Niggli约化胞在确定Bravais点阵中的应用

Ｎｉｇｇｌｉ约化胞的唯一性使其能应用于确定晶体的Ｂｒａｖａｉｓ点阵。本文在Ｎｉｇｇｌｉ约化条件的基础上，讨论了Ｎｉｇｇｌｉ约化胞到Ｂｒａｖａｉｓ点阵的转换关系，通过电子衍射方法，介绍了Ｎｉｇｇｌｉ约化胞在确定未知晶体Ｂｒａｖａｉｓ点阵中的应用。     

图解法标定未知点阵的电子衍射图

报道了标定未知点阵的电子衍射图的图解法。当某一衍射点列与倾转轴基本一致时，绕此点列转动晶体，得到不同转角下的一系列衍射图。将这些衍射图画在以倾转轴为Ｘ轴的直角坐标系内，由各衍射点的坐标可以求出倒易点阵初基胞的参数，经变换、约化得出相应的布喇菲点阵的类型和参数，同时对电子衍射图指标化。此图解法直观、简便，适用于任意晶体点聩，尤其是对称性不高的晶体点阵使用起来较为方便。     

新型闪烁晶体NaBi(WO4)2的研制及闪烁性能的研究

报道了新型闪烁晶体ＮａＢｉ（ＷＯ４）２的生长。测试了晶体的透过率，抗辐照损伤能力和发光效率，ＮａＢｉ（ＷＯ４）２晶体的衰减为３ｎｓ，是闪烁晶体中最短者之一。     

OH^—和氧离子杂质在BaF2晶体辐照损伤的影响

从理论和实验两方面研究ＯＨ＾－和氧离子杂质为ＢａＦ２晶体辐照损伤的影响，并对其机理进行讨论，理论上用ＨＦＳ－ＤＶＭ－Ｘα）局域密度离散变分法计算ＯＨ＾－和氧离子杂质心在ＢａＦ２中的电子结构，得到ＯＨ＾－，Ｈｓ＾－（Ｕ心），Ｏｓ＾－，Ｏｓ＾２－和（Ｏｓ＾２－－Ｆ＾＋）都可能是引起辐照损伤的源泉。实验发现，ＢａＦ２晶体水解处理后，ＯＨ＾－和氧离子杂质很容易进入ＢａＦ２，在晶体中的存在形式主要是：ＯＨ＾     

OH~－和氧离子杂质对BaF_2晶体辐照损伤的影响

从理论和实验两方面研究ＯＨ－和氧离子杂质对ＢａＦ２晶体辐照损伤的影响，并对其机理进行讨论．理论上用（ＨＦＳ－ＤＶＭ－Ｘα）局域密度离散变分法计算ＯＨ－和氧离子杂质心在ＢａＦ２中的电子结构，得到ＯＨ－，Ｈｓ－（Ｕ心），Ｏｓ－，Ｏｓ２－和（Ｏｓ２－－Ｆ＋）都可能是引起辐照损伤的源泉．实验发现，ＢａＦ２晶体水解处理后，ＯＨ－和氧离子杂质很容易进入ＢａＦ２，在晶体中的存在形式主要是：ＯＨ－占据阴离子位置，氧离子以Ｏｓ２－的形式占据Ｆ－的晶格位置，并由氟空位（Ｆ＋）作电荷补偿，较大可能以（Ｏｓ２－Ｆ＋）形式存在．γ辐照前后水解处理样品的光吸收谱（ＶＵＶ，ＵＶ，ＩＲ）和电子顺磁共振谱（ＥＰＲ）验证了理论计算的正确性．综合理论和实验，我们认为ＯＨ－氧离子杂质引起ＢａＦ２晶体辐照损伤的主要原因是：ＯＨ－和（Ｏｓ２－Ｆ＋）辐照分解成Ｈｓ－（Ｕ心）和Ｏｓ－．上海硅酸盐研究所在晶体生长工艺中，有意识地对ＯＨ－和氧离子给予特别的注意，在改进辐照损伤上获得较好的效果．     

HL—1M装置硼化,硅化和锂化壁的出气特性

借助ＱＭＳ诊断技术，完成了ＨＬ－１Ｍ装置Ｂ化，Ｓｉ化与Ｌｉ化壁的出气特性的对比实验研究。ＧＤＣ期间，Ｈ２出气占支配地位，杂质分压与Ｈ２分压成正比，ＧＤＣ后的本底放气率较同时间烘烤去气后低；ＧＤＣ（Ｈｅ＋Ｈ２）是去除膜的有效方法；Ｂ化、Ｓｉ化和Ｌｉ化壁相比，托卡马了放电后的Ｈ２出量绋１：０．１３：０．２１。其中Ｂ化壁Ｈ２出气量主于ｓｓ壁，Ｓｉ化和Ｌｉ化壁则低于ｓｓ壁，出Ｈ２量／送Ｈ２量，ｓｓ壁为０．     

同步辐射的基本知识 第三讲 同步辐射中的散射术及其应用（四）

3．3 动力学结构的非弹性散射研究 3．3．1 晶体内的点阵动力学 晶体点阵动力学理论已有专著,试验研究的文献很多。由于还无法就晶体结构类型作总结分析,这里选一些典型例子作简单介绍。由3．1节可知,声子的特征矢量e（Q,j）描述了晶体单胞中原子的振动方向。在晶体的两个主要对称轴上存在两个不同的振动模式,     

Ca_4RO(BO_3)_3(R=Gd,Y)晶体空间方向的确定

介绍了高质量Ｃａ4ＧｄＯ（ＢＯ3）3（ＧｄＣＯＢ）、Ｃａ4ＹＯ（ＢＯ3）3（ＹＣＯＢ）晶体的生长，报道了两种晶体空间方向的确定方法；对于该类低对称性非线性光学晶体的实用化具有推动意义．     

YCOB晶体生长与激光倍频性能研究

坩埚下降法沿＜０１０＞和＜００１＞方向生长了直径达到２５ｍｍ的完整透明的Ｃａ４ＹＯ（ＢＯ３）３ （ＹＣＯＢ）晶体．化学腐蚀结果表明，所生长晶体无孪晶或亚晶界等缺陷，晶体尾部的位错密度 不超过１８００／ｃｍ２测量了ＹＣＯＢ的透射光谱，其截止波长为２００ｎｍ进行了ＹＣＯＢ晶体对 Ｎｄ：ＹＡＧ激光的二次倍频实验．通过与ＫＤＰ晶体对比，计算出ＹＣＯＢ晶体的有效非线性系数 在Ⅰ型相位匹配方向（θ，φ）＝（６６．３°，１４３．５°）和（６５．９°，３６．５°）上分别为１．４５ｐｍ／Ｖ和０．９１ｐｍ／Ｖ； 大于ＫＤＰ和 ＬＢＯ晶体．在脉冲宽度１０ｎｓ的 Ｎｄ：ＹＡＧ激光单脉冲辐射下ＹＣＯＢ晶体出现体 损伤的激光损伤阈值不低于８５ＧＷ／ｃｍ２．     

可调蓝光皮秒脉冲

利用３５１ｎｍ紫外光泵浦非临界相位匹配ＬｉＢ３Ｏ５光参量振荡器（ＬＢＯ－ＯＰＯ）,直接获得了在４７２￣４５３ｎｍ范围内波长连续可调的蓝图光皮秒脉冲序列。ＯＰＯ所用ＬＢＯ晶体的两端面切割成Ｂｒｅｗｓｔｅｒ角,避开了晶体表面增透膜的稳定性问题,保证了ＬＢＯ晶体端面低反射损耗和ＯＰＯ长期稳定工作。     

Formation of all fourteen Bravais lattices of three-dimensional photonic crystal structures by a dual beam multiple-exposure holographic technique

We make use of a dual beam multiple-exposure (DBME) holographic technique for the formation of all 14 Bravais lattices of three-dimensional photonic crystal microstructures. For simplicity of experimental implementation, the DBME method has been modified such that, prior to each exposure, once the proper angle between the wave vectors of the interfering beams is chosen, a single axis rotation of the recording medium gives the desired results. The parameters required for the generation of the lattice structures have been derived by appropriate modification of interference of four noncoplanar beams (IFNB) analysis for corresponding implementation in the DBME technique, and the results have been verified by computer simulations. After giving a comparative study of the results with the IFNB method, recording geometries for the DBME approach are also proposed in order to realize all 14 Bravais lattices experimentally.     

Nuclei for heterogeneous formation of graphite spheroids in ductile cast iron

Abstract

The crystal structure and composition of nuclei for graphite spheroids in a ductile cast iron containing small amounts of magnesium and traces of aluminium have been studied in a transmission electron microscope equipped with EDX and parallel electron energy loss spectrometer and in an electron microprobe analyser. The particles were identified as Al–Mg–Si nitrides, having a trigonal superlattice crystal structure derived from a hexagonal AlN type fundamental cell. The superlattice can be indexed according to a hexagonal Bravais lattice, with parameters a=0.544 nm and c=0.482 nm. The parameters of the fundamental cell are a _f=0.314 nm and c _f=0.482 nm, deviating only 1–3% from the parameters of hexagonal AlN. Based on the compositional analysis, the chemical formula of the nitride particles is suggested to be AlMg_2.5Si_2.5N₆.     

Formation of three-dimensional periodic microstructures by interference of four noncoplanar beams

A newly reported method of making three-dimensional microstructures or photonic crystals by holographic lithography has some obvious advantages over other techniques with the same purpose. A systematic and comprehensive analysis of interference of four noncoplanar beams (IFNB) is provided. It shows that all 14 Bravais lattices can be formed by means of IFNB and gives explicit relationships between each lattice and the corresponding recording geometry. The concept of pattern contrast is extended to the case of IFNB, and it is indicated that a uniform contrast for each interference term can be obtained by properly choosing the beam ratio and polarization. A calculation algorithm is then developed to optimize the direction of polarization of each beam to ensure maximum uniform contrast. These results, verified by computer simulations, may lay a theoretical foundation for fabrication of photonic crystals with the approach of IFNB.     

Layer-like cation ordering in a perovskite-type compound Ca3(CaTa2)O9 with monoclinic symmetry

A structural scheme to fit in a monoclinic cell of the ordered perovskites Ca₃(XZ₂)O₉ is proposed based on a layer-like ordering of X and Z ions. According to this scheme, the structure of compound Ca₃(CaTa₂)O₉ was analyzed with X-ray powder method. The structure is found to involve an alternate stacking of one Ca-layer and two Ta-layers in the octahedral cation sites, and has a base-centered monoclinic Bravais lattice with the constants a=9.808 A,b=5.533 A,c=7.061 A, and ß=89°1′. The space group is $\text{C}\text{2}\text{m-C}{}_{\mathrm{2h}}{}^3$. The unit cell contains six triclinic perovskite units with the lattice constants a′=b′=4.000 A, c′=3.996 A, α′=ß′=89°32′, and γ′=87°32′.     

Microstructural mechanics of granular media

A geometrically and physically linear micromechanical theory for elastic granular media is presented, based on the identification of the constituent grains with the nodes of a Bravais lattice. Adjacent particles are permitted to displace normally and transversely to each other, and to rotate with respect to the doublet axes. Thus, microstrains of the axial, torsional, and shear type are generated. The conjugate microstresses are then defined. Through a variational formulation, the microstress equations of motion are derived, together with natural boundary conditions and the transition from the microstresses to the macrostresses. The principles of thermodynamics are employed to derive the most general, invariant, and appropriately symmetric microconstitutive equations, and to close the system of field equations for the granular medium, subject to both adiabatic and non-adiabatic processes. The problem of a granular semispace loaded by compressive boundary force is solved as an application, and the existence of locally tensile microstresses is determined, while the associated macrostresses are computed to coincide with the well-known Flamant's solution and thus to be compressive everywhere.     

Material design of elastic structures using Voronoi cells

New tools for the design of metamaterials with periodic microarchitectures are presented. Initially, a two‐scale material design approach is adopted. At the structure scale, the material effective properties and their spatial distribution are obtained through a Free Material Optimization technique. At the microstructure scale, the material microarchitecture is designed by appealing to a Topology Optimization Problem (TOP). The TOP is based on the topological derivative and the level set function. The new proposed tools are used to facilitate the search of the optimal microarchitecture configuration. They consist of the following: (i) a procedure to choose an adequate shape of the unit cell domain where the TOP is formulated and shapes of Voronoi cells associated with Bravais lattices are adopted and (ii) a procedure to choose an initial material distribution within the Voronoi cell being utilized as the initial configuration for the iterative TOP.     

Lattice Symmetry and Identification—The Fundamental Role of Reduced Cells in Materials Characterization

In theory, physical crystals can be represented by idealized mathematical lattices. Under appropriate conditions, these representations can be used for a variety of purposes such as identifying, classifying, and understanding the physical properties of materials. Critical to these applications is the ability to construct a unique representation of the lattice. The vital link that enabled this theory to be realized in practice was provided by the 1970 paper on the determination of reduced cells. This seminal paper led to a mathematical approach to lattice analysis initially based on systematic reduction procedures and the use of standard cells. Subsequently, the process evolved to a matrix approach based on group theory and linear algebra that offered a more abstract and powerful way to look at lattices and their properties. Application of the reduced cell to both database work and laboratory research at NIST was immediately successful. Currently, this cell and/or procedures based on reduction are widely and routinely used by the general scientific community: (i) for calculating standard cells for the reporting of crystalline materials, (ii) for classifying materials, (iii) in crystallographic database work (iv) in routine x-ray and neutron diffractometry, and (v) in general crystallographic research. Especially important is its use in symmetry determination and in identification. The focus herein is on the role of the reduced cell in lattice symmetry determination.     

Lattice Matching (LM)—Prevention of Inadvertent Duplicate Publications of Crystal Structures

Lattice-matching techniques have proved to be extremely effective for the identification of unknown crystalline materials. A commonly employed lattice-matching strategy is based on matching the reduced cell of an unknown against a database of known materials represented by their respective standard reduced cells. The success of the method relies on the fact that the lattice or the lattice plus chemical information (e.g., element types) is highly characteristic of a material—like a fingerprint. Because of its intrinsic power, the procedure has many and diverse applications—in materials characterization, in nano-technology, in epitaxial growth, in materials design, etc. An especially fruitful role for the method is in the journal publication process as the quality of the scientific literature can be enhanced. The focus herein is on the major role that lattice matching can play in the prevention of inadvertent duplicate publications of the same structure and in the determination of key cross-references.     

Unit cell parameters of wurtzite InP nanowires determined by x-ray diffraction

High resolution x-ray diffraction is used to study the structural properties of the wurtzite polytype of InP nanowires. Wurtzite InP nanowires are grown by metal-organic vapor phase epitaxy using S-doping. From the evaluation of the Bragg peak position we determine the lattice parameters of the wurtzite InP nanowires. The unit cell dimensions are found to differ from the ones expected from geometric conversion of the cubic bulk InP lattice constant. The atomic distances along the c direction are increased whereas the atomic spacing in the a direction is reduced in comparison to the corresponding distances in the zinc-blende phase. Using core/shell nanowires with a thin core and thick nominally intrinsic shells we are able to determine the lattice parameters of wurtzite InP with a negligible influence of the S-doping due to the much larger volume in the shell. The determined material properties will enable the ab initio calculation of electronic and optical properties of wurtzite InP nanowires.     

Dislocations in crystals and in continua: A confrontation

We consider Bravais crystals under volume-conserving plastic deformation. The crystal is the prominent prototype of a metric solid. In fact, it allows the measurement of lengths by counting atomic steps and applying Pythagoras' law. This remains true even when , in a thought experiment, the lattice spacing is made smaller and smaller. If in this process care is taken for the conservation of the local dislocation content, then the distance between the scaled down dislocations, when measured in (scaled down) atomic distances goes to infinity so that the dislocations remain discrete objects in the now-called continuized crystal. In particular Frank's definition of the dislocation remains valid. Since numbers and types of discrete dislocations can be determined, at least in thought experiments, the dislocation density can be introduced as an explicit state quantity in the continuized crystal. By explicit state quantities we understand those which enter the energy expression explicitly as independent variables.

On the other hand, in ordinary structureless continua dislocations, as defined by Burgers, are not state quantities. As a result, these dislocations do not appear as dynamical variables in the pertaining field theory, whereas dislocations in crystals do. Hence the mechanical theory of (ordinary structureless) continua, proven most valuable in many applications, is not appropriate in situations where the internal mechanical state (here the dislocation state) suffers significant changes.