Atom movement on a dislocated surface

The standard picture of growth at a screw dislocation assumes that the movement of adatoms on a dislocation loop is the same as on an ideal plane. We have examined this proposition by investigating the movement of a single tungsten adatom on a W(110) plane intersected by a screw dislocation. Surprisingly enough, adatom movement was entirely different than on a normal (110) plane: the overall diffusivity was higher, and the mobility varied with the location of the adatom relative to the dislocation core. This study demonstrates that surface transport is strongly affected in the vicinity of dislocations.     

Growth morphology of isotactic polybutene-1

Isotactic polybutene-1 has been crystallized from solution by the film formation method. Well-defined single crystals have been grown from solutions of the polymer in amyl acetate and its mixtures with n-butanol in all the three modifications by varying the concentration and crystallization temperature. Multilayered orthorhombic, tetragonal and hexagonal crystals with spiral ramps and terraces have been obtained from concentrated solutions near the turbidity temperature. Rotation of the screw dislocation as well as interlacing multilayered growth have been observed in the tetragonal morphology. Different types of twinning are commonly observed in the orthorhombic and hexagonal modifications. It is suggested that (a) spiral ramps grow by the mechanism of screw dislocation, (b) terraced crystals grow by the mechanism of Frank-Read source or the basal lamellae being hairy on the molecular level nucleating the next lamella, (c) movement of screw dislocation is very common in the orthorhombic and hexagonal modifications, (d) the solution temperature does not affect the morphology but the solvent concentration and the crystallization temperature decide the growth habit of the single crystals and (e) the growth mechanism of multilayered crystals and twinning are similar to those of monomeric substances.     

Growth and dislocation studies of β-HMX

Background

The defect structure of organic materials is important as it plays a major role in their crystal growth properties. It also can play a subcritical role in “hot-spot” detonation processes of energetics and one such energetic is cyclotetramethylene-tetranitramine, in the commonly used beta form (β-HMX).

Results

The as-grown crystals grown by evaporation from acetone show prismatic, tabular and columnar habits, all with {011}, {110}, (010) and (101) faces. Etching on (010) surfaces revealed three different types of etch pits, two of which could be identified with either pure screw or pure edge dislocations, the third is shown to be an artifact of the twinning process that this material undergoes. Examination of the {011} and {110} surfaces show only one type of etch pit on each surface; however their natural asymmetry precludes the easy identification of their Burgers vector or dislocation type. Etching of cleaved {011} surfaces demonstrates that the etch pits can be associated with line dislocations. All dislocations appear randomly on the crystal surfaces and do not form alignments characteristic of mechanical deformation by dislocation slip.

Conclusions

Crystals of β-HMX grown from acetone show good morphological agreement with that predicted by modelling, with three distinct crystal habits observed depending upon the supersaturation of the growth solution. Prismatic habit was favoured at low supersaturation, while tabular and columnar crystals were predominant at higher super saturations. The twin plane in β-HMX was identified as a (101) reflection plane. The low plasticity of β-HMX is shown by the lack of etch pit alignments corresponding to mechanically induced dislocation arrays. On untwinned {010} faces, two types of dislocations exist, pure edge dislocations with b?=?[010] and pure screw dislocations with b?=?[010]. On twinned (010) faces, a third dislocation type exists and it is proposed that these pits are associated with pure screw dislocations with b?=?[010].

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Graphical abstract Etch pits on the twinned (010) face of β-HMX.

     

Molecular and Crystal Structure of 4-[2-(4-Dimethy aminophenyl)ethenyl]-1-methypyridinium tetraphenylborate

The title compound, C40H39N2B, Mr=558.54, crystallizes in space group P21/c, monoclinic, a=18.013(3), b=10.821(2), c=17.321(3),β=112.20(1)°, V=3125.9(9)3, Z=4, Dc=1.187g/cm3, μ(MoKα)=0.068mm-1, F(000)=1192. The final R and wR(F2) are 0.0467 and 0.0972 for 2353 observed reflections (I>2σ(I)), respectively. In the title molecule, the pyridyl ring of the cation makes dihedral angle of 19.94° with the phenyl ring. The anion takes a slightly distorted tetrahedral geometry. In the crystal, along the screw axis b, the centrosymmetric anion pairs form double chains, which are in turn related by the glide plane c, forming double layers. The centrosymmetric cation pairs are related by screw axis b and located between the double sheets of the anion.     

Molecular and Crystal Structure of 4-〔2-(4-Dimethy aminophenyl)ethenyl〕-1-methypyridinium tetraphenylborate

1　INTRODUCTIONAccordingtotheorganic“saltmethodology”principle〔1〕,whichsuggeststhatCoulombicinteractionsinorganicsaltscouldoverridedipolarinteractionswhichpro-videastrongdrivingforceforcentrosymmetriccrystallizationindipolarmolecularcrystals,asystematicsearchforSHG(SecondHarmonicGeneration)organiccrys-talswasconducted.AsearchontheCambridgeStructuralDatabase〔2〕(Version5.18)wasdonefororganicsaltshavinganionBPh4-(47cmpoundsintotal)orBF4-(147compoundsintotal)andcationcontainingatle…     

Magneto-optical specifications of Rosen-Morse quantum dot with screw dislocation

This is the first study to consider a quantum dot with screw dislocation that has Rosen-Morse (RM) confinement potential, generated by a GaAs/GaAlAs heterostructure. An external magnetic field and Aharonov-Bohm (AB) flux field were also applied on RM quantum dot (RMQD) in order to stave the effects of a screw dislocation defect. The combined effect of the screw dislocation defect, the external magnetic field, and AB flux field on the total refractive index changes (TRICs) and the total absorption coefficients (TACs) of RMQD are thus investigated. Cylindrical coordinates are used due to the direction of application of the torsion and the external fields, as well as due to the structure's symmetry. The effective mass approximation and tridiagonal matrix methods are used in order to obtain the subband energy spectra and electronic wave functions of RMQD. The nonlinear optical specifications of RMQD are checked using compact-density-matrix formalism within the framework of the iterative method. Reviews without screw dislocation are also carried out in order to be able to clarify the effects of a screw dislocation defect on the optical properties, and then, both cases are deliberated. This study is the first attempt to analyze the AB flux field for RMQD without screw dislocation. In the present study, the influences of a screw dislocation defect on RMQD's TRICs and TACs are probed by considering different values of the external magnetic field and AB flux field, and the ranges of corresponding parameters on the optimum of the structure are specified. Moreover, the study also elucidates how to rule out the effects of screw dislocation on optical specifications by means of the external fields. Despite a certain screw dislocation, the frequency range is determined where the structure behaves as if it is perfect (namely, without screw dislocation) for its optimum, which in turn is crucial for experimental applications.     

Fractal MTW Zeolite Crystals: Hidden Dimensions in Nanoporous Materials

Screw dislocation structures in crystals are an origin of symmetry breaking in a wide range of dense‐phase crystals. Preparation of such analogous structures in framework‐phase crystals is of great importance in zeolites but is still a challenge. On the basis of crystal‐structure solving and model building, it was found that the two specific intergrowths in MTW zeolite produce this complex fractal and spiral structure. With the structurally determined parameters (spiral pitch h, screw angle θ, and spatial angle ψ) of Burgers circuit, the screw dislocation structure can be constructed by two different dimensional intergrowth sections. Thus the reported complexity of various dimensions in diverse crystals can be unified.     

Thermal conditions of growth and the necking evolution of Si,GaSb AND GaAs

The configuration of thermal gradient is illustrated for various types of crucible rotation, which is important for the creation of dislocations, which decreases along the grown axis of crystal. A new mechanism for dislocation elimination during the growth is proposed to explain this phenomenon, which provides a good agreement with the experimental results. The concentration of etch pits rapidly decreased from the beginning to the end of the crystals and the dislocation densities in the middle portion of all investigated crystals were found less than 102 cm^-2. The shallow vertical temperature gradients and virtually flat solidification interface prevented thermal stress from their building up in the crystals. As a result, the dislocation formation had random distribution. Using good necking procedures and choosing an appropriately oriented starting crystal with the shoulder angle <38.94° (assuming growth in <111> direction) it is possible to produce almost dislocation-free crystals without resorting to additional doping normally employed to reduce dislocation formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

KDP晶体台阶生长动力学的激光干涉实验研究

采用迈克尔逊干涉技术,通过测量KDP晶体生长的法向速率和台阶斜率来研究其台阶生长的动力学系数、台阶自由能、溶质在边界层内的扩散特征以及激发晶体生长台阶的位错活性.实验表明, KDP中不同活性位错的台阶动力学系数差异较大,例如高活性和低活性台阶动力学系数分别为10.3×10-2和5.21×10-2 cm/s,位错源在晶体表面的形状、面积的变化,以及Burgers矢量的变化是造成晶体生长动力学测量数据重复性差的主要原因.     

Optical and electrochemical studies of the underpotential deposition of metals Part I. Thallium deposition on single crystal silver electrodes

The underpotential deposition of thallium on single crystal silver cathodes, orientations 100, 110 and 111, was investigated optically and voltammetrically. Careful chemical polishing of the silver surfaces was necessary to reveal the fine structure on the voltammograms reproducibly. The deposition process for the first monolayer was shown to be the formation of a layer of adsorbed atoms initially, followed by a phase transformation to form a crystal plane by two-dimensional nucleation and growth. The extent of the initial adsorption was related to the number of favourable sites on each crystal face such that the adsorbed layer formed a superlattice. The influence of the substrate structure was seen on the formation of the second thallium monolayer. The formation of this layer also involved adsorption followed by transformation into a crystal plane.     

Synthesis of Anisotropic ZnSe Nanorods with Zinc Blende Crystal Structure

Compared with the well‐explored cadmium‐based one‐dimensional nanorods (NRs), it is still a challenge to produce heavy‐metal‐free II–VI semiconductor analogues with a controlled size, shape, and crystal structure. Herein, a synthetic strategy towards ZnSe NRs with a zinc blende crystal structure is presented, where use of the anisotropic nuclei produced via a high‐temperature selenium injection favors anisotropic growth. Elongated ZnSe NRs were produced from anisotropic ZnSe nuclei, while spherical ZnSe nanocrystals were obtained starting from isotropic nuclei. The different free energy at (111) and (220) planes in anisotropic ZnSe nuclei induces the anisotropic growth of (111) plane for ZnSe NRs. Proper choice of the capping ligand (1‐dodecanethiol) has an important implication for the formation of anisotropic ZnSe nuclei and also allows the control of the diameter of the final ZnSe NRs by limiting the growth of the (220) crystal plane of anisotropic ZnSe nuclei.     

Landau-de Gennes theory of the core structure of a screw dislocation in smectic A liquid crystals

We present details of calculations of the core structure of a screw dislocation in a smectic A liquid crystal, using the phenomenological Landau-de Gennes free energy functional. The order parameter frustration created by topological constraints far from the dislocation core is resolved in one of three qualitatively different ways. The three types of dislocation core solution are the DT (double twist), CL (classical), and BP (broken polar symmetry) solutions, respectively. The stability requirements for these structures are discussed, as a function of temperature, smectic elastic properties, and coupling between smectic and nematic order. The effect of possible inhomogeneity between left- and right-handed conformers is also examined.     

AN UNUSUAL ADDITION AND RING-CLOSURE REACTION OF 1-(2-BROMOETHYL)-2,3-DIHYDRO- 3-PROPYL-1,3,2-BENZO-DIAZAPHOSPHORIN-4(1H)-ONE 2-OXIDE WITH CARBON DISULFIDE FOR A NEW AND CONVENIENT SYNTHESIS OF THE FUSED PHOSPHORUS HETEROCYCLIC COMPOUND

ABSTRACT

The reaction of 1-(2-bromoethyl)-2,3-dihydro-3-propyl-1,3,2-benzodiazaphosphorin-4(1H)-one 2-oxide with carbon disulfide takes an alternative pathway in the use of different bases. The sodium hydride mediated reaction leads to the formation of the tricyclic fused 1,2,3,4,4a,4b,5,6-octahydro-6-oxo-5-propyl-4-thia-3,4b,4a-thiazphosphaphenanthridine 4a-oxide via addition of H-P bond across the double bond of carbon disulfide followed by intramolecular cyclization. In the presence of triethylamine, refluxing a mixture of 1-(2-bromoethyl)-2,3-dihydro-3-propyl-1,3,2-benzodiazaphosphorin-4(1H)-one 2-oxide with carbon disulfide in benzene takes an unusual course with formation in excellent yield of the first example of fused phosphorus heterocyclic 4-[1′-(β-bromoethyl)-4′-oxo- 3′-propyl-1′,2′,3′,4′-tetrahydro-1,3,2-benzodiazaphosphorin-2′- sulfide]-1,2,3,4,4a,4b,5,6-octahydro-6-oxo-5-propyl-3,4b, 4a-thiazphosphaphenanthridine 4a,2′-dioxide, which was confirmed by spectroscopic methods, microanalyses and single crystal X-ray structure determination.     

Unusual Pathway Taken in the Reaction of Bis(alkynyl)diisopropylaminoboranes with B(C6F5)3

The bis(alkynyl)diisopropyl‐aminoboranes 7 were prepared by treatment of iPr₂NBCl₂ with two molar equivalents of 1‐pentynyl lithium or lithium phenylacetylenide, respectively. Their reaction with one molar equivalent of B(C₆F₅)₃ resulted in the formation of the 1,1‐carboboration products 8 that were subsequently stabilized by adduct formation ( 9 ) with tert‐butyl isocyanide. Thermolysis of 8a (R=nPr) proceeded with hydride transfer from a N‐isopropyl substituent to the distal carbon atom of the remaining pentynyl unit at boron to give the zwitterionic five‐membered heterocyclic product 10a in good yield. The analogous product 10b (R=Ph) was obtained upon photolysis of 8b . The compounds 7b , 9b , 10a , and 10b were characterized by X‐ray crystal structure analysis.     

片晶折叠表面成核机制与结晶温度的相关性研究

利用自晶种方法,在邻二氯苯稀溶液中培养聚丁二酸丁二醇酯(PBS)晶体,系统研究了结晶温度对其晶体形貌的影响.使用PBS单晶作为研究对象,有效避免小尺寸观察不具有统计意义的缺点.在结晶过程中,通过改变结晶温度和自晶种温度,可有效调控稀溶液中生长的PBS晶体尺寸大小和晶体中缺陷的数量,得到了单层无缺陷的单晶、双层晶体和多层晶体等一系列PBS片晶.基于对不同实验条件下得到片晶的形貌和表面粗糙度的统计结果,提出晶体中可容忍的缺陷数量与结晶温度和晶种温度密切相关这一结论,通过建立热力学模型,定性分析了晶体中缺陷数量和结晶温度的依赖关系,从片晶表面粗糙度统计结果出发,提出高分子片晶折叠表面成核机制,较好地解释了实验中观察到的不同PBS晶体的形貌.     

对—乙酰苯基二茂铁的晶体结构

单取代二茂铁化合物,尤其是单取代芳基二茂铁化合物,当芳基为带有各种不同取代基苯时,中心铁原子的氧化还原电势会产生相应的变化,茂环上屹化学位移也明显地与Hammett常数相关。在对-乙酰苯基二茂铁的电化学研究中已经证明,它的氧化势     

ZnO multipods, submicron wires, and spherical structures and their unique field emission behavior

A simple method of vapor deposition for the shape selective synthesis of ZnO structures, namely, multipods, submicron wires, and spheres, has been successfully demonstrated. A plausible growth mechanism based on the studies of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) is proposed. Our studies suggest that the growth of a multipod structure is governed by the screw dislocation propagation while the vapor-liquid-solid (VLS) mechanism is responsible for the formation of submicron wires and spheres. Moreover, the flow rate of the carrier gas plays a crucial role in governing the morphology. Further, these structures exhibit an enhanced field emission behavior. The nonlinearity in the Fowler-Nordheim (F-N) plot, a characteristic feature of electron emission from semiconductors, is explained by considering the contributions from both the conduction and the valence bands of ZnO.     

R、S-三[N-(邻-氧α-萘亚甲基)-α-苯乙胺]合钴的合成、晶体结构及分子绝对构型

合成了R,S一三[N-(邻-氧α-萘亚甲基)-α-苯乙胺]合钴。x射线单晶衍射衍射测定了晶体结构及分子的绝对构型。晶体属正交晶系,D_2~4-P2_12_12_1空间群,晶胞参数:α=12.587(3),b=14.166(3),c=25.652(14)(?),v=4574.2(?)~3,z=4。用重原子法及差值Fourier合成解出结构,最后的R值为0.074。利用Co~( )对CuKa反常散射测定了分子的绝对构型。Co~( )为六配位,d~2sp~3杂化近正八面体构型。     

Dislocation approach to the plastic deformation of semicrystalline polymers: Kinetic aspects for polyethylene and polypropylene*

The plasticity of semicrystalline polymers is analyzed in the framework of Young's dislocation model under the assumption of nucleation of screw dislocations from the lateral surface of the crystalline lamellae. It is proposed that the driving force for the nucleation and propagation across the crystal width of these screw dislocations relies on chain twist defects that migrate along the chains stems and allow a step‐by‐step translation of the stems through the crystal thickness. Such defects are identified as thermally activated conformational defects responsible for the so‐called crystalline relaxation. Dislocation kinetic equations are derived. Plastic flow rates attainable by dislocation motion in polyethylene and polypropylene are assessed with frequency–temperature data of the crystalline relaxation. Comparisons are made with experimental strain rates that enable homogeneous plastic deformation. In addition to temperature, the crystal lamellar thickness, which is a basic factor of the plastic flow stress in Young's dislocation model, is a major factor in dislocation kinetics through its influence on chain twist activation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 593–601, 2002; DOI 10.1002/polb.10118     

Growth Defects in Cubic KTa1-xNbxO3 Crystal

Potassium tantalate niobate （KTa1-xNbxO3, KTN） crystals with different dimensions and quality situations were grown by Czochralski method. Crystal growth process and morphology properties of KTN are presented in this paper. It was found that some defects, such as bubble, inclusion, crack, dislocation etc., can all appear if the crystal is grown in an improper condition. The character and formation mechanism of such defects in macro growth are discussed. We consider that the CO2, which was not released absolutely during the sintering process and dissolved in the melt, led to bubbles. The composition of the inclusion caused by high pulling and rotation rates is KTN polycrystalline. The crack and dislocation in KTN crystal mainly come from improper temperature field. Etching and high-resolution X-ray diffraction （HRXRD） experiment results indicate that the central area is the defects concentrated.