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Cubic Structure (cubic + structure)
Selected AbstractsInfluence of substrate temperature on the properties of electron beam evaporated ZnSe filmsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 4 2010M. G. Syed Basheer Ahamed Abstract ZnSe films were deposited on glass substrates keeping the substrate temperatures, at room temperature (RT), 75, 150 and 250 °C. The films have exhibited cubic structure oriented along the (111) direction. Both the crystallinity and the grain size increased with increasing deposition temperature. A very high value of absorption co-efficient (104 cm -1) is observed. The band gap values decrease from a value of 2.94 eV to 2.69 eV with increasing substrate temperature. The average refractive index value is in the range of 2.39 , 2.41 for the films deposited at different substrate temperatures. The conductivity values increases continuously with temperature. Laser Raman spectra showed peaks at 140.8 cm -1, 246.7 cm -1and 204.5 cm -1which are attributable to 2TA LO phonon and TO phonon respectively. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Characterization of reactive DC magnetron sputtered TiAlN thin filmsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 10 2008B. Subramanian Abstract Thin films of about 1,m Titanium Aluminum Nitride (TiAlN) were deposited onto mild steel substrates by reactive direct current (DC) magnetron sputtering using a target consisting of equal segments of titanium and aluminum. X-ray diffraction (XRD) analysis showed that the TiAlN phase had preferred orientations along 111 and 200 with the face-centered cubic structure. Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) analyses indicated that the films were uniform and compact. Photoluminescence (PL) spectra reveal that TiAlN thin films are of good optical quality. Laser Raman studies revealed the presence of characteristic peaks of TiAlN at 312.5, 675, and 1187.5 cm,1. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Nanostructured copolymer gels for dsDNA separation by CEELECTROPHORESIS, Issue 23 2008Fen Wan Abstract Pluronics are triblock copolymers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) that are able to form many different ordered nanostructures at appropriate polymer concentrations and temperatures in selective solvents. These nanostructured "gels" showed desirable criteria when used as DNA separation media, especially in microchip electrophoresis, including dynamic coating and viscosity switching. A ternary system of F127 (E99P69E99)/TBE buffer/1-butanol was selected as a model system to test the sieving performance of different nanostructures in separating dsDNA by CE. The nanostructures and their lattice constants were determined by small-angle X-ray scattering. Viscosity measurements showed the sol,gel transition phenomena. In addition to the cubic structure, successful electrophoretic separation of dsDNA in 2-D hexagonally packed cylinders was achieved. Results showed that without further optimization, ,X174 DNA,Hae III digest was well separated within 15,min in a 7-cm separation channel, by using F127/TBE/1-butanol gel with a 2-D hexagonal structure. A mechanism for DNA separations by those gels with both hydrophilic and hydrophobic domains is discussed. [source] Nonquenchable Chemical Order,Disorder Phase Transition in Yttrium OxyfluorideEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 1 2005Igor Levin Abstract A chemical order,disorder polymorphic phase transition in yttrium oxyfluoride (YOF) was studied in situ by X-ray and neutron powder diffraction. The high-temperature form of YOF crystallizes with a cubic Fmm fluorite structure in which the O and F atoms are disordered among the tetrahedrally coordinated sites. The low-temperature form of YOF exhibits rhombohedral Rm symmetry and evolves from the high-temperature form by the phase transition associated with the ordering of the O and F atoms. The transition occurs around 560 °C. The superstructure contains layers of [OY4] and [FY4] tetrahedra alternating along the c -axis of the trigonal cell (parallel to the <111> direction of the parent cubic structure). The ordering of the O and F atoms is accompanied by the significant displacements of the Y, O, and F atoms from their ideal positions in the cubic phase. Bond valence sum calculations indicate considerable bond strain for both O and F in the cubic structure; the strain is relieved in the ordered low-temperature phase. The order,disorder transition in YOF is completely reversible and exhibits fast nonquenchable kinetics. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Coaxial Nanorods of MgO Core with Si Shell LayersADVANCED ENGINEERING MATERIALS, Issue 1-2 2007W. Kim This article demonstrates an approach to the synthesis of MgO/Si core-shell nanorods. The Si shell was close to amorphous, whereas the MgO core was crystalline with a cubic structure. The PL of the Si-coated products under excitation at 325 nm exhibited a visible light emission, which was almost identical to that of the uncoated ones. [source] A Novel Narrow Band Red-Emitting Phosphor for White Light Emitting DiodesINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 4 2009Sivakumar Vaidyanathan Research on down conversion phosphor materials is the key for the development of solid-state lighting (SSL). Especially finding alternative red phosphor for white light emitting diodes (LEDs) based on blue or near ultraviolet (NUV) LEDs is important research task. In this view, we have synthesized a series of Eu3+ -substituted La2W2,xMoxO9 (x=0,2, in step of 0.3) red phosphor and characterized by X-ray diffraction (XRD) and photoluminescence. XRD results reveal a phase transition from triclinic to cubic structure for x>0.2. All the compositions show broad charge transfer (CT) band due to CT from oxygen to tungsten/molybdenum and red emission due to Eu3+ ions. Select compositions show high red emission intensity compared with the commercial red phosphor under NUV/blue ray excitation. Hence, this candidate can be a possible red phosphor for white LEDs. [source] Single-Crystalline In2O3 Nanotubes Filled with InADVANCED MATERIALS, Issue 7-8 2003Y. Li Single-crystalline In2O3 nanotubes with cubic structure were synthesized via a physical vapor depostion process. The nanotubes grew along the ,111, direction and were continuously filled with metallic In (see Figure). Photoluminescence measurement results displayed strong orange emission. Indium-filled In2O3 nanotubes represent an exciting system and offer great potential for application in electronic devices or "nanothermometers". [source] Hexagonal and cubic TiOF2JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2010Samuel Shian The chemical, electrochemical, optical and electro-optical properties of titanium oxyfluoride, TiOF2, have led to interest in this compound for a number of applications. Prior analyses have indicated that TiOF2 possesses a simple cubic structure (space group Pmm) at room temperature. Three-dimensional nanostructured assemblies of polycrystalline TiOF2 have recently been synthesized via chemical conversion of intricate SiO2 structures by metathetic reaction with TiF4(g). Rietveld analysis has been used to evaluate the structure of the TiOF2 product formed by such reaction at 623,K. Unlike prior reports, this TiOF2 product possessed a hexagonal structure (space group Rc) at room temperature. Upon heating through 333,338 K, the hexagonal TiOF2 polymorph converted into cubic (Pmm) TiOF2. Differential scanning calorimetry and X-ray diffraction analyses have been used to evaluate this thermally induced phase transformation. [source] Metal Objects Mapping After Small Charge Explosions.JOURNAL OF FORENSIC SCIENCES, Issue 3 2006A Study on AISI 304Cu Steel with Two Different Grain Sizes ABSTRACT: Evidence of exposure of a metal component to a small charge explosion can be detected by observing microstructural modifications; they may be present even if the piece does not show noticeable overall plastic deformations. Particularly, if an austenitic stainless steel (or another metal having a face-centered cubic structure and a low stacking fault energy) is exposed to an explosive shock wave, high-speed deformation induces primarily mechanical twinning, whereas, in nonexplosive events, a lower velocity plastic deformation first induces slip. The occurrence of mechanical twins can be detected even if the surface is damaged or oxidized in successive events. In the present research, optical metallography (OM) and scanning electron microscopy (SEM), and scanning tunneling microscopy (STM) were used to detect microstructural modifications caused on AISI 304Cu steel disks by small-charge explosions. Spherical charges of 54.5 or 109 g TNT equivalent mass were used at explosive-to-target distances from 6.5 to 81.5 cm, achieving peak pressures from 160 to 0.5 MPa. Explosions induced limited or no macro-deformation. Two alloy grain sizes were tested. Surface OM and SEM evidenced partial surface melting, zones with recrystallization phenomena, and intense mechanical twinning, which was also detected by STM and X-ray diffraction. In the samples' interior, only twins were seen, up to some distance from the explosion impinged surface and again, at the shortest charge-to-sample distances, in a thin layer around the reflecting surface. For forensic science locating purposes after explosions, the maximum charge-to-target distance at which the phenomena disappear was singled out for each charge or grain size and related to the critical resolved shear stress for twinning. [source] Synthesis and characterization of cobalt nano-particles through microwave polyol processAICHE JOURNAL, Issue 6 2009Wen-Tung Cheng Abstract This article aims to synthesize and character cobalt nano-particles through microwave polyol process. At first, cobalt acetate is used as precursor to synthesize cobalt nano-particles by microwave assisted ethylene glycol in the presence of polyvinylpyrrolidone (PVP) as capping agent. As measured from ion-coupled plasma (ICP) and thermo-gravimetric analysis (TGA), the yields of as-synthesized cobalt particles in average diameters of 40 and 81 nm verified by transmission electronic microscopy (TEM), are 71 and 85%, respectively; and the structure of as-synthesized cobalt nano-particle is a face-center cubic structure analyzed by X-ray diffraction (XRD). Finally, the as-synthesized cobalt nano-particle modified by oleic acid is dispersed in kerosene to character magnetic property. As measured by vibration sample magnetometer (VSM), the saturation magnetisms of cobalt nano-particles with average diameters of 40 and 81 nm are 39.1 and 29.5 emu/g, respectively, in solution. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Synthesis of hafnium tungstate by a CO2 laser and its microstructure and Raman spectroscopic studyJOURNAL OF RAMAN SPECTROSCOPY, Issue 7 2008E. J. Liang Abstract Densely packed hafnium tungstate blocks were synthesized by rapid solidification with a CO2 laser. It is shown that the optimum synthesis conditions for HfW2O8 are around 700 W laser power and 1 mm/s scan speed. Scanning electron microscopy (SEM) observation shows that the blocks consist of oriented nano-threads/rods that grew horizontally on the surface region and vertically in the interior. The orientations of the nanostructures are governed by the heat transfer directions on the surface and in the interior. Raman spectroscopic and X-ray diffraction studies show that the samples solidified in the cubic structure with minor contents of the orthorhombic phase. This is explained by a compressive stress induced during the rapid solidification process due to a sudden drop of temperature of the molten pool to the ambient. The stress is estimated to be about 0.6 GPa by comparison with high-pressure Raman study. Some specific Raman bands appear in the samples synthesized with the laser synthetic route but not in the sample by solid-state reaction. Copyright © 2008 John Wiley & Sons, Ltd. [source] Raman spectroscopic study on the structure, phase transition and restoration of zirconium tungstate blocks synthesized with a CO2 laserJOURNAL OF RAMAN SPECTROSCOPY, Issue 9 2007E. J. Liang Abstract Densely packed zirconium tungstate blocks were synthesized by rapid solidification with a CO2 laser. The structure and phase transition properties of the samples were studied by Raman spectroscopy and X-ray diffraction. Raman spectroscopic study reveals that zirconium tungstate solidifies with an orthorhombic structure. This is attributed to the pressure encountered by the samples during rapid solidification. Several Raman bands change discontinuously at about 390 K, indicating a phase transition from the ,- to the ,-phase occurring at this temperature. In the ,-phase, most of the Raman modes give rise to negative Grüneisen parameters, suggesting contribution of the corresponding optical phonons to the negative thermal expansion coefficient of the material, at least for the ,-phase. A recovery of the ,-phase was observed when the samples were cooled to room temperature. This suggests that the cubic structure is metastable only at temperatures above 390 K, and at room temperature the ,-phase is preferred. Copyright © 2007 John Wiley & Sons, Ltd. [source] Photoluminescent Properties of SrTiO3:Pr, Al Nanophosphors Synthesized by Microemulsion,Microwave HeatingJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2007Qi Pang A novel approach for the preparation of SrTiO3:Pr, Al nanophosphors by microemulsion,microwave heating is reported in this paper. In comparison with the conventional solid-state sintering processes, this novel method provides a limited small space in a micelle for the formation of nanosized precursors and requires a very short heating time, thus reducing the energy consumption. As a result, small-sized particles with a narrow size distribution and high purity were produced. Transmission electron microscopy characterizations indicated that the synthesized particles were almost spherical with an averaging diameter of ,24 nm and agglomerates slightly. The X-ray diffraction analysis revealed the perovskite cubic structure of SrTiO3:Pr3+ nanoparticles. The luminescent of SrTiO3:Pr, Al phosphors were investigated by photoluminescence. Under 350 nm excitation, SrTiO3:Pr, Al showed a strong red emission, peaking at around 615 nm. The photoluminescence excitation intensity was enhanced by the addition of Al3+ ions. [source] Photoluminescence from Boron-Doped Titanium Nitride Nanocomposite Thin Films Prepared by the Magnetron Sputtering MethodJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2007Sheng-Guo Lu Boron-doped titanium nitride (TiBN) thin films with nanosized grains were prepared by a magnetron sputtering method. X-ray diffraction and transmission electron microscopy observation indicated that TiBN thin films have a cubic structure with grains ,5 nm in size. The photoluminescence (PL) of the films was investigated as a function of temperature over a wavelength range of 350,900 nm. Two PL peaks near 3.20 and 2.38 eV were conisdered to have resulted from the recombination of the donor-bound excitons and deep-trap defects with the holes in the valence band, respectively. An energy transfer from bound electrons to deep-trap defects was observed in the nanocomposite thin film. [source] Structural and Dielectric Characterization of Nanocrystalline (Ba, Pb)ZrO3 Developed by Reverse Micellar SynthesisJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2006Tokeer Ahmad Nanocrystalline zirconates of barium and lead have been synthesized using a modified reverse micellar route (avoiding alkoxides). The entire solid solution of Ba1,xPbxZrO3 (0,x,1) has been synthesized for the first time. Powder X-ray diffraction studies show the monophasic nature of the powders after heating at 800°C except minor impurities of ZrO2 (2%,3%) at a higher lead content (x=0.50 and 0.75). The oxides crystallize in the cubic structure till x=0.25; for higher values, they crystallize in the orthorhombic structure. The particle size obtained from X-ray line-broadening studies and transmission electron microscopic studies is found to be in the range of 20,60 nm for all the oxides obtained after heating at 800oC. The grain size of the solid solution of Ba1,xPbxZrO3 (0,x,1) was found to increase with the lead content. The dielectric constant of the solids corresponding to Ba1,xPbxZrO3 (0,x,1) was found to be a maximum at x=0.50. Note that the cubic to orthorhombic transition is also observed between x=0.25 and 0.5. Dielectric properties with respect to variation in frequency and temperature are reported for these nanocrystalline oxides for the first time. [source] Crystal Morphology of Mesoporous Silica Thin Films Synthesized by the Spin-Coating Method Using PEO,PPO,PEO Triblock CopolymerJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2002Gyeong-Su Park Mesoporous thin films on Si substrates with thicknesses of about 460,610 nm have been synthesized by the spin-coating method using a Pluronic EO77PO29EO77 (F68), EO104PO39EO104 (F88), and EO133PO50EO133 (F108) triblock copolymer system. The triblock copolymers were preserved within the synthesized mesoporous thin films. Transmission electron microscopy (TEM) characterization of these films clearly demonstrates that long-range mesostructural ordering strongly depends on the molecular weight of the poly(ethylene oxide),poly(propylene oxide),poly(ethylene oxide) (PEO,PPO,PEO) triblock copolymer, with lower molecular weight producing higher degrees of order. Plane and cross-sectional high-resolution TEM studies coupled with X-ray diffraction (XRD) analysis also show that highly ordered F68 mesoporous silica thin film forms a cubic structure with a lattice spacing a= 6.70 nm. [source] Comparative Lattice-Dynamical Study of the Raman Spectra of Monoclinic and Tetragonal Phases of Zirconia and HafniaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2002Pierre E. Quintard An interpretation of the Raman spectra of monoclinic ZrO2 and monoclinic HfO2 is made by analyzing the results of the zirconia,hafnia substitution jointly with a lattice dynamical treatment of both structures. The Raman spectra of tetragonal ZrO2 and tetragonal HfO2 are also interpreted. Emphasis is put on their relations to the spectrum of the parent cubic structure and on the position of the soft mode. The band assignment proposed earlier by other researchers is critically reconsidered. [source] Ultra-incompressible and hard technetium carbide and rhenium carbide: First-principles predictionPHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 3 2008Yuan Xu Wang Abstract Using density functional theory, the author predicts that the compounds ReC and TcC with the hexagonal WC-like structure are ultra-incompressible and hard materials. This is concluded from the very large bulk and shear moduli. The phonon dispersion reveals no soft modes indicating the stability of the two materials. The calculated density of states shows that ReC and TcC are metallic. The structural and elastic properties of OsC, IrC, and PtC with hexagonal and cubic structure are also investigated for comparison. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Error propagation in multiscale approaches to the elasticity of polycrystalsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 12 2008Martin Friák Abstract The error propagation properties of the Voigt, Reuss, Voigt,Reuss,Hill,Gilvarry, and Hershey schemes for the determination of the integral elastic response of texture free polycrystalline aggregates with cubic structure were studied. The sensitivity of the homogenized polycrystalline shear modulus was tested (i) analytically on the partial derivatives of the shear modulus with respect to the individual elastic constants within extremal Voigt and Reuss schemes, and (ii) numerically for all four methods. The sensitivity of the Hershey shear modulus on the input parameters, the single crystalline elastic constants B, C ,, C44, is shown to be within the limiting values found for the Voigt and Reuss schemes. This conclusion is illustrated numerically on a set of five cubic materials with very different physical properties. The influence of the bulk modulus was found to be approximately two orders of magnitude smaller than that of C , and C44. The Hershey modulus was also found to be non-linear, asymmetric, and strongly dependent on the level of the elastic anisotropy of the studied system. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Phase transitions and transport phenomena in Li0.25Cu1.75Se superionic compoundPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 15 2004M. Kh. Abstract Phase transformation points in Li0.25Cu1.75Se mixed electronic,ionic conductor have been determined by calorimetric, conductometric and thermoelectric measurements. The phase transformation (PT) from triclinic to monoclinic occurs at 403,413 K. At 503,515 K the monoclinic phase is followed by a rhombohedral modification. Both of these PTs are accompanied by drops on the calorimetric curve. At about 653 K observed anomalies in the temperature dependencies of the ionic conductivity, of the chemical diffusion coefficient and the jump of the ionic Seebeck coefficient have been induced by the PT to hexagonal phase. Neutron diffraction studies reveal the cubic structure of Li0.25Cu1.75Se compound (with space group Fm3m) at 773 K. The corresponding PT causes anomalies in the electrical and diffusion properties at 703,713 K. Cu ions are statistically distributed over tetrahedral and trigonal voids in an Fm3m cage; lithium ions randomly occupy 32(f) positions. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Natural oxidation of InN quantum dots: the role of cubic InNPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2010David González Abstract The natural aging process occured in indium nitride quantum dots (QDs) heterostructures as a consequence of exposure to the atmosphere has been studied by means of transmission electron microscopy and electron beam related techniques. The comparison between GaN-capped and uncapped InN QDs kept at room conditions during 36 months indicates the structural changes that take place. While the capping layer seems to act in a protective way avoiding any change in the QDs, the uncapped structures suffer a series of phase transformations, where the original wurtzite structure is replaced by a layer of cubic phases. The main constituent of this layer is shown to be bcc-In2O3 formed by the substitution of the nitrogen atoms by oxygen from the atmosphere. This supposes a transformation from a hexagonal to a cubic structure, explained by the existence of an oxygen-rich cubic InN acting as an intermediate phase. The difference in the formation enthalpy between the original and the final product, together with the good match between the crystals would explain this transformation that shows the high instability of InN at environmental conditions. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Spontaneous stratification of InGaN layers and its influence on optical propertiesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2009Z. Liliental-Weber Abstract Transmission Electron Microscopy, high resolution X-ray diffraction and reciprocal space maps, Rutherford Back Scattering and photoluminescence were applied to study InGaN layers grown by MOCVD with increasing layer thickness (100 nm to 1000 nm) and nominally constant In concentration of 10%. Spontaneous stratification of the layer has been found. A strained layer with lower than nominal In content was found in direct contact with the underlying GaN followed by relaxed layers with a nominal or higher In concentration. A high density of randomly distributed stacking faults as well as domains with cubic structure and closely distributed stacking faults (polytype-like) were present in the thicker layers. Strong corrugation of the thicker sample surface was observed. The appearance of multiple photoluminescence line positions was related not only to the spontaneously formed layers with different In content, but also to the structural planar defects formed in the thicker layers. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effect of annealing temperature on the crystalline quality and phase transformation of Chemically Deposited CdSe filmsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2005M. Zapata-Torres Abstract Polycrystalline CdSe thin films were grown on glass substrates by chemical bath deposition at 50 ºC. The samples were annealed in air atmosphere at different temperatures and characterized by X-ray diffraction and Raman spectroscopy. It was found that the as-grown films have cubic structure. These samples maintain their cubic structure for annealing temperatures between 60 ºC and 300 ºC. For annealing temperatures higher than 300 ºC we obtain a mixture of cubic and hexagonal phases. The analysis made by X-ray diffraction and Raman dispersion show that the samples annealed at temperatures under the phase-transition temperature increase their crystalline quality. In order to determinate the temperature for the complete transition of the cubic phase, we used the precipitated material obtained during the grown of the CdSe films. This material was annealed on air atmosphere between 300 °C and 500 °C with 50 ° intervals. The samples were measured by X-ray diffraction. The samples maintained the cubic structure if the annealing temperature is under 300 °C. For temperatures between 300 °C and 450 °C we found a mixture of cubic and hexagonal phase. For an annealing temperature of 500°C we obtain only the hexagonal phase. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Solid C60 layer growth on AlN (0001) surface for C60 FET structure by MBEPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7 2003D. Yokoyama Abstract One of the most important problems is how to form an excellent interface between the insulator and the C60 layers in the C60 field-effect doping technique. In this paper, we propose a new structure of the C60 field-effect transistor which consists of an epitaxial AlN layer as the insulator and a C60 epitaxial layer grown on an AlN (0001) surface. We have mainly investigated solid C60 growth on the AlN (0001) surface by the MBE technique in the present work. C60 grains with face centred cubic structure have been obtained on a smooth AlN surface. Four types of the C60 grains were obtained, that is, the 0° orientation and its twin grains and the 30° orientation and its twin grains. We have found that the 0° orientation grains only can be obtained by controlling the growth temperature. The results strongly indicate that an excellent interface between the AlN insulator and the C60 layer can be formed for device applications. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Structure determination of A2M3+TaO6 and A2M3+NbO6 ordered perovskites: octahedral tilting and pseudosymmetryACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2006Paris W. Barnes The room-temperature crystal structures of six A2M3+M5+O6 ordered perovskites have been determined from neutron and X-ray powder diffraction data. Ba2YNbO6 adopts the aristotype high-symmetry cubic structure (space group Fmm, Z = 4). The symmetries of the remaining five compounds were lowered by octahedral tilting distortions. Out-of-phase rotations of the octahedra about the c axis were observed in Sr2CrTaO6 and Sr2GaTaO6, which lowers the symmetry to tetragonal (space group = I4/m, Z = 2, Glazer tilt system = a0a0c,). Octahedral tilting analogous to that seen in GdFeO3 occurs in Sr2ScNbO6, Ca2AlNbO6 and Ca2CrTaO6, which lowers the symmetry to monoclinic (space group P21/n, Z = 2, Glazer tilt system = a,a,c+). The Sr2MTaO6 (M = Cr, Ga, Sc) compounds have unit-cell dimensions that are highly pseudo-cubic. Ca2AlNbO6 and Ca2CrTaO6 have unit-cell dimensions that are strongly pseudo-orthorhombic. This high degree of pseudosymmetry complicates the space-group assignment and structure determination. The space-group symmetries, unit-cell dimensions and cation ordering characteristics of an additional 13 compositions, as determined from X-ray powder diffraction data, are also reported. An analysis of the crystal structures of 32 A2MTaO6 and A2MNbO6 perovskites shows that in general the octahedral tilt system strongly correlates with the tolerance factor. [source] Investigation of the structure and phase transitions in the novel A-site substituted distorted perovskite compound Na0.5Bi0.5TiO3ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2002G. O. Jones Rietveld neutron powder profile analysis of the compound Na0.5Bi0.5TiO3 (NBT) is reported over the temperature range 5,873,K. The sequence of phase transitions from the high-temperature prototypic cubic structure (above 813,K), to one of tetragonal (673,773,K) and then rhombohedral structures (5,528,K) has been established. Coexisting tetragonal/cubic (773,813,K) and rhombohedral/tetragonal (with an upper temperature limit of 145,K between 528 and 673,K) phases have also been observed. Refinements have revealed that the rhombohedral phase, space group R3c, with aH = 5.4887,(2), cH = 13.5048,(8),Å, V = 352.33,(3),Å3, Z = 6 and Dx = 5.99,Mg,m,3, exhibits an antiphase, a,a,a, oxygen tilt system, , = 8.24,(4)°, with parallel cation displacements at room temperature. The tetragonal phase, space group P4bm, with aT = 5.5179,(2), cT = 3.9073,(2),Å, V = 118.96,(1),Å3, Z = 2 and Dx = 5.91,Mg,m,3, possesses an unusual combination of in-phase, a0a0c+ oxygen octahedra tilts, , = 3.06,(2)°, and antiparallel cation displacements along the polar axis. General trends of cation displacements and the various deviations of the octahedral network from the prototypic cubic perovskite structure have been established and their systematic behaviour with temperature is reported. An investigation of phase transition behaviour using second harmonic generation (SHG) to establish the centrosymmetric or non-centrosymmetric nature of the various phases is also reported. [source] X-ray structure determination of the monoclinic (121,K) and orthorhombic (85,K) phases of langbeinite-type dithallium dicadmium sulfateACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2000A. Guelylah The structures of the monoclinic and the orthorhombic phases of type I langbeinite Tl2Cd2(SO4)3 have been determined at 121 and 85,K, respectively, by X-ray diffraction. A precise analysis of these structures shows the existence of some differences compared to langbeinites of type II. The monoclinic structure differs very little from the high-temperature cubic structure and the distortion relating the monoclinic structure to the cubic one is very small. SO4 tetrahedra seem to rotate under orthorhombic symmetry in the monoclinic phase. A symmetry distortion analysis of the ferroelectric monoclinic distortion discloses the importance of the secondary modes with orthorhombic symmetry, especially for the O atoms of the SO4 groups. [source] Hexacopper(I) phosphorus(V) bromide penta(selenide/sulfide), Cu6P(Se0.7S0.3)5BrACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2008A. Gagor This work illustrates possible diffusion paths for CuI ions in a highly disordered structure of a superionic conductor of the argyrodite family. The Cu6P(Se0.7S0.3)5Br cubic structure is built from a [P(Se0.7S0.3)5Br] framework in which CuI ions are distributed in various tetrahedral, triangular and linear sites. There are two types of disorder in the structure. The first type results from the fact that there are fewer CuI ions than the number of positions available for them in the unit cell. The second type is due to the static distribution of Se and S atoms in the [P(Se0.7S0.3)5Br] framework. The title compound is a solid solution of two efficient ionic conductors, namely Cu6PSe5Br and Cu6PS5Br, in which high ionic conductivity results from order,disorder phenomena in the copper substructure. To shed light on the distribution of CuI ions in disordered Cu6P(Se0.7S0.3)5Br, we refined their positions using a combination of a nonharmonic approach and a split-atom model. At room temperature, CuI ions show strong anharmonic vibrations along the edge of the (Br)4 tetrahedra. The probability density functions of the CuI ions overlap and reveal possible diffusion paths. [source] Tetranuclear titanium 7,7,-modified binaphtholate cluster as a novel chiral Lewis acid catalyst,CHIRALITY, Issue 9 2001Koichi Mikami Abstract Chiral tetranuclear Ti cluster, a cubic structure constituted of four Ti atoms and OHs, and six (R)-binaphthols (BINOL) bridged two Ti atoms as ligands, is shown to be a novel chiral Lewis acid catalyst for the [2+3] cycloaddition reaction with nitrones. The chiral Ti clusters with 7,7,-substituted (R)-BINOL ligands was synthesized to give enhanced enantiomeric excesses up to 78% ee. Chirality 13:541,544, 2001. © 2001 Wiley-Liss, Inc. [source] Thermal behaviour of cubic phases rich in 1-monooleoyl- rac -glycerol in the ternary systemFEBS JOURNAL, Issue 1 2003- d -glucoside/water, 1-monooleoyl- rac -glycerol/n -octyl- Using synchrotron X-ray diffraction the thermal behaviour was studied of the cubic phases in the 1-monooleoyl- rac -glycerol (MO)/n -octyl-,- d -glucopyranoside (OG)/2H2O system with 58 or 45 wt % MO concentration and varying OG/2H2O contents. These MO contents correspond to a Pn3m cubic single-phase or a Pn3m cubic phase in excess water on the binary MO/water axis of the ternary phase diagram. The cubic liquid crystalline phases are stable with small fractions of OG, while higher OG concentrations trigger a cubic-to-lamellar phase transition. Moreover, with increasing OG concentration the initial Pn3m structure is completely converted to an Ia3d structure prior to the L, phase being formed. Upon heating this effect is reversed, resulting in an Ia3d -to- Pn3m phase transition. For some samples additional peaks were observed in the diffractograms upon heating, resulting from the metastability notoriously shown by bicontinuous cubic phases. This judgement is supported by the fact that upon cooling these peaks were absent. Remarkably, both the Ia3d and the Pn3m cubic structures could be in equilibrium with excess water in this ternary system. A comparison is made with previous results on n -dodecyl-,- d -maltoside (DM), showing that cubic phases with OG have higher thermal and compositional stability than with DM. [source] | ||||||||||||||||