PubMed Health⌕ Search
Home/Archive/PubMed

PubMed · 2809768

Convergent beam diffraction studies of interfaces, defects, and multilayers.

D Cherns·A R Preston
View original source ↗Export metadata ↓

Abstract

This paper explains how the convergent beam electron diffraction (CBED) and large angle convergent beam (LACBED) techniques can be used to study crystal defects, bicrystals, and multilayers. It is shown how the LACBED technique in particular can be used to derive the magnitude and sign of the Burgers vectors of dislocations and displacements at stacking faults. For bicrystals and multilayers examined in plan-view, LACBED gives the rocking curve for a chosen reflection. This enables layer strains to be measured to approximately 0.1% and composition profiles derived for both periodic and aperiodic structures. It is shown that a simple kinematic approach can be used to interpret the essential results in most cases.

Explore related subjects

Chemical PhenomenaChemistry, PhysicalCrystallographyMicroscopy, ElectronJournal Article

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

D Cherns, A R Preston. 1989. Convergent beam diffraction studies of interfaces, defects, and multilayers.. https://doi.org/10.1002/jemt.1060130204

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related citations

PubMed2001

Classification of CD and absorption spectra in the Soret band of H(2)TMPyP bound to various synthetic polynucleotides.

The binding mode of porphyrins, namely meso-tetrakis(N-methyl pyridinium-4-yl)porphyrin (H(2)TMPyP), was classified in this work by absorption and circular dichroism(CD) spectroscopy. The three binding modes of intercalation, minor groove binding and external stacking exhibit their own characteristic absorption and CD spectra. Intercalation occurs for this porphyrin when bound to GC-rich polynucleotides at a low mixing ratio, as expected. This binding mode produces hypochromism and a red shift in the absorption band and a negative CD band in the Soret absorption region. When it is complexed with AT-rich polynucleotides at a low mixing ratio, hypochromism and a red shift in the absorption band and a positive CD peak is apparent, and this species can easily be assigned to the minor groove-binding mode. For both AT- and GC-rich polynucleotides at a high binding ratio, an excitonic CD was apparent. The sign of excitonic CD depends on the order of the DNA bases; the CD spectra of H(2)TMPyP complexed with non-alternating homopolymer (disregarding the nature of base pairs, i.e. AT or GC) are characterized by a positive band at short wavelengths followed by a negative band at long wavelengths. In contrast, those complexed with alternating polynucleotide were opposite to those of non-alternating homopolymers.

Chemical Phenomena↗
PubMed2001

Hydrogen-bonding cavities about metal ions: synthesis, structure, and physical properties for a series of monomeric M-OH complexes derived from water.

The tripodal ligand N[CH2CH2NHC(O)NHC(CH3)3]3 ([H61]) was used to synthesize a series of monomeric complexes with terminal hydroxo ligands. The complexes [Co(II/III)H31(OH)](2-/1-), [Fe(II/III)H31(OH)](2-/1-), and [Zn(II)H31(OH)](2-) have been isolated and characterized. The source of the hydroxo ligand in these complexes is water, which was confirmed with an isotopic labeling study for [Co(III)H31(OH)](1-). The synthesis of [M(II)H31(OH)](2-) complexes was accomplished by two routes. Method A used 3 equiv of base prior to metalation and water binding, affording yields of < or = 40% for [Co(II)H31(OH)](2-). When 4 equiv of base was used (method B), yields ranged from 50% to 70% for all of the M(II)H31(OH)](2-) complexes. This improvement is attributed to the presence of an intramolecular basic site within the cavity, which scavenges protons produced during formation of the M(II)-OH complexes. The molecular structures of [Zn(II)H31(OH)](2-), [Fe(II)H31(OH)](2-), [Co(II)H31(OH)](2-), and [Co(III)H31(OH)](1-) were examined by X-ray diffraction methods. The complexes have trigonal bipyramidal coordination geometry with the hydroxo oxygen trans to the apical nitrogen. The three M(II)-OH complexes crystallized with nearly identical lattice parameters, and each contains two independent anions in the asymmetric unit. The complexes have intramolecular H-bonds from the urea cavity of [H31](3-) to the coordinated hydroxo oxygen. All the complexes have long M-O(H) bond lengths (>2.00 A) compared to those of the few previously characterized synthetic examples. The longer bond distances in [M(II)H31(OH)](2-) reflect the intramolecular H-bonds in the complexes. The five-coordinate [Zn(II)H31(OH)](2-) has an average Zn-O(H) distance of 2.024(2) A, which is similar to that found for the zinc site in carbonic anhydrase II (2.05(2) A). The enzyme active site also has an extensive network of intramolecular H-bonds to the hydroxo oxygen. [Co(II)H31(OH)](2-) and [Fe(II)H31(OH)](2-) have one-electron redox processes at -0.74 and -1.40 V vs SCE. Both complexes can be chemically oxidized to yield their corresponding M(III)-OH complexes. [Co(III)H31(OH)](1-), with an S = 1 ground state, is a rare example of a paramagnetic Co(III) complex.

Chemical Phenomena↗
PubMed2001

Characterization of binary solvent mixtures of DMSO with water and other cosolvents.

Binary mixtures of DMSO with nine different cosolvents were characterized in light of the pure solvent scales, using suitable probe/homomorph couples. Various physical (vapor pressure, surface tension, viscosity, and enthalpy of mixing) and spectroscopic (IR and NMR) properties of the DMSO/water mixtures are described in terms of their polarity, acidity, and basicity, and the descriptions are examined with a view to establishing their potential physical significance.

Chemical Phenomena↗