Abstract: Basic results concerning oxygen ordering in the superconducting compound YBa2Cu3Oz are briefly summarized. It is shown that, at equilibrium, only infinite-chain structures can be stabilized and those models based on hypothetical (and actually nonphysical) screened Coulomb interactions cannot produce stable ground states. It is suggested that diffraction data (neutrons, X-rays, electrons) from oxygen-lean samples are indicative of metastable displacive transformations, and are not directly related to oxygen ordering.

Abstract: In this study of the changes induced by exposing MWCNTs to a nitrogen plasma, it was found by HRTEM that the atomic nitrogen exposure does not significantly etch the surface of the carbon nanotube (CNT). Nevertheless, the atomic nitrogen generated by a microwave plasma effectively grafts amine, nitrile, amide, and oxime groups onto the CNT surface, as observed by XPS, altering the density of valence electronic states, as seen in UPS. (C) 2008 Elsevier Ltd. All fights reserved.

Abstract: Ti-MCM-41 nanoparticles 80-160 nm in diameter (Ti-MCM-41 NP) were successfully prepared by a dilute solution route in sodium hydroxide medium at ambient temperature. Ti-MCM-41 NP were characterized by X-ray diffraction, nitrogen adsorption/desorption isotherms, SEM, TEM. FT-IR, and UV-vis spectroscopy. The characterization results showed the existence of highly ordered hexagonal mesoporous structure and tetrahedral Ti species in Ti-MCM-41 NP. In the epoxidation of cyclohexene with aqueous H2O2, Ti-MCM-41 NP displayed higher conversion and initial reaction rate than a Ti-MCM-41 sample with normal particle size (Ti-MCM-41 LP). Diffusion of the reactants was accelerated and the accessibility to the catalytic Ti species was enhanced in the shorter channels in Ti-MCM-41 NP samples. Ti-MCM-41 NP showed much higher selectivity for cyclohexene oxide compared with Ti-MCM-41 LP, suggesting reduced hydrolysis of cyclohexene oxide with water in the former case. The increased selectivity for cyclohexene oxide can be attributed to the lower concentration of residual surface silanols in Ti-MCM-41 NP and the shorter residence time of epoxide in the shorter mesoporous channels. Ti-MCM-41 NP also appears to be a suitable catalyst in the epoxidation of a bulky substrate, like cholesterol, with tert-butyl hydroperoxide. (c) 2007 Elsevier Inc. All rights reserved.

Abstract: Structural, chemical, and physical properties of whitlockite-type Ca9Bi(VO4)(7) were studied by X-ray powder diffraction (XRD), electron diffraction (ED), second-harmonic generation (SHG), thermogravimetry, differential scanning calorimetry, dielectric, and electrical-conductivity measurements. A new phase-transition of the ferroelectric type was found in Ca9Bi(VO4)(7) with a transition temperature, T-c of 1053 +/- 3 K. The polar phase, beta-Ca9Bi(VO4)(7), is stable below T-c down to at least 160 K. The centrosymmetric beta'-phase is stable above T-c up to 1273 +/- 5 K. Above 1273 K, it decomposes to give BiVO4 and whitlockite-type solid solutions of Ca9+1.5xBi1-x(VO4)(7). The beta<---->beta' phase transition is reversible and of second order. Electrical conductivity of beta'-Ca9Bi(VO4)(7) is rather high (sigma = 0.6 x 10(-3) S/cm at 1200 K) and obeys the Arrhenius law with an activation energy of 1.0 eV. Structure parameters of Ca9Bi(VO4)(7) are refined by the Rietveld method from XRD data measured at room temperature (space group R3c; Z = 6; a = 10.8992(1) Angstrom, c = 38.1192(4) Angstrom, and V = 3921.6(1) Angstrom(3); R-wp = 3.06% and R-p = 2.36%). Bi3+ ions together with Ca2+ ions are statistically distributed among the M1, M2, M3, and M5 sites. Ca9Bi(VO4)(7) has a SHG efficiency of about 140 times that of quartz. Through the powder SHG measurements, we estimated the nonlinear optical susceptibility, Digital, at about 6.1-7.2 pm/V. This value for Ca9Bi(VO4)(7) is comparable with that for known nonlinear optical materials such as LiNbO3 and LiTaO3.

Abstract: The present paper describes the synthesis, characterization of mixed-valence bismuthates with three- or two-dimensional perovskite-like structures and structural criteria that influence superconductivity in these compounds. Single-phase samples of Sr1-xKxBiO3 were prepared for the broad range of K-content: 0.25 less than or equal to x less than or equal to 0.65. For these bismuthates the symmetry of the structure changes from monoclinic to orthorhombic and finally to tetragonal upon increasing the K-content thus resulting in the decrease of the Bi-O distances and reduction of the network distortions. Superconductivity with maximum T-c = 12 K exists in the narrow range (x approximate to 0.5-0.6) within the stability field of the tetragonal phase (0.33 less than or equal to x less than or equal to 0.65), when the three-dimensional octahedral framework has close to the ideal perovskite structure arrangement. The layered type (Ba,K)(3)Bi2O7 and (Ba,K)(2)BiO4 bismuthates belonging to the A(n+1)B(n)O(3n+1) homologous series were investigated. Buckling of the (BiO2) layers in the structure of the n = 2 member occurs due to the ordering of alkaline- and alkaline-earth cations between two independent positions. The formation of the one-layer bismuthate was revealed by Electron Microscopy and XRPD studies. Both types of compounds are considered to be possible candidates for new superconducting materials among bismuthates. (C) 2002 Published by Elsevier Science B.V.

Abstract: Results of dielectric and resonance frequency (f(r)) measurements below room temperature are presented for Pb(ZrxTi1-x)O-3, x = 0.515 and 0.520. It is shown that the temperature coefficient of f(r) changes sign from negative to positive around 210 and 265 K for x = 0.520 and 200 and 260 K for x = 0.515. Anomalies in the real part of the dielectric constant (epsilon') are observed around the same temperatures at which the temperature coefficient of f(r) changes sign because of the electrostrictive coupling between the elastic and dielectric responses. Low-temperature powder x-ray-diffraction (XRD) data, however, reveal only one transition from the tetragonal to monoclinic phase similar to that reported by Noheda et al. [Phys. Rev. B, 61, 8687 (2000)]. Electron-diffraction data, on the other hand, reveal yet another structural transition at lower temperatures corresponding to the second anomaly in the epsilon' vs T and f(r) vs T curves. This second transition is shown to be a cell-doubling transition not observed by Noheda et al. in their XRD studies. The observation of superlattice reflections raises doubts about the correctness of the Cm space group proposed by Noheda et al. for the monoclinic phase of Pb(ZrxTi(1-x))O-3 below the second transition temperature.

Abstract: YCuO2 delafossite crystallizes into two stacking variants; hexagonal 2H or rhombohedral 3R, depending on the preparation conditions. The structure of the fully oxygenated material YCuO2.50 has been determined as orthorhombic (a(O) = 6.1961 Angstrom; b(O) = 11.2158 Angstrom; c(O) = 7.1505 Angstrom; space group Pnma). The structure is based on the hexagonal 2H structure (a(O) = a(H)root3; b(O) = c(H); c(O) = 2a(H)). Upon incomplete oxidation, a different YCuOZ phase with ideal composition YCuO2.33 and lattice parameters a(H root)3, a(H)root3, c(H) is also formed. Diffraction patterns are often very complex because of the presence of planar defects and intergrowth of both phases. Under electron beam irradiation, oxygen is released from the structure and one phase gradually transforms into the other. (C) 2001 Academic Press.

Abstract: We present a detailed analysis of the point-defect clustering in strained Si/Si(1-x)Ge(x)/(001)Si structures, including the interaction of the point defects with the strained interfaces and the sample surface during 400 kV electron irradiation at room temperature. Point-defect cluster formation is very sensitive to the type and magnitude of the strain in the Si and Si(1-x)Ge(x) layers. A small compressive strain (-0.3%) in the SiGe alloy causes an aggregation of vacancies in the form of metastable [110]-oriented chains. They are located on {113} planes and further recombine with interstitials. Tensile strain in the Si layer causes an aggregation of interstitial atoms in the forms of additional [110] rows which are inserted on {113} planes with [001]-split configurations. The chainlike configurations are characterized by a large outward lattice relaxation for interstitial rows (0.13 +/-0.01 nm) and a very small inward relaxation for vacancy chains (0.02+/-0.01 nm). A compressive strain higher than -0.5% strongly decreases point-defect generation inside the strained SiGe alloy due to the large positive value of the formation volume of a Frenkel pair. This leads to the suppression of point-defect clustering in a strained SiGe alloy so that SiGe relaxes via a diffusion of vacancies from the Si layer, giving rise to an intermixing at the Si/SiGe interface. In material with a 0.9% misfit a strongly increased flow of vacancies from the Si layer to the SiGe layer and an increased biaxial strain in SiGe bath promote the preferential aggregation of vacancies in the (001) plane, which relaxes to form intrinsic 60 degrees dislocation loops.

Abstract: We compare the magnetic properties of pure and oxidized Co nanoparticles embedded in an amorphous Al2O3 matrix. Nanoparticles with diameters of 2 or 3 nm were prepared by alternate pulsed laser deposition in high vacuum conditions, and some of them were exposed to O-2 after production and before being embedded. The nanoparticles are organized in layers, the effective edge-to-edge in-depth separation being 5 or 10 nm. The lower saturation magnetizations per Co atom for the samples containing oxidized nanoparticles provide evidence for the formation of antiferromagnetic CoO shells in the nanoparticles. None of the samples with Co/CoO nanoparticles show exchange bias, while vertical hysteresis loop shifts and enhanced coercivities (as compared to samples with pure Co nanoparticles) are observed. This constitutes evidence for the nanoparticles size being in all cases smaller than the critical size for exchange bias. The difference in coercivity versus temperature dependences for the samples with pure and oxidized Co nanoparticles shows that the exchange anisotropy in Co/CoO nanoparticles appears at temperatures lower than 50 K. (c) 2007 American Institute of Physics.

Abstract: High-quality single-walled carbon nanotubes (SWNTs) are synthesized in gram amount on Fe-Mo/MgO catalysts by catalytic decomposition of CH4 in H-2 or N-2. Raman data reveal that the as-prepared SATNTs have a diameter of about 0.74-1.29 nm. It is found that the diameter of the as-prepared SWNTs can be controlled mainly by adjusting the molar ratio of Fe-Mo versus the MgO support. Several other factors that potentially influence the growth of SWNTs have been studied in detail. The experimental results show that the nature of the catalyst determines the diameter of the as-prepared SWNTs. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: The dissociation of 60degrees and screw dislocations in diamond is modeled in an approach combining isotropic elasticity theory with ab initio-based tight-binding total-energy calculations. Both dislocations are found to dissociate with a substantial lowering of their line energies. For the 60degrees dislocation, however, an energy barrier to dissociation is found. We investigate the core structure of a screw dislocation distinguishing “shuffle,” “mixed,” and “glide” cores. The latter is found to be the most stable undissociated screw dislocation. Further, the glide motion of 90degrees and 30degrees partials is discussed in terms of a process involving the thermal formation and subsequent migration of kinks along the dislocation line. The calculated activation barriers to dislocation motion show that the 30degrees partial is less mobile than the 90degrees partial. Finally, high-resolution electron microscopy is performed on high-temperature, high-pressure annealed natural brown diamond, allowing the core regions of 60degrees dislocations to be imaged. The majority of dislocations are found to be dissociated. However, in some cases, undissociated 60degrees dislocations were also observed.

Abstract: The large-scale production of single-wall carbon nanotubes (SWNTs) is reported. Large quantities of SWNTs can be synthesised by catalytic decomposition of methane over well-dispersed metal particles supported on MgO at 1000 degrees C. The thus produced SWNTs can be separated easily from the support by a simple acidic treatment to obtain a product with high yields (70-80%) of SWNTs. Because the typical synthesis time is 10 min, 1 g of SWNTs can be synthesised per day by this method. The SWNTs are characterized by high-resolution transmission electron microscopy and by Raman spectroscopy, showing the quality and the quantity of products. (C) 2000 Elsevier Science B.V. All rights reserved.

Abstract: Insulating and metallic stripes above and below the Curie temperature, T-C, respectively, were observed by a high-resolution scanning tunneling microscopy (STM) and/or spectroscopy (STS) in A-site ordered and macroscopically strain free epitaxial La0.75Ca0.25MnO3 film grown on MgO substrate. The “insulating” stripes were found to be incommensurable to the lattice and aligned along (110) direction. Metallic stripes were commensurable with periodicity 2a(p)similar to 0.8 nm and aligned parallel to the crystallographic a/b-axis. Formation of these stripes involves competing charge, orbital, and lattice orders and is an outcome of an overlapping of electron wave functions mediated by the local lattice-strain distribution, existed even in A-site ordered film due to the difference in cation radii of La and Ca. (C) 2007 Elsevier B.V. All rights reserved.

Abstract: The atomic structure of single-wall carbon nanotube bundles produced by three different techniques has been characterized by electron diffraction and microscopy. Small bundles produced by Catalytical Chemical Vapor Deposition (CCVD) exhibit only one or two tube chiralities within a single bundle while bundles produced by arc-discharge or laser-ablation exhibit more chiralities. A detailed analysis of the central line of diffraction is also presented. The CCVD nanotubes present more intense spots around 1.7 Angstrom(-1) < k < 2Angstrom(-1) (k is the momentum transfer) compared to what is observed for nanotubes produced by other methods. Amongst the possible explanation for such an anomaly, we put forward that in this range of momentum transfer, the relative tube orientations and translations are important for what concerns the interpretation of the diffraction peaks intensities.

Abstract: Yittria-stabilized zirconia (YSZ) films doped with 3 and 9 vol%. Y(2)O(3), respectively, are epitaxially deposited on (0 0 1) silicon substrates by means of pulsed laser deposition (PLD) technique. Transmission electron microscopy (TEM) and X-ray diffraction are mainly combined to study the film microstructure. It is: found that the film structure strongly depends on the amount of Y(2)O(3) dopant. 99/0 Y(2)O(3)-doped films display a near cubic structure; 45 degrees 1/2(1 1 0) dislocations are the main defects in the film and thermal cracks are formed during cooling. The 3% Y(2)O(3)-doped films are dominated by {1 1 0} twin-related tetragonal domains in which monoclinic phase is found. The films are free of thermal cracks even for films thicker than 2 mum. (C) 2001 Elsevier Science B.V. All rights reserved.