Domain population in SrTiO3 below the cubic-to-tetragonal phase transition

High-angle double-crystal X-ray diffractometry (HADOX) has been applied to measure the intensity around three reciprocal lattice points: 400, 040 and 004 of SrTiO₃. Each point splits into two in the tetragonal phase according to the appearance of three kinds of domains; the temperature dependence of the intensity of component peaks at these points was measured under identical conditions. The results show that the domain population varies with temperature in a range of about 20 K below the transition. It is found, however, that the total intensities of 400 and 004 of the tetragonal phase are constant; this is observed by summing up the relevant intensity at the three reciprocal lattice points.     

A New Method for the Measurement of Thickness in Single Crystals

A new method for thickness determination of single-crystal thin samples at exact zone axis orientation, based on pattern recognition in convergent beam electron diffraction (CBED), is presented. The method is especially well suited to materials with a large unit cell in zone axis directions where the reciprocal lattice is uniformly dense with diffraction points. The new method is based on comparison of a measured CBED zone axis pattern with a set of calculated ones. Its accuracy was estimated to be around 10% in the 5–100 nm thickness range as checked for garnets at the [111] zone orientation.     

Phase transitions in RbCaF3. II: Neutron scattering

The 196 K phase transition of RbCaF₃ has been identified as a phonon condensation at the R point (cubic [111] zone boundary). Above the transition temperature, a ridge of scattering extending from the [110] zone boundary (M point) to the R point was observed, corresponding to a line of soft phonons from M₃ to R₂₅. Below the transition the scattering at M decreases rapidly with temperature, indicating a lattice stabilization which causes the ridge of scattering to disappear.     

透射式GaAs光电阴极AlGaAs/GaAs外延层在热应力作用下晶格弯曲的倒易点二维图研究

本文介绍了倒易点及其倒易点二维图的概念、倒易点形状及其倒易点二维图同晶格形变的关系,推导了热应力作用下双层膜发生弯曲的理论公式,阐明了晶格弯曲的原因和晶格倒易点二维图展宽的方向,分析了GaAs光电阴极AlGaAs/GaAs外延层的实测倒易点二维图,最后提出了降低AlGaAs/GaAs外延层中的热应力的技术途径.     

New Form for the Coulomb Potential in Complex Crystals. Part III

The present paper continues our works [1, 2] devoted to the construction of the Coulomb potential at an arbitrary point of the unit cell of crystals with a close-packed hexagonal (CPH) lattice by the Green's function method. Convergence of the real space and reciprocal lattice sums in the Green's function is investigated and the optimal convergence parameter is chosen. The method is used to construct the Coulomb potential in Ti, Co, and Zn metals. Calculations are performed for four directions. Nonmonotonic behavior of nuclear and electron components of the potential in the cell is established. The crystal potential is also calculated for these metals with allowance for the exchange interaction in the Slater approximation.     

Selective adsorption of 4He on clean LiF (001) surfaces

The selective adsorption of ⁴He on in-situ cleaved LiF surfaces has been studied under improved resolution. The main results are as follows: (1) There are four bound states in the surface potential well, at energies of ?5.8, ?2.2, ?0.6 and ?0.1 meV. The lowest three levels were reported previously. (2) Most of the structure previously designated as “fine structure” is due either to transitions to these four levels via various small reciprocal lattice vectors or to the opening of diffraction channels. (3) The transitions involving the [01] and [01?] reciprocal lattice vectors (i.e., the ones nearly perpendicular to the incident wave vector) are strong; as much as 85% of the specular intensity may be removed. Transitions via the other small reciprocal lattice vectors are much weaker. (4) The widths of the lines are consistent with the velocity distribution, which has a half-width of about 2%. (5) The observed energies agree fairly well with those calculated by Tsuchida for a zeta-function potential, but are not consistent with a Morse potential.     

High-resolution X-ray diffraction analysis of SnTe/Sn1−xEuxTe superlattices grown on (1 1 1) BaF2 substrates

A set of SnTe/Sn_1−xEu_xTe superlattice (SL) samples with increasing nominal Eu content x up to 0.28 was successfully grown on (1 1 1) BaF₂ substrates by molecular beam epitaxy. A complete structural characterization was performed by triple-axis X-ray diffractometry and reciprocal space mapping. The X-ray results showed that, despite the phase separation that normally occurs for unstrained Sn_1−xEu_xTe layers with x0.02, an SL stack with homogeneous individual layers can be formed for SL samples with a nominal Eu content up to 0.16. No SL satellite peak structure could be identified for samples with x values higher than 0.24. The structural parameters of the individual layers that compose the SL were determined using a best-fit simulation procedure which compared the calculated X-ray spectra to the measured (2 2 2) ω/2Θ scans. The strain information used in the simulation was obtained from the reciprocal space maps measured around the (2 2 4) lattice point.     

Neutron scattering study of phonons associated with indirect band-gap transition in AgBr

Phonons in AgBr are studied by neutron inelastic scattering along the [111] direction of reciprocal lattice. From the phonon dispersion and the optical data, it is concluded that the valence band maximum is at the L-point and the phonons participating in the indirect band-gap transition are identified as TO(L) and LA(L).     

不同时效状态的含铜4%的铝合金在疲劳载荷下溶质原子与位错的交互作用

为了进一步研究在疲劳载荷下含铜4％的铝合金中的位错钉扎过程,进行了经过不同时效的试样的扭转疲劳试验,测定了经过各种应力循环数N以后的滞后迴线的形状和面积,从而算出了在每次循环中的能量消耗ΔE和最大抗扭矩T_m。所选择的时效温度和时效时间是使试样中分别有G.P.[1]区,G.P.[2]区,θ′相和稳定的θ相出现。把所得的ΔE-N曲线和T_m-N曲线的变化情况作比较时可以看出,在疲劳载荷的起始阶段引起位错钉扎的并不是由于相变产物如G.P.[1]或[2]区的作用。比较并分析了在各种时效状态下的第一周能量消耗值(ΔE)₁的变化,结果指出,在所研究的铝铜合金的情形,产生ΔE的原因是由于在位错附近的点阵中有起伏的内应力场出现,因为位错在这种内应力场中往复运动需要作功。产生这种起伏的应力场的因素有点缺陷(空位和溶质原子)、原子簇、G.P.[1]区和G.P.[2]区,或者其他种不在位错线上聚集或成核的缺陷。根据上述分析,可以认为,在疲劳载荷中,使位错钉扎的是由于溶质原子气团的形成。溶质原子在疲劳过程中通过空位的帮助进入位错,形成气团,使位错被钉扎。被钉扎的位错的动性减低,因而ΔE下降。在时效过程中,在位错线上成核的θ′和θ相,对于位错线也起着一定的钉扎作用。由上述的图象还可以推知,G.P.[1]区和G.P.[2]区不是在位错线上成核的,而θ′相和θ相则是在位错线上成核的。     

Study on the lattice distortion of the As-prepared nanosized TiO2 particles via detonation method

Quantitative XRD measurements of the nanosized TiO₂ particles obtained from the detonation soot have been carried out. The lattice parameters, such as grain size, cell volume, lattice constants and lattice strain were obtained. The relationships between the change ratio of cell volume (the reciprocal of the particles size, or the mass ratio of explosive and TiO₂ precursor) and the lattice strain of the different TiO₂ phases were also discussed. The relationship between the change ratio of cell volume and the particle size of TiO₂ particles was also studied. The results demonstrated that with the decreasing of the particles size, the lattice strain of anatase phase increased, while the lattice strain of rutile phase increased firstly and then decreased to some extent. It is different from the linear relationship between the lattice distortion and the reciprocal of the particles size reported in other literatures. In the meantime, the lattice strains were different with the different mass contents of RDX in the microstructures of the TiO₂ particles. The direct reflection of microstructure changes is the changes of the particle size of TiO₂ particles. Based on the XRD results, the particular characteristics of the detonation process and interfacial effects of nanocrystalline materials, a crude explanation was also given.     

碲镉汞辐射损伤的微观过程

利用高分辨率的电子显微技术,在原子和晶格尺度上,研究了碲镉汞晶体结构在大剂量高能电子作用下,晶体结构产生损伤、扩展和重构的过程。研究结果表明,碲镉汞结构损伤是在强电子束作用下,[(Hg,Cd)Te₄]四面体的形变,在{111}面产生并沿{111}面呈阶梯形扩展的过程。从而在[110]方向观察到了新的更大周期的两维重构网格。 关键词：     

Influence of surface disorder on RHEED patterns from GaAs(001)−2 × 4 surfaces

Reflection high-energy electron diffraction measurements have been carried out for a nearly perfect and a disordered GaAs(001)−2 × 4 surface in the [ 10] azimuth. The surfaces were prepared in-situ by molecular beam epitaxy and the measurements were made at a substrate temperature of 560 °C under different flux conditions, giving the different degrees of surface ordering. Diffracted intensity distributions along various reciprocal surface lattice rods were measured for different angles of incidence using a primary beam energy of 15 keV. Drastic differences with surface order were found for both the (quasi) elastically diffracted beams (rocking curves) and the diffuse beams. The rocking curves can be interpreted largely by Bragg reflection from the bulk crystal and a number of causes for the intensity changes are given. In the case of the disordered surface the intensity distributions along the reciprocal lattice rods are dominated by broad diffuse peaks which occur at the intersection of reciprocal rods and Kikuchi lines. It is shown that while a kinematical model is unable to explain the experimental observations, a previously published model provides a very satisfactory interpretation.     

用赝势微扰法计算某些半导体的能带结构(用于GaAs,GaP和Ga[As1-xPx]合金)

用赝势微扰法计算了GaAs,GaP和Ga[As_1-xP_x]合金的能带。赝势选择的原则是使计算所得直接能隙和间接能隙与实验值相符合。计算结果表明,不但能带次序准确,而且与室温下的实验值符合得很好。基于由GaAs到GaP晶格常数和赝势是线性变化的假设,计算了GaP含量为20％,50％和80％时Ga[As_1-xP_x]合金的能带。当GaP含量为41％时,直接能隙和间接能隙相等,这一数值刚好是Spitzer和Fenner的实验值的平均值。此时,由于很好满足光激射器p-n结所要求的条件,因此可望在光激射器中得到应用,它们的能带也就有一定的参考价值。     

混合自旋(S=1,1/2)模型具有再次近邻反铁磁相互作用晶格基态

本文发展文献[1]的方法到两种不同自旋的原子(S_a=1,S_b=1/2)构成的晶格中,计算了简单立方晶格在具有再次近邻反铁磁相互作用下在外磁场中的基态自旋结构、能量和相界。从文中给出的相图可知:这种晶格有两种反铁磁自旋结构,有四种亚铁磁自旋结构。 关键词：     

单斜晶系多晶X射线衍射图指标化的一种解析法

本文提出一种比较简便且适用性强的用电子计算机进行单斜晶系化合物衍射图指标化的解析法。根据各衍射线对晶面间距倒数平方值Q之差δ值的重复数m(δ-m图)及其比例关系确定晶胞参数b。经退化步骤将各倒易阵点投影到a^*c^*平面上,每三个倒易阵点组合可求出一对倒易矢量间的夹角。根据夹角重复数M(β-M图)确定β角。从而可求出晶胞参数a,c。最后用叠代修正逼近指标化方法进行全衍射图的指标化。给出了计算机程序流程图。用四水醋酸镍和钨酸钾等作为实例,证明了本指标化方法是适用的。 关键词：     

Self-consistent inhomogeneity of quantum wells in II–VI semiconductors

An X-ray diffraction method that uses a slightly diverging (3′) beam and maximally attainable diffraction angles ? _B (as large as 77°) was developed to study quantum wells (QWs) with widths of 5–8 nm separated by wide (100–220 nm) barrier layers. The advantage of this method compared to the use of a parallel beam is an increase by two orders of magnitude in the intensity of the beam incident on the sample and an increase in the probability of diffraction for all QWs as a unified single crystal. It is found that the growth on GaAs substrates misoriented by 10° from the (001) plane in the [111]_II direction brings about monoclinization of crystal lattices of the QW layers and barrier layers in opposite directions. Inhomogeneity of composition over the thickness of each well is observed. In the case of growth of a ZnSe/ZnMgSSe structure in which the layers have a crystal-lattice period close to the lattice period of the GaAs substrate, the QWs are inhomogeneously doped with elements from the composition of the barrier layers. The inhomogeneity of QW composition observed in the growth of mismatched layers in ZnCdSe/ZnSSe and ZnCdS/ZnSSe structures is caused by the fact that mismatch between the lattice parameters of QWs and barriers stimulates the growth of self-consistent compositions; this occurs due to a decrease in the Cd concentration in the Zn_1?x Cd _x Se QW in the initial stages of growth compared to the Cd concentration in the flow of gases and an increase in the Zn concentration in the Cd_1?x Zn _x S QW at small values of x up to the concentration matching GaAs (x = 0.4). The mismatch stresses are partially relaxed via dislocations with the (111)_II glide planes, as a result of which is observed the combination of rotation of the crystal planes of the layers and QW around the [1\(\overline 1 \)0] axis and almost cylindrical bending of the entire sample around the perpendicular [110] axis. Mismatch between lattice parameters of the ZnMgSSe barrier layers and the substrate brings about decomposition of these layers into two phases; this decomposition is caused by thermodynamic instability of the alloy.     

A reexamination of theI.S. results for the cluster compound Au55(PPh3)12Cl6

The observed lowI.S. values [1] for the four distinct gold sites in the cluster compound Au₅₅(PPh₃)₁₂Cl₆ have been explained in terms of a decrease of the 6s electron density at the nucleus, due either to a lattice expansion or to a delocalization of 6s electrons over the ligand shell surrounding the gold core. Recent EXAFS measurements [2] indicate a single average distance between the gold atoms, about 3.5% less than for bulk gold. This not only excludes the lattice expansion hypothesis, it effectively increases the density of 6s electrons, making an explanation for the observedI.S. even more difficult. For an understanding of theI.S. values it is necessary to reconsider the probable occupation of the 5d orbitals within the framework provided by the XPS surface atom core level shifts developed by Citrin and Wertheim [3]. Partial confirmation has been found in preliminary XPS results [5]. The consequences for thef-factor and specific heat results [1] will also be examined.     

Detection of faceting on (hk0) surfaces of thin gold films by diffraction in the electron microscope at 100 kV

A series of single crystal thin films of gold with (hk0) surfaces ranging between (100) and (310), grown by vapor deposition, was studied in transmission in the electron microscope at 100 kV. For specimens intermediate between (100) and (310) both bright and dark field images contained line structure running parallel to [001]. In certain cases segments were observed lying at 45° and 90° to these lines. Diffraction patterns from these specimens revealed relrods at each reciprocal lattice point running parallel to [100] and [310]. In certain cases [301] and [301?] relrods were also present. No line structure or relrods were detectable for the specimens with (100) or (310) surfaces. The results are consistent with a model in which the (100) and (310) surfaces are essentially flat but surfaces with intermediate orientations consist of appropriate mixtures of (100) and (310) facets plus (301) and (301?) facets in certain cases. The line structure in the images is attributed to “thickness contrast” arising from variations in the specimen thickness due to the facets. The relrods are well explained by the specimen form factor as originally proposed by von Laue. The results prove that faceted structures on thin film surfaces can be conveniently and systematically studied under certain conditions by conventional transmission electron microscopy at 100 kV.     

Lattice stability and the effect of Co and Re on the ideal strength of Ni:First-principles study of uniaxial tensile deformation

Using first-principles density functional calculations, lattice stability of γ-Ni under [001], [110], and [111] uniaxial tensions and the effect of alloying elements Co and Re on the uniaxial tensile behavior of γ-Ni were studied in this paper.With elastic constants and phonon spectra calculations, we examined the mechanical stability and phonon stability of Ni during the uniaxial tensions along the three characteristic directions. The results show that the mechanical stability and phonon stability of a lattice occurs before the maximum stress–strain point under the [001] and [111] tension, respectively.The effects of Co and Re on the ideal tensile strength of γ-Ni show a significant directivity: Co and Re have little effect on the stresses in [001] and [111] directions, but increases the ideal strength of the system in the weakest uniaxial tensile direction. Moreover, the strengthening effect of Re is significantly better than that of Co. By further analyzing electronic structure, it is found that the effect of alloying elements on the uniaxial tensile behavior of γ-Ni comes from their interactions with host atoms.