relative to the fibra axes relative to the fibre axis, which is preferentially parallel in graphoepitaxially grown films. The electrical anisotropy was found to be rather weak. Optical measurements on doped (CH)_x indicate that both the characteristic IR modes at 0.11 eV and 0.17 eV and the mid-gap absorption around 0.5 eV, which have been attributed to solitonic defects, persist up to the metallic level. This result shows that both the IR modes and the mid-gap absorption are not necessarily signatures for solitonic defects. furthermore we conclude that the impurity potential is more effective than hitherto assumed.     

Short-range order in non-stoichiometric amorphous silicon oxynitride and silicon-rich nitride

By de-convoluting the Si 2p X-ray photoelectronic spectra, it was found that the short-range order in amorphous silicon oxynitride (SiO_xN_y) films with different compositions can be quantitatively described by the random bonding model. In this model the SiO_xN_y consists of five types of randomly distributed tetrahedra and it indicates that metal-oxide-semiconductor transistor with this gate dielectric will not result in any gigantic potential fluctuation in the conduction channel. On the contrary, the structure of silicon-rich silicon nitride SiN_x can only be described by the random mixture model where the local composition fluctuations in this film will result in gigantic potential contra-variant fluctuation.     

Quasicrystal as a locally deformed crystal structure

Lattice models for a two-dimensional octagonal quasicrystal and a three-dimensional icosahedral quasicrystal with the icosahedral-dodecahedral local order are introduced. It is shown that an octagonal quasicrystal may be obtained as a result of the local deformation of an ideal square atomic lattice. Possible paths of structural transformations between various phases of an AlPd crystal are determined. It is shown that icosahedral quasicrystals of aluminum alloys with transition metals can be considered to be a result of the local distortions of an ideal fcc lattice of elemental aluminum induced by a transition metal impurity.