Abstract: 'Natural' platelets are planar defects in {001} planes found in natural type-IaA/B diamonds. 'Man-made' platelets are platelets formed in the laboratory by annealing type-IaA diamonds at temperatures over 2500 degrees C. Careful study shows that the infrared (IR) spectra of the 'man-made' platelets are different from the IR spectra of 'natural' platelets. High-temperature (T greater than or equal to 2000 degrees C) annealing of platelets containing type-IaA/B diamonds modifies the IR absorption spectrum owing to the 'natural' platelets and makes it similar to the IR spectrum of the 'man-made' platelets. It is suggested that such high-temperature annealing changes the structure of the 'natural' platelets. The changes are too subtle to be detected by electron microscopy techniques. Topographic electron-energy-loss spectroscopy shows that platelets contain nitrogen at an average density of 0.7 atoms per a(0)(2); however, high-temperature annealing does not seem to affect the concentration of the nitrogen in the platelets.

Abstract: Ultralong Cu(OH)(2) and CuO nanowire bundles with lengths ranging from tens to hundreds of micrometers have been selectively synthesized on a large scale by a facile solution-phase method, using PEG200 as growth-directing agent. The growth mechanisms were investigated by monitoring the nanowire evolution process. The results showed that under the action of PEG200 molecules, the Cu(OH)(2) and CuO nanowires were first formed through oriented attachment of colloidal particles, then through side self-assembly leading to nanowire bundles, and finally to CuO nanoleaves. PEG200 plays a critical role in the synthesis of nanowires as it not only prevents the random aggregation of colloidal particles toward CuO nanoleaves but also helps to orientate nanowire growth by the coalescence and alignment in one direction of the colloidal particles. The concentration of OH(-) in the reaction system is also important for nanowire growth. In the absence of PEG200, nanoleaves are formed by an Ostwald ripening process. The band-gap value estimated from a UV-Vis absorption spectrum of CuO nanowire bundles is 2.32 eV. The photodegradation of a model pollutant, rhodamine B, by CuO nanowires and nanoleaves was compared with commercial nanopowders, showing that the as-synthesized ultralong CuO polycrystalline nanowire bundles have an enhanced photocatalytic activity with 87% decomposition of rhodamine B after an 8-h reaction, which was much higher than that of single-crystal nanoleaves (61%) and commercial nanopowders (32%). The origin of the high photocatalytic activity of these new polycrystalline CuO nanowire bundles has been discussed. This present work reveals that the (0 0 2) crystallographic surface is more favorable for photocatalytic decomposition of organic compounds and that these ultralong CuO nanowire bundles are potential candidates for photocatalysts in wastewater treatment. (C) 2010 Elsevier Inc. All rights reserved.

Abstract: By means of electron diffraction the 2 root 2 a(p) x 2 root 2 a(p) phase, well-known in YBa2Cu3O7-delta was observed in two other perovskite-based materials (Y0.75Ce0.25)(2)(Sr0.85Y0.15)(2)AlCU2O9 and Bi1.8Pb0.4Sr2Ca2Cu3O10+x. Highly correlated ordering is observed in the ab-plane, the correlation along the c-direction being weak. The plane group of the superstructure symmetry elements was determined on the basis of observed reflection conditions in diffraction patterns. Our results unambiguously rule out oxygen ordering as a possible origin of the superstructure. Experimental evidence points out that the superstructure is associated with the CuO2 layers, that are the only structural elements common to the three compounds studied. A model is proposed where the CuO2 sheet is displacively modulated. Experimental evidence suggests a correlation between adjacent CuO2 sheets. Comparison of simulated and experimental [001] zone diffraction patterns strongly supports our model.

Abstract: Tetrahedral chain ordering in the Sr2MnGaO5 structure is studied using electron diffraction (ED) and high-resolution electron microscopy. The ED patterns show the presence of satellite reflections, which indicate a commensurately modulated structure with a = 5.4056(8) Angstrom b 16.171(3) Angstrom, c = 5.5592(7) Angstrom, q – 1/2c*, superspace group Immma(00gamma,)s00. The Superstructure arises due to ordering of the two types of symmetry related tetrahedral chains (L and R) according to a ... LRLR ... sequence, where L and R chains alternate along the c-axis within the same (GaO) layer. Numerous defects at different structural levels were observed, comprising interleaving L and R chains, violation of the ... LRLR ... chain sequence within one layer, different stacking modes of the ... LRLR ... ordered layers with subsequent alternation of blocks of different width along the h-axis of the brownmillerite subcell and island fragmentation of the modulated superstructure. By in situ heating ED experiments it is found that the long-range ordering of the tetrahedral chains is stable tip to 665degreesC and is completely suppressed at 905degreesC. (C) 2003 Elsevier Inc. All rights reserved.

Abstract: The structure and defect structure of Bi2-xPbxSr3.5Cu2 (CO3)O8-delta compounds with 0 less-than-or-equal-to x less-than-or-equal-to 0.75 are carefully investigated by electron diffraction and high-resolution electron microscopy. All compounds have an orthorhombic structure with a almost-equal-to b almost-equal-to 5.4 angstrom and c almost-equal-to 39.5 angstrom. The length of the b-axis decreases monotonically with increasing x. The space group for the basic structure is Abm2. The structure can be considered as an intergrowth of Bi2Sr2CuO6 lamellae with Sr2CuO2 (CO3) lamellae along the c-axis. CO3 groups behave as bridges connecting the CuO6 octahedra. In the x = 0 compound the carbon atoms are shifted away from their symmetry positions; the orientational ordering of the CO3 groups (or the carbon shift) in successive CO planes alternates along +b and -b. Typical Bi-type and Pb-type modulations are found along the b-axis up to a Pb content x = 0.5. Electron beam irradiation destroys the ordering of the CO3 groups and alters the modulated structure. (C) 1994 Academic press, inc.