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Cubic Phase (cubic + phase)
Selected AbstractsNonquenchable 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] 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] Synthesis of Hexagonal Yb3+,Er3+ -Doped NaYF4 Nanocrystals at Low TemperatureADVANCED FUNCTIONAL MATERIALS, Issue 19 2009Helmut Schäfer Abstract Nanocrystals of NaYF4 doped with Yb3+ and Er3+ are synthesized in oleylamine using Y2(CO3)3, Yb2(CO3)3, Er2(CO3)3, Na2CO3, and NH4F as precursors. In contrast to other starting materials normally used for such syntheses, these precursors react even at room temperature to form hexagonal-phase (, -phase) NaYF4:Er,Yb nanoparticles. Cubic-phase (, -phase) NaYF4:Yb,Er particles are formed only at elevated temperatures (>250,°C). The formation of the cubic phase at high temperatures can be suppressed by replacing pure oleylamine with oleic acid/oleylamine mixtures. Under optimized reaction conditions, particles with an average particle size of about 7,nm are generated in 84% yield. Heat treatment (30,min, 280,°C) of the particles significantly increases the luminescence efficiency. A transparent solution of the heat-treated, nanometer-sized phosphor in toluene shows intense visible light emission upon excitation in the near infrared. [source] A Stimuli-Responsive, Photoluminescent, Anthracene-Based Liquid Crystal: Emission Color Determined by Thermal and Mechanical ProcessesADVANCED FUNCTIONAL MATERIALS, Issue 12 2009Yoshimitsu Sagara Abstract Here, a photoluminescent liquid crystal that exhibits a change of emission color on the metastable,stable phase transition induced by external stimuli is prepared. A 2,6-diethynylanthracene derivative with amide groups and dendritic side chains exhibits a columnar phase on slow cooling from the isotropic phase and shows blue emission in this columnar phase. In contrast, a cubic phase is obtained by rapid cooling from the isotropic phase. In the cubic phase, the 2,6-diethynylanthracene cores form excimers, resulting in yellow emission. While the columnar phase is a stable liquid-crystalline (LC) phase, the cubic phase is a metastable LC phase. It is found that a change of the photoluminescent color from yellow to blue is observed on the cubic-columnar phase transition induced by heating or mechanical shearing for this 2,6-diethynylanthracene derivative in the cubic phase. This change of photoluminescent color is ascribed to the inhibition of excimer formation on the metastable,stable LC phase transition. [source] Synthesis and Optical Properties of KYF4/Yb, Er Nanocrystals, and their Surface Modification with Undoped KYF4,ADVANCED FUNCTIONAL MATERIALS, Issue 19 2008Helmut Schäfer Abstract KYF4/Yb3+, Er3+ nanocrystals with a mean diameter of approximately 13,nm were synthesized at 200,°C in the high boiling organic solvent N -(2-hydroxyethyl)ethylenediamine (HEEDA). The particles crystallize in the cubic phase known from , -NaYF4 and form transparent colloidal solutions in tetraethylene glycol (TEG) or propanol. Solutions containing 1,wt % of the nanocrystals in TEG display visible upconversion emission upon continuous wave (CW) excitation at 978,nm. Growing undoped KYF4 on the surface of the KYF4/Yb3+, Er3+ nanocrystals increases the upconversion efficiency by more than a factor of 20. The XRD data of these particles, display a slight increase in the mean particle size from 13 to 15.5,nm, indicating that only a part of the subsequently added KYF4 shell material is deposited onto the particle surface. Nevertheless the performed surface modification obviously leads to core/shell structured particles. [source] Crosslinked Bicontinuous Cubic Lyotropic Liquid-Crystal/Butyl-Rubber Composites: Highly Selective, Breathable Barrier Materials for Chemical Agent Protection,ADVANCED MATERIALS, Issue 24 2006X. Lu A breathable material that can act as a barrier to chemical agents is presented. The material is composed of butyl rubber and a crosslinkable lyotropic liquid crystal. Under the right synthesis conditions, a cubic phase with the desired selectivity properties results (see figure). The material is highly permeable to water vapor, but strongly impermeable to a mustard gas simulant. Application of the material for effective personal protection against such chemical agents is envisaged. [source] X, -DVM modelling of oxygen transport in YBa2Cu3O7-, and SrFeO3INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 10 2006Elmira I. YuryevaArticle first published online: 4 APR 200 Abstract The reasons for oxygen transport in YBa2Cu3O7-, and SrFeO3 were investigated, using the X, -discrete variation method. Reduced binding energy and other parameters of the electronic energy structure were calculated for a number of crystal fragments within the T and O phases of YBa2Cu3O7-, and the cubic phase of SrFeO3. The following results were obtained: (i) test calculations of reduced binding energy confirmed that high temperature should be a condition of existence of the T phase, as compared with the O phase of YBa2Cu3O7-,; (ii) an increase in temperature up to ,400°C points to the beginning of the Jahn,Teller dynamic effect in SrFeO3; and (iii) transport of neutral and charged oxygen atoms results in distributions of stable and unstable oxygen vacancies, respectively. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] Phase relations in the Ba,Sr,Co,Fe,O system at 1273,K in airJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2009Zhèn Yáng Selected compositions of the Ba,Sr,Co,Fe,O system were synthesized from powders by the solid-state reaction method. Samples were equilibrated at 1273,K for 36,000,s in air. The resulting powders were characterized by X-ray diffraction (XRD) at room temperature and by high-temperature in situ XRD. The phases present in the BaxSr1,xCoyFe1,yO3,, system are outlined for 1273,K in air. For most of the quaternary compositions, the cubic perovskite is formed, except for the compositions with x = 1 (excluding y = 0.4), y = 1 and x, y = 0.8, where the phases mainly show hexagonal distortions, and x, y = 0, for which a predominant cubic phase is mixed with other phases. [source] Preparation and microstructure characterization of ball-milled ZrO2 powder by the Rietveld method: monoclinic to cubic phase transformation without any additiveJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 5 2002S. Bid The phase transformation kinetics of high-energy ball-milled monoclinic ZrO2 have been studied in detail by Rietveld powder structure refinement analysis. In the present study, no stabilizing compound was required to obtain the cubic phase. The fine-grain powder was milled in a planetary ball mill for up to several hours at different BPMRs (ball to powder mass ratios): 10:1, 20:1, 35:1 and 40:1. During the process of ball milling, the monoclinic phase is gradually transformed to the cubic phase. The relative phase abundances of the respective phases, the particle sizes, the r.m.s. strains, the lattice parameter changes, etc., have been estimated from Rietveld analysis of X-ray powder diffraction data. It has been found that a higher BPMR exerts more influence on rapid phase transformation. In the m - to c -ZrO2 phase transformation, no formation of an intermediate tetragonal ZrO2 phase has been found. The small change in the lattice volume of m -ZrO2, which is very close to the lattice volume of c -ZrO2, caused by ball milling may be attributed to this phase change. The formation of the c phase is noticed, in general, after just 1,h of ball milling, and the particle size of the m phase is reduced to a large extent at the first stage of milling and remains almost unchanged with increasing milling time. However, the particle size of the c phase increases with increasing milling time for the samples milled with higher BPMRs (35:1 and 40:1), suggesting that quenching caused by a high impact energy followed by an annealing effect may play a vital role, which is further manifested in the agglomeration of small particles. [source] Synthesis of metal/polymer colloidal composites by the tailored deposition of silver onto porous polymer microspheresJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 10 2004Jin-Woong Kim Abstract A new colloidal silver system is presented in which a fine colloidal silver is in situ deposited onto functionalized porous poly(ethylene glycol dimethacrylate) [poly(EGDMA)] microspheres. The effectiveness of the silver deposition has been investigated through an examination of the surface characteristics of poly(EGDMA) microspheres. The result reported in this study demonstrate that the control of the surface area and surface functionality (in this study, a hydroxyl group) of poly(EGDMA) microspheres is an important factor that practically determines the degree of deposition of colloidal silver. X-ray analysis has shown that silver nanoparticles are dispersed evenly on inner and outer surfaces and have a face center cubic phase. Preservation testing has shown that silver-containing poly(EGDMA) microspheres have powerful antibacterial properties and, therefore, have significant potential as new preservatives. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2551,2557, 2004 [source] Crystal Structure and Compressibility of Pb(Zr1,xTix)O3 Ceramics Under High PressureJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2010Young Ho Ko X-ray diffraction patterns of Pb(Zr0.3Ti0.7)O3 and Pb(Zr0.7Ti0.3) O3 have been measured at pressures up to 36 GPa with good signal-to-noise ratio using synchrotron radiation. In order to refine the crystal structure, Rietveld analysis has been performed. The structure of Pb(Zr0.3Ti0.7)O3 is tetragonal in space group P4mm with lattice parameters a=b=3.9843±0.0003 Å and c= 4.1545±0.0003 Å at ambient pressure. A pressure-induced phase transition was found in the pressure range higher than ,7.4 GPa at room temperature. The high-pressure phase is cubic in space groupwith lattice parameter a=3.9600±0.0003 Å at 7.4 GPa. The Birch,Murnaghan equation of state fit to the experimental data yields B0=83.9±1.7 GPa (B,0=4.0 is fixed) for tetragonal phase and B0=102.5±4.4 GPa (B,0=4.0 is fixed) for cubic phase with V0=65.95±0.01 Å3. The structure of Pb(Zr0.7Ti0.3)O3 is monoclinic in space group Cc with lattice parameters a=10.0627±0.0164 Å, b=5.8511±0.0024 Å, c= 5.7949±0.0020 Å, and ,=125.185±0.119 ° at ambient pressure. No pressure-induced phase transition for Pb(Zr0.7Ti0.3)O3 was observed over the pressure range of this study. The Birch,Murnaghan equation of state fit to the experimental data yields B0=73.6±18.9 GPa (B,0=4.0 is fixed) with V0= 278.81±0.63 Å3. [source] Sintering Behavior and Dielectric Properties of Bi3NbO7 Ceramics Prepared by Mixed Oxides and High-Energy Ball-Milling MethodsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2007Di Zhou The sintering behavior and dielectric properties of Bi3NbO7 ceramics prepared by the high-energy ball milling (HEM) method and conventional mixed oxides method with V2O5 addition were investigated. All the samples were sintered between 840° and 960°C. For the ceramics prepared by the mixed oxides method, the pure tetragonal Bi3NbO7 phase formed without any cubic phase. With changing sintering temperature, the dielectric constant ,r lies between 79 and 92, while the Q×f values are between 300 and 640 GHz. The samples sintered at 870°C have the best microwave dielectric properties with ,r=79, Q×f=640 GHz, and the temperature coefficients of resonant frequency ,f between 0 and ,20 ppm/°C. For the ceramics prepared by the HEM, a pure cubic phase was obtained. The ,r changes between 78 and 80 and Q×f were between 200 and 290 GHz. [source] Analysis of Phase Coexistence in Fe2O3 -Doped 0.2PZN,0.8PZT Ferroelectric Ceramics by Raman Scattering SpectraJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2006Man-Kang Zhu In this work, we suggested a method to evaluate quantitatively the effect of doping oxide on the phase coexistence of PbZr1,xTixO3 (PZT)-based ceramics through the analysis of the Raman scattering spectra. Theoretically, the degenerated T3u mode in the cubic phase of PZT will transform as A1(3) and E(4) modes in the tetragonal phase or as rhombohedral (R) modes in the rhombohedral phase below the Curie temperature, which set up the theoretical base to study the phase coexistence in ferroelectric materials. Through separation by fitting of the Raman bands, the shifts and intensities of different Raman vibration modes were determined. A calculation equation representing the phase coexistence was put forward based on the theoretical analysis of the degenerated T3u modes. The results showed that a turning point appears at the Fe2O3 addition of 0.3%. The variation in the electrical properties of the Fe2O3 -doped Pb(Zn1/3Nb2/3)O3 (PZN),PZT ceramics also affirmed the turning point of the phase evolution as the addition of Fe2O3. [source] Preparation of a Monodispersed Suspension of Barium Titanate Nanoparticles and Electrophoretic Deposition of Thin FilmsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2004Juan Li A transparent and stable monodispersed suspension of nanocrystalline barium titanate was prepared by dispersing a piece of BaTiO3 gel into a mixed solvent of 2-methoxyethanol and acethylacetone. The results of high-resolution transmission electron microscopy (HR-TEM) and size analyzer confirmed that the BaTiO3 nanoparticles in the suspension had an average size of ,10 nm with a narrow size distribution. Crystal structure characterization via TEM and X-ray diffraction indicated BaTiO3 nanocrystallites to be a perovskite cubic phase. BaTiO3 thin films of controlled thickness from 100 nm to several micrometers were electrophoretic deposited compactly on Pt/Ti/SiO2/Si substrates. The deposited thin film had uniform nanostructure with a very smooth surface. [source] Cubic-Formation and Grain-Growth Mechanisms in Tetragonal Zirconia PolycrystalJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2003Koji Matsui The microstructure in Y2O3 -stabilized tetragonal zirconia polycrystal (Y-TZP) sintered at 1300°,1500°C was examined to clarify the role of Y3+ ions on grain growth and the formation of cubic phase. The grain size and the fraction of the cubic phase in Y-TZP increased as the sintering temperature increased. Both the fraction of the tetragonal phase and the Y2O3 concentration within the tetragonal phase decreased with increasing fraction of the cubic phase. Scanning transmission electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) measurements revealed that cubic phase regions in grain interiors in Y-TZP generated as the sintering temperature increased. High-resolution electron microscopy and nanoprobe EDS measurements revealed that no amorphous layer or second phase existed along the grain-boundary faces in Y-TZP and Y3+ ions segregated at their grain boundaries over a width of ,10 nm. Taking into account these results, it was clarified that cubic phase regions in grain interiors started to form from grain boundaries and the triple junctions in which Y3+ ions segregated. The cubic-formation and grain-growth mechanisms in Y-TZP can be explained using the grain boundary segregation-induced phase transformation model and the solute drag effect of Y3+ ions segregating along the grain boundary, respectively. [source] Morphotropic Phase Boundary in the Pb(Zn1/3Nb2/3O3)-BaTiO3 -PbTiO3 SystemJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2001Weizhong Zhu The morphotropic phase boundary (MPB) in the relaxor ferroelectric system Pb(Zn1/3Nb2/3O3)-BaTiO3 -PbTiO3 (PZN-BT-PT) with 15 mol% BT was investigated through dielectric permittivity and high-temperature X-ray diffraction measurements. It was revealed that MPB is a broad composition region extending from 12 to 18 mol% PT, within which the temperatures of the permittivity maximum are close to the ending temperatures for the phase transformation from coexisting rhombohedral and tetragonal phases to cubic phase on heating. When the specimen is cooled, the starting temperatures for the rhombohedral-to-tetragonal phase transition increase with increasing PT content. The large thermal hysteresis observed by X-ray diffraction is caused by the phase transformation between rhombohedral and tetragonal phases. On cooling, the MPB curves toward the PT-rich side, so that ceramics within this composition range undergoe successive phase transitions from cubic to rhombohedral and from rhombohedral to tetragonal phase. The diffuseness of the paraelectric-to-ferroelectric phase transition is remarkably decreased by the addition of PT. The enhanced dielectric permittivity peak values for the MPB compositions are correlated with the reduced lattice distortion and phase coexistence. [source] Exciton relaxation in bulk wurtzite GaN: the role of piezoelectric interactionPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 3 2003G. Kokolakis Abstract A theoretical study on the relaxation of coupled free carriers and excitons after non-resonant optical excitation in bulk wurtzite GaN is presented. In particular the effect of the acoustic piezoelectric scattering is taken under consideration, and the respective rates have been calculated, including screening effects. Results show that the piezo-acoustic rates are bigger in the wurtzite phase of GaN with respect to the cubic phase, and they are really sensitive to the background doping of the sample. Simulations of the full dynamics of the system are performed by using an Ensemble Monte Carlo method under which all the relevant scattering mechanisms are included. The set of semiclassical Boltzmann equations for electron and hole populations is complemented by an additional equation for the exciton distribution and is coupled by reaction terms describing phonon-mediated exciton binding and dissociation. The temporal evolution is studied in the short range time (100 ps) after photo-excitation. It shows that a high background doping prevents the electrons to relax toward low energy states. [source] Temperature dependence of structural parameters in the perovskite Na0.74WO3PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 4 2005C. N. W. Darlington Abstract NaxWO3 undergoes three phase transitions involving tilting of the oxygen octahedra about ,100, pseudocubic directions. We have measured the temperature dependence of the lattice parameters between 200 and 750 K , a temperature range that spans the three transitions , and determined the evolution of the structure within each phase as temperature changes. The experiments were performed on the high-resolution powder diffractometer HRPD, Rutherford Appleton Laboratory, UK, using time-of-flight neutron diffraction. The critical exponent that describes the temperature dependence of the total tilt angle has been evaluated. An unexpected change in the thermal expansion coefficient in the cubic phase is not understood: an argument based on Landau's theory of phase transitions is used to demonstrate that the change is not the result of co-existence of two phases. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] DFT LCAO and plane wave calculations of SrZrO3PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2005R. A. Evarestov Abstract The results of the density functional (DFT) LCAO and plane wave (PW) calculations of the electronic and structural properties of four known SrZrO3 phases (Pm3m, I4/mcm, Cmcm and Pbnm) are presented and discussed. The calculated unit cell energies and relative stability of these phases agree well with the experimental sequence of SrZrO3 phases as the temperature increases. The lattice structure parameters optimized in the PW calculations for all four phases are in good agreement with the experimental neutron diffraction data. The LCAO and PW results for the electronic structure, density of states and chemical bonding in the cubic phase (Pm3m) are discussed in detail and compared with the results of previous PW calculations. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Deposition of nano-crystalline lead chalcogenide thin films using a simple electrochemical techniquePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2008Nillohit Mukherjee Abstract Nanocrystalline thin films of lead chalcogenides (PbS, PbSe and PbTe) were deposited on transparent conducting oxide (TCO) coated glass substrates at room temperature using a simple electrochemical (galvanic) technique, from suitable electrolytic solutions and without the application of any external bias. X-ray characterization revealed that PbS, PbSe and PbTe thin films were all of cubic phase. The formation of nanocrystallites with dense and compact surface morphology was detected from FESEM and AFM measurements. The nanocrystalline films showed ,blue shifted' IR absorption. Hall measurements showed the materials to be p-type in nature with carrier concentration in the range 1019 , 1020/cm3. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Characterization and electrical properties of new perovskite films of Ba(Ti,Zr)O3 type doped with lanthanum (BLZT)PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2007E. Delgado Abstract In the current work, we characterized a new family of compounds with perovskite-type structure with the general formula: Ba1,yLa2y/3Ti1,xZrxO3 (BLZT). Through XRD on crystalline powder we detected a single perovskite cubic phase, Pm3m (221), for the compounds with zirconium content (x) lower than 0.15 and lanthanum content (La) lower than 0.17. Upon increasing the x value in BLZT compounds, the value of La falls to 0.13. Morphology studies through SEM revealed that the incorporation of lanthanum into the composition of Ba(Ti,Zr)O3 (BZT) compounds produces the formation of bigger grains and materials with greater density. The stoichiometry for each compound was corroborated by atomic emission spectroscopy (AES-ICP). The lanthanum-doped compounds demonstrated higher relative permittivity values with respect to the BZT and a decrease of the Curie temperature (Tc) with relation to the amount of La3+ ion present in the structure. Finally, we grew thin films by using a target of the compound Ba0.90La0.067Ti0.91Zr0.09O3 over three different substrates under two atmospheres (Ar and O2), using the Magnetron Sputtering technique by Radio Frequency (RF), showing the best results when growth is over the substrate SrTiO3.Nb 0.1% in the [00l] direction under an O2 atmosphere. (© 2007 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] Normal-mode analysis of the structures of perovskites with tilted octahedra.ACTA CRYSTALLOGRAPHICA SECTION A, Issue 3 2002Erratum There is an error in the mode assignment for hettotype 9, [a,b+a,], discussed in the paper by Darlington [Acta Cryst. (2002). A58, 6671], which has been pointed out by Dr Kevin Knight, Rutherford Appleton Laboratory, Didcot, Oxon, England. In this paper, a mode involving displacements of the anions of hettotype 9 was labelled [(½,,0,,½),,M1] rather than [(½,,0,,½),,M2]. Both modes involve plus-like distortion of the octahedra. In the corrected Tables 1,4 shown below, this mode, which is only found in hettotype 9, has been labelled K2 rather than H2. Therefore, there are not seven but eight normal modes of the cubic phase required to describe the displacements found in the nine hettotypes considered. The weights of K2 in all the materials examined in the original paper with the structure of hettotype 9 [labelled W(H1) in the original Table 4] are correct, unaltered by the change in the labelling of the mode. It should be noted that [(½,,0,,½),,M2] is a longitudinal mode , the seven other modes are all transverse. The weights of K2 are not significantly different from zero in the 15 structures examined. [source] X-ray diffraction study of the phase transition of K2Mn2(BeF4)3: a new type of low-temperature structure for langbeinitesACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2001A. Guelylah The potassium manganese tetrafluoroberyllate langbeinite compound has been studied in the temperature range 100,300,K. Using DSC measurements, a phase transition has been detected at 213,K. The space group of the low-temperature phase was determined to be P1121 using X-ray diffraction experiments and optical observations of the domain structure. The b axis is doubled with respect to the prototypic P213 cubic phase. Lattice parameters were determined by powder diffraction data [a = 10.0690,(8), b = 20.136,(2), c = 10.0329,(4),Å, , = 90.01,(1)°]. A precise analysis of the BeF4 tetrahedra in the low-temperature phase shows that two independent tetrahedra rotate in opposite directions along the doubled crystallographic axis. A symmetry mode analysis of the monoclinic distortion is also reported. This is the first report of the existence of such a phase transition in K2Mn2(BeF4)3 and also of a new type of low-temperature structure for langbeinite compounds. [source] A convenient synthesis of nanocrystalline chalcopyrite, CuFeS2 using single-source precursorsAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 12 2009Sujit D. Disale Abstract Nanocrystalline chalcopyrite (CuFeS2) have been synthesized using single-source precursors, CuL2 and Cu(LH)2Cl2 (where LH = monoacetylferrocene thiosemicarbazone) by pyrolysis and solvothermal decomposition methods. The nanocrystals were characterized by UV,visible spectroscopy, X-ray powder diffraction, transmission electron microscopy and energy dispersive X-ray analysis. The magnetic character was determined by the hysteresis loop. CuFeS2 nanocrystallites prepared by the pyrolysis of CuL2 and Cu(LH)2Cl2 have a cubic phase and rod-like morphology with diameters of about 18 and 15 nm, respectively, and lengths of about 195,390 and 100,145 nm, respectively. However, CuFeS2 nanoparticles obtained by solvothermal route from CuL2 and Cu(LH)2Cl2, which are capped with ethylene glycol, possess a nearly spherical shape with an average grain size of 16 and 11 nm, respectively. Copyright © 2009 John Wiley & Sons, Ltd. [source] Structure and mechanical properties of nanocrystalline boron nitride thin films ,APPLIED ORGANOMETALLIC CHEMISTRY, Issue 5 2001Paolo M. Ossi Abstract Boron nitride thin films have been deposited on (100) Si wafers, kept at low temperature, by radio frequency (r.f.) magnetron sputtering. The r.f. target power was fixed at 150,W and the substrate bias voltage ranged between ,50 and ,130,V. Film composition was checked by Auger electron spectroscopy; the structure was investigated by Fourier transform IR spectroscopy, glancing-angle X-ray diffraction and micro-Raman spectroscopy. Film hardness and Young's modulus were measured by nanoindentation. Film composition is nearly equiatomic, with a low degree of gaseous contamination. All samples are very fine grained, and nanocrystalline. Film coordination is mixed sp2,sp3, and the fraction of tetrahedral coordination depends critically on the bias voltage value. In hexagonal sp2 -bonded films the hardnesses and Young's moduli are low and increase considerably with the content of sp3 -coordinated cubic phase. Copyright © 2001 John Wiley & Sons, Ltd. [source] Effects of impurities on membrane-protein crystallization in different systemsACTA CRYSTALLOGRAPHICA SECTION D, Issue 10 2009Christopher A. Kors When starting a protein-crystallization project, scientists are faced with several unknowns. Amongst them are these questions: (i) is the purity of the starting material sufficient? and (ii) which type of crystallization experiment is the most promising to conduct? The difficulty in purifying active membrane-protein samples for crystallization trials and the high costs associated with producing such samples require an extremely pragmatic approach. Additionally, practical guidelines are needed to increase the efficiency of membrane-protein crystallization. In order to address these conundrums, the effects of commonly encountered impurities on various membrane-protein crystallization regimes have been investigated and it was found that the lipidic cubic phase (LCP) based crystallization methodology is more robust than crystallization in detergent environments using vapor diffusion or microbatch approaches in its ability to tolerate contamination in the forms of protein, lipid or other general membrane components. LCP-based crystallizations produced crystals of the photosynthetic reaction center (RC) of Rhodobacter sphaeroides from samples with substantial levels of residual impurities. Crystals were obtained with protein contamination levels of up to 50% and the addition of lipid material and membrane fragments to pure samples of RC had little effect on the number or on the quality of crystals obtained in LCP-based crystallization screens. If generally applicable, this tolerance for impurities may avoid the need for samples of ultrahigh purity when undertaking initial crystallization screening trials to determine preliminary crystallization conditions that can be optimized for a given target protein. [source] Heavy-atom derivatives in lipidic cubic phases: results on hen egg-white lysozyme tetragonal derivative crystals with Gd-HPDO3A complexACTA CRYSTALLOGRAPHICA SECTION D, Issue 8 2004Éric Girard Gd-HPDO3A, a neutral gadolinium complex, is a good candidate for obtaining heavy-atom-derivative crystals by the lipidic cubic phase crystallization method known to be effective for membrane proteins. Gadolinium-derivative crystals of hen egg-white lysozyme were obtained by co-crystallizing the protein with 100,mM Gd-HPDO3A in a monoolein cubic phase. Diffraction data were collected to a resolution of 1.7 Å using Cu,K, radiation from a rotating-anode generator. Two binding sites of the gadolinium complex were located from the strong gadolinium anomalous signal. The Gd-atom positions and their refined occupancies were found to be identical to those found in derivative crystals of hen egg-white lysozyme obtained by co-crystallizing the protein with 100,mM Gd-HPDO3A using the hanging-drop technique. Moreover, the refined structures are isomorphous. The lipidic cubic phase is not disturbed by the high concentration of Gd-HPDO3A. This experiment demonstrates that a gadolinium complex, Gd-HPDO3A, can be used to obtain derivative crystals by the lipidic cubic phase crystallization method. Further studies with membrane proteins that are known to crystallize in lipidic cubic phases will be undertaken with Gd-HPDO3A and other Gd complexes to test whether derivative crystals with high Gd-site occupancies can be obtained. [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] , -Conjugated Oligothiophene-Based Polycatenar Liquid Crystals: Self-Organization and Photoconductive, Luminescent, and Redox PropertiesADVANCED FUNCTIONAL MATERIALS, Issue 3 2009Takuma Yasuda Abstract A series of liquid-crystalline (LC) ,- -conjugated oligothiophenes bearing three or two alkoxy chains at their extremities has been designed and synthesized. These polycatenar oligothiophenes form various LC nanostructures including smectic, columnar, and micellar cubic phases. These properties depend on the number and length of the terminal alkoxy chains. The hole mobilities for the oligothiophenes have been measured. The layered smectic and columnar structures are capable of transporting holes, leading to mobilities of up to 0.01,cm2 V,1 s,1. The columnar LC assemblies have also been explored to produce linearly polarized light-emission. Fine red polarized fluorescence is observed from a uniaxially aligned film of the oligothiophenes. The redox properties of the oligothiophenes both in solutions and in films have been examined. The oligothiophenes exhibit electrochromism upon applying an oxidative potential. The present design strategy is useful for fabricating a variety of functional electro-active molecular assemblies. [source] | ||||||||||||||||