Evolution of the premartensitic state in the NiAl phase of a NiCoCrAlY bond coat during thermal cycling

Transmission electron microscopy (TEM) is used to investigate the evolution of the premartensitic state in the NiAl-based β-phase in NiCoCrAlY bond coats during thermal cycling. Bright-field imaging and diffraction analysis confirm that the lattice modulation is in the form of {110} ?110? transverse displacement waves. High resolution electron microscopy (HREM) revealed an increase in modulation wavelength during thermal cycling due to the β-phase becoming progressively richer in Ni through superalloy–bond coat interdiffusion. Transverse modulations extending over large regions of the crystal were observed in the HREM images, inconsistent with previous theories attributing the origin of the premartensitic state to point defects that couple to the dynamic instability of the lattice.     

Charge ordering modulations in a Bi_0.4Ca_0.6MnO₃ film with a thickness of 110 nm

The low temperature sample stage in a transmission electron microscope is used to investigate the charge ordering behaviours in a Bi 0.4 Ca 0.6 MnO 3 film with a thickness of 110 nm at 103 K.Six different types of superlattice structures are observed using the selected-area electron diffraction(SAED) technique,while three of them match well with the modulation stripes in high-resolution transmission electron microscopy(HRTEM) images.It is found that the modulation periodicity and direction are completely different in the region close to the Bi 0.4 Ca 0.6 MnO 3 /SrTiO 3 interface from those in the region a little further from the Bi 0.4 Ca 0.6 MnO 3 /SrTiO 3 interface,and the possible reasons for this are discussed.Based on the experimental results,structural models are proposed for these localized modulated structures.     

Formation and microstructure of carbon encapsulated superparamagnetic Co nanoparticles

Carbon encapsulated magnetic cobalt nanoparticles have been synthesized by the modified arc-discharge method. Both high resolution transmission electron microscopy (HREM) and powder X-ray diffraction (XRD) profiles reveal the presence of 8–15 nm diameter crystallites coated with 1–3 carbon layers. In particular, HREM images indicate that the intimate and contiguous carbon fringe around those Co nanoparticles is good evidence for complete encapsulation by carbon shell layers. The encapsulated phases are identified as hcp α-Co, fcc β-Co and cobalt carbide (Co₃C) nanocrystals using X-ray diffraction (XRD), nano-area electron diffraction (SAED) and energy dispersive X-ray analysis (EDX). However, some fcc β-Co particles with a significant fraction of stacking faults are observed by HREM and confirmed by means of numerical fast Fourier transform (FFT) of HREM lattice images. The carbon encapsulation formation and growth mechanism are also reviewed.     

Electron microscopy study of incommensurate modulated structures in misfit ternary chalcogenides

The study of misfit structures by means of transmission electron microscopy and associated techniques is reviewed. It is complementary to X-ray crystallography and provides a local and direct view of the beautiful complexity of these systems. Three different types are presented in this work. Misfit columnar structures can be contemplated as a case of one-dimensional misfit structures while misfit layer structures are considered as two-dimensional misfit structures. Some extra dimensionality is added when these misfit layer structures wrap to give rise to tubular crystals. Electron microscopy/diffraction shows clearly in many examples the presence of structural modulations as weak satellite reflections that are very difficult to detect by X-ray diffraction methods. Besides, high-resolution images show the presence of stacking defects in some of these misfit layer structures.     

Diffraction pattern of modulated structures described by Bessel functions

We performed detailed analysis of 1D modulated structure (MS) with harmonic modulation within the statistical approach. By applying two-mode Fourier transform, we were able to derive analytically the structure factor for MS with single harmonic modulation component. We confirmed in a very smooth way that ordinary Bessel functions of the first kind define envelopes tuning the intensities of the diffraction peaks. This applies not only to main reflections of the diffraction pattern but also to all satellites. In the second part, we discussed in details the similarities between harmonically modulated structures with multiple modulations and 1D model quasicrystal. The Fourier expansion of the nodes’ positions in the Fibonacci chain gives direct numerical definition of the atomic arrangement in MS. In that sense, we can define 1D quasicrystal as a MS with infinite number of harmonic modulations. We prove that characteristic measures (like v(u) relation typical for statistical approach and diffraction pattern) calculated for MS asymptotically approach their counterparts for 1D quasicrystal as large enough number of modulation terms is taken into account.     

Direct observation of the CuO chains adsorbed on a ultra-thin film of Cu(110) using a high resolution electron microscope

We have, for the first time, observed directly the CuO chains adsorbed on a Cu(110) surface using a high resolution electron microscope (HREM). This observation was confirmed by a multi-slice image simulation based on electron diffraction and imaging theory. Our calculations indicate quantitatively under what conditions an adsorption layer can be observed by HREM with sufficient contrast. This study reveals the potential of HREM to investigate surface structures of a ultra-thin film with thicknesses of nanometers at atomic level.     

Sr0.6Ba0.2Ca0.2Al2O4 ∶Eu,Dy的制备与长余辉发光性能研究

采用高温固相法制备了Ca,Ba共掺的Sr_0.6Ba_0.2Ca_0.2Al₂O₄ ∶Eu²⁺_0.01, Dy³⁺_0.02和单掺Ba的Sr_0.6Ba_0.4Al₂O₄ ∶Eu²⁺_0.01, 关键词： 长余辉 铝酸锶 稀土掺杂 陷阱能级     

Charge ordering modulations in Bi0.4Ca0.6MnO3 film with a thickness of 110 nm

Low temperature sample stage in transmission electron microscope is used to investigate the charge ordering behaviours in Bi_0.4Ca_0.6MnO₃ film with a thickness of 110 nm at 103 K. Six different types of superlattice structures are observed using selected-area electron diffraction (SAED) technique, while three of them match well with the modulation stripes in high-resolution transmission electron microscopy (HRTEM) images. It is found that the modulation periodicity and direction are completely different in the region close to the Bi_0.4Ca_0.6MnO₃/SrTiO₃ interface from those in the region a little far from the Bi_0.4Ca_0.6MnO₃/SrTiO₃ interface, and the possible reasons are discussed. Based on the experimental results, structural models are proposed for these localized modulated structures.     

Sb2O3的丰度对（1-x）La0.6Dy0.1Sr0.3MnO3/（x/2）（Sb2O3）电输运性质的影响

用固相反应法制备(1-x)La0.6Dy0.1Sr0.3MnO3/x/2(Sb2O3)(x=0.00,0.02,0.15)样品,通过X射线衍射(XRD)图谱,扫描电子显微镜(SEM)照片及SEM能谱(EDS),ρ～T曲线研究样品的结构及电输运性质.结果表明:Sb离子没有进入Mn位,Sb2O3包覆在La0.6Dy0.1Sr0.3MnO3颗粒表面,Sb2O3起助熔剂作用,使得复合样品的颗粒变大且大小相对均匀;复合样品的绝缘体-金属转变温度TP较纯的La0.6Dy0.1Sr0.3MnO3的TP提高20K左右,对x=0.15的样品电阻高峰值比纯的La0.6Dy0.1Sr0.3MnO3的峰值电阻率增大两个数量级,用自旋极化隧穿理论予以解释.     

Domain configurations of the "incommensurate" structures of 2H-TaSe2 and Bi2Sr2(Ca1-xLn(x))Cu2O8+delta studied by HRTEM

High-resolution transmission electron microscopy (HRTEM) images of the "incommensurate" structures of 2H-TaSe2 and Bi2Sr2Ca1-xLnx Cu2O8+delta (Ln: rare earth metal) are shown. They were taken from a wide specimen area with homogeneous thickness. For the former, a configuration of two domains was found by a scrutiny of HRTEM images. For the latter, many configurations of two domains were extracted from the photometric density distribution in the one-dimensional contrast modulations in HRTEM images. One domain of the two in both configurations is created by a phase slip occurring in the primary atomic displacement longitudinal wave.     

A structural model of the orthorhombic Ga7Mn5 derived from its HREM image

The base-centered orthorhombic approximant Ga7Mn5 (a = 4.547(6) nm, b = 1.256(9) nm, c = 1.482(1) nm) consisting of flattened hexagon and concave octagon (called "crown" in this paper) subunits shown by high-resolution electron microscopy (HREM) was found to coexist with the Ga-Mn decagonal quasicrystal in the Ga58Mn42 alloy. The atomic sites in the flattened hexagon subunit have been deduced recently from a structural study of the orthorhombic Ga-Mn (1/1,1/1) approximant that consists only of the hexagon subunits. Similarly, the atomic sites in the crown subunit have been deduced from another orthorhombic Ga-Mn (2/1,1/1) approximant consisting only of the crown subunits. A structural model of the orthorhombic Ga7Mn5 can thus be constructed from these structural subunits. There are 332 Ga atoms and 232 Mn atoms in a unit cell, with a composition of Mn41.2Ga58.8, satisfying the symmetry of the space group Bmm2. The simulated electron diffraction patterns, [010] HREM images, and X-ray powder diffraction pattern based on this structural model agrees mainly with the experimental observations.     

Magnetotransport in a modulated two-dimensional electron gas

We propose a semiclassical theory of dc magnetotransport in a two-dimensional electron gas modulated along one direction with weak electrostatic modulations. We show that oscillations of the magnetoresistivity ρ_∥ corresponding to the current driven along the modulation lines observed at moderately low magnetic fields can be explained as commensurability oscillations.     

On the theory of magnetotransport in a periodically modulated two-dimensional electron gas

A semiclassical theory based on the Boltzmann transport equation for a two-dimensional electron gas modulated along one direction with weak electrostatic or magnetic modulations is proposed. It is shown that oscillations of the magnetoresistivity ρ_∥ corresponding to the current driven along the modulation lines observed at moderately low magnetic fields, can be explained as classical geometric resonances reflecting the commensurability of the period of spatial modulations and the cyclotron radius of electrons.     

Incommensurate modulations in Ba2TiSi2O8, Sr2TiSi2O8, and Ba2TiGe2O8

Incommensurate modulations in Ba₂TiSi₂O₈, Sr₂TiSi₂O₈, and Ba₂TiGe₂O₈ are compared based on their corresponding electron diffraction patterns. The dependence of the modulations on chemical composition provides a suitable model system for the investigation of incommensurations in framework structures using high-resolution transmission electron microscopy (HRTEM). A widening of quantitative HRTEM utilising the concept of rigid units is proposed allowing for a determination of atomistic displacements responsible for the formation of incommensurately modulated structures.     

Modulated crystals and almost periodic measures

Modulated crystals and quasicrystals can simultaneously be described as modulated quasicrystals, a class of point sets introduced by de Bruijn in 1987. With appropriate modulation functions, modulated quasicrystals themselves constitute a substantial subclass of strongly almost periodic point measures. We re-analyze these structures using methods from modern mathematical diffraction theory, thereby providing a coherent view over that class. Similar to de Bruijn’s analysis, we find stability with respect to almost periodic modulations.

     

Eu3+掺杂MMoO4(M=Ca,Sr)荧光粉的表征及发光性质

本文采用简易的Pechini方法制备了Eu3+掺杂MMoO4(M=Ca,Sr)红色荧光粉.产物的结构、形貌和性质分别通过X射线衍射(XRD)、扫描电子显微镜(SEM)、光致发光发射谱(PL)和激发谱(PLE)进行表征分析.XRD表明反应得到了纯相目标掺杂产物,SEM照片显示两种体系掺杂产物形貌均匀性不同,Eu3+掺杂C...     

Influence of the incommensurability in Sr14-xCaxCu24O41 Family Compounds

We study the influence of the structural modulation on the low energy physics of the Sr14-xCaxCu24O41 oxides, using ab initio determination of the on-site and nearest-neighbor effective parameters. The structural modulations appear to be the key degree of freedom responsible for the low energy properties, such as the electron localization, the formation of dimers in the x=0 compound, or the antiferromagnetic order in the x=13.6 compound.     

A TEM and XRD study of the SbNbSe3 misfit layer structures

Two new misfit layer structures have been synthesized within the Sb-Nb-Se system. Powder X-ray diffraction and electron microscopy techniques (electron diffraction, HREM, XEDS) have been used to determine the nature of their structure. According to TEM and XEDS data (for more than 15 crystals studied) both phases are monolayer type, i.e. (SbSe)1+delta (NbSe2). Electron microscopy reveals a composite modulated structure that consists of the periodical intergrowth of a pseudotetragonal SbSe layer, denominated as Q, and a pseudohexagonal layer NbSe2, denominated as H. Both layers fit along b, stack along c and do not fit along a (misfit) giving rise to an incommensurate modulation along this direction. The two phases differ in the symmetry of the Q layers being in one case orthorhombic (for delta = 0.17) and monoclinic in the other (for delta = 0.19). After the characterization of the sample by electron microscopy the unit cells of the basic layers could be refined for both phases by powder X-ray diffraction: aQ = 5.824(2) A, bQ = 5.962(5) A, cQ = 23.927(6) A, alpha = 90 degrees, beta = 90 degrees and gamma = 90 degrees and aH = 3.415(5) A, bH = 5.962(6) A,, cH = 11.962(1) A, alpha = 90 degrees, beta = 90 degrees and gamma = 90 degrees for the orthorhombic phase; aQ = 5.844(2) A, bQ = 5.981(1) A, cQ = 23.919(5) A, alpha = 90 degrees, beta = 90 degrees and gamma = 96.00(3)degrees and aH = 3.439(1) A, bH = 5.994(2) A, cH = 11.956(3) A, alpha = 90 degrees, beta = 90 degrees and gamma = 90 degrees for the monoclinic phase. The phase with the monoclinic Q-sublattice often appears as twinned crystals. The more abundant crystals are disordered intergrowths of both monolayer phases.     

Hydrogen distribution in Nb/Ta superlattices

The distribution of hydrogen in Nb/Ta superlattices has been investigated by combined neutron reflectivity and x-ray scattering. We provide evidence to support that strain modulations determined with x-ray diffraction can be interpreted as modulations in hydrogen content. We show that the hydrogen concentration is modulated and favors Nb, in agreement with previous studies. We measure the concentration directly using neutron reflectivity and demonstrate no detectable change in the distribution of hydrogen with temperature, in stark contrast to previous studies.     

Preparation and characterisation of the double perovskite Sr2TaMnO6

The presented compound, Sr₂TaMnO₆, has a weak, disordered magnetic structure. The metal oxide was prepared under high isostatic oxygen pressure. The doubling of the perovskite structure was proven with electron diffraction and powder neutron diffraction. Combining neutron- and X-ray diffraction data, the room-temperature structure was modelled with the Rietveld method. Both octahedral positions are partially occupied by Mn and Ta, but with different Mn/Ta ratios. AC- and DC-magnetic measurements indicate a magnetic transition at about 17 K and the AC-magnetic susceptibility, both real and imaginary part, is frequency dependent, suggesting that the material has a spin-glass feature. The magnetic spins freeze during a wide temperature range and a possible explanation is a competative situation between the double exchange (ferromagnetism) and the super-exchange (anti-ferromagnetism).