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Crystal Symmetry (crystal + symmetry)
Selected AbstractsCrystal Symmetry and the Reversibility of Martensitic Transformations.CHEMINFORM, Issue 19 2004Kaushik Bhattacharya No abstract is available for this article. [source] Dielectric Properties of the Perovskite System Pb(Mg1/3Nb2/3)O3 -PbTiO3 Modified by Pb(Mg1/3Ta2/3)O3 and Pb(Zn1/3Nb2/3)O3JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2001Dong-Hwan Suh Effects of Pb(Mg1/3Ta2/3)O3 and Pb(Zn1/3Nb2/3)O3 substitution in the Pb(Mg1/3Nb2/3)O3 -PbTiO3 ceramic system on structure formation, crystallographic aspects, and dielectric properties are discussed. Developed phases in the B-site precursor and perovskite systems were studied by X-ray diffraction. Crystal symmetries and dimensions of the perovskite unit cell of the two systems are compared. Changing rates of the lattice parameter with substituent PbTiO3 concentration in the two modified systems are virtually identical to that of the unmodified Pb(Mg1/3Nb2/3)O3 -PbTiO3 system. Weak-field low-frequency dielectric responses of the ceramics were investigated. The dielectric maximum temperatures of the two perovskite systems shifted almost linearly with compositional change. Dielectric constant spectra at low concentrations of PbTiO3 exhibited typical diffuse phase transition behavior, whereas those at high PbTiO3 concentrations were rather sharp. The phase transition modes reflected on the dielectric spectra were quantitatively analyzed in terms of diffuseness parameters. [source] 51V magic angle spinning NMR in VOPO4 phasesMAGNETIC RESONANCE IN CHEMISTRY, Issue 12 2004Renée Siegel Abstract 51V magic angle spinning NMR was applied to the ,II, , and , phases of VOPO4 at three magnetic field strengths (4.7, 7.1, and 11.7 T). The 51V quadrupole and chemical shift tensors were determined by iterative fitting of the NMR lineshapes at the three magnetic field strengths. The applicability of the method is illustrated by comparison with literature data. Although determined chemical shift tensors are completely axially symmetric and of the same magnitude, all studied phases can clearly be distinguished by their quadrupole coupling tensor. Relationships between the 51V NMR data and structural characteristics such as crystal symmetries are discussed. Copyright © 2004 John Wiley & Sons, Ltd. [source] New Organic Nonlinear Optical Polyene Crystals and Their Unusual Phase Transitions,ADVANCED FUNCTIONAL MATERIALS, Issue 11 2007O-P. Kwon Abstract A series of new nonlinear optical chromophores based on configurationally locked polyenes (CLPs) with chiral pyrrolidine donors are synthesized. All CLP derivatives exhibit high thermal stability with decomposition temperatures Td at least > ,270,°C. Acentric single crystals of enantiopure D - and L -prolinol-based chromophores with a monoclinic space group P21 exhibit a macroscopic second-order nonlinearity that is twice as large than that of analogous dimethylamino-based crystal. This is attributed to a strong hydrogen-bonded polar polymer-like chain built by these molecules, which is aligned along the polar crystallographic b -axis. Five ,-phase CLP crystals with different donors grown from solution exhibit a reversible or irreversible thermally induced structural phase transition to a ,-phase. These phase transitions are unusual, changing the crystal symmetry from higher to lower at increasing temperatures, for example, from centrosymmetric to non-centrosymmetric, enhancing their macroscopic second-order nonlinear optical properties. [source] Gyroid Single Crystals: Nanostructured Calcite Single Crystals with Gyroid Morphologies (Adv. Mater.ADVANCED MATERIALS, Issue 38-39 200939/2009) Single crystals typically assume a crystallographically distinct shape. Many biological organisms, however, synthesize single crystals with an intricate mescoscopic morphology that does not reflect the crystal symmetry. The cover shows a calcite single crystal with a bicontinuous gyroid morphology, which was obtained by calcite nucleation in a self-assembled polymer matrix in work reported by Ulli Steiner and co-workers on p. 3928. The characteristic size of the biomimetic structure is ,30 nm. The pattern in the title is the characteristic 211 plane of the gyroid morphology. [source] Transformations for monoclinic crystal symmetry in texture analysisJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2009Siegfried Matthies Monoclinic crystals can be described in two settings: in the first setting the C2 rotation axis is parallel to the z axis and in the second setting it is parallel to the y axis. Transformations of lattice parameters, Miller and zone indices, and atomic coordinates is straightforward; the situation is far more complex for texture analysis with orientation distributions and corresponding representations. This article gives explicit transformations that need to be applied, not only for texture analysis but also for calculations of physical properties of materials with preferred orientation. In texture research the relationship between the Cartesian crystal coordinate system and the unit cell must be unambiguously defined and a uniform convention is desirable. [source] Synchrotron texture analysis with area detectorsJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2003H.-R. Wenk The wide availability of X-ray area detectors provides an opportunity for using synchrotron radiation based X-ray diffraction for the determination of preferred crystallite orientation in polycrystalline materials. These measurements are very fast compared to other techniques. Texture is immediately recognized as intensity variations along Debye rings in diffraction images, yet in many cases this information is not used because the quantitative treatment of texture information has not yet been developed into a standard technique. In special cases it is possible to interpret the texture information contained in these intensity variations intuitively. However, diffraction studies focused on the effects of texture on materials properties often require the full orientation distribution function (ODF) which can be obtained from spherical tomography analysis. In cases of high crystal symmetry (cubic and hexagonal) an approximation to the full ODF can be reconstructed from single diffraction images, as is demonstrated for textures in rolled copper and titanium sheets. Combined with area detectors, the reconstruction methods make the measurements fast enough to study orientation changes during phase transformations, recrystallization and deformation in situ, and even in real time, at a wide range of temperature and pressure conditions. The present work focuses on practical aspects of texture measurement and data processing procedures to make the latter available for the growing community of synchrotron users. It reviews previous applications and highlights some opportunities for synchrotron texture analysis based on case studies on different materials. [source] Modeling high pressure reactivity in unsaturated systems: Application to dimethylacetyleneJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 3 2009C. Mediavilla Abstract A general model is introduced to study pressure-induced reactivity on unsaturated systems in the condensed state. The model is applied here to dimethylacetylene (DMA) in the solid phase II (C/2m) because it has been proposed that two DMA molecules can react to form tetramethyl-cyclobutadiene (TMCBD). The proposed reaction process has been modeled by studying the structural and electronic changes undergone by two DMA molecules as they approach each other preserving the crystal symmetry of phase II. Both monodeterminantal (MP2 and DFT) and multideterminantal (CASSCF and MRMP2) methodologies were used to check the reliability of our model in predicting the reactivity of the system under compression. In all cases, structural results are in agreement with low-temperature diffraction experiments for the solid phase II. Our model indicates that DMA is expected to form the TMCBD dimer at intermolecular distances close to 2 Å. This value is in excellent agreement with previous calculations on the existence of long carbon,carbon bonds. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2009 [source] Polar phonon mode selection rules in tip-enhanced Raman scatteringJOURNAL OF RAMAN SPECTROSCOPY, Issue 10 2009Samuel Berweger Abstract We discuss the use of the symmetry selectivity of phonon Raman scattering to determine nanocrystallographic information of solids using tip-enhanced Raman scattering (TERS). The necessary degrees of freedom arise from the combination of the Raman selection rules reflecting crystal symmetry superimposed by the polarization and k -vector-dependent field enhancement and scattering of the scanning probe tip. The resulting phonon TERS selection rules are discussed, including the use of the crystal Raman tensor and momentum conservation for polar phonon modes. We demonstrate the selection rules for both far-field and tip-enhanced near-field Raman scattering from bulk and nanocrystalline LiNbO3. Copyright © 2009 John Wiley & Sons, Ltd. [source] Neutron Powder Diffraction Study of a Phase Transition in La0.68(Ti0.95Al0.05)O3JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2006Roushown Ali Crystal structures and structural changes of the compound La0.68(Ti0.95Al0.05)O3 have been studied using neutron powder diffraction data and the Rietveld method in the temperature range from 25° to 592°C. The Rietveld profile-fitting analyses of the neutron data and the synchrotron diffraction profile revealed that the crystal symmetry of the low-temperature phase of La0.68(Ti0.95Al0.05)O3 is orthorhombic Cmmm (2ap× 2ap× 2ap; p: pseudo-cubic perovskite). The unit-cell and structural parameters were successfully refined with the orthorhombic Cmmm for the intensity data measured at 25°, 182°, and 286°C, and with the tetragonal P4/mmm (ap×ap× 2ap) for intensity data obtained at 388° and 592°C. The P4/mmm -to- Cmmm phase transition was found to be induced by tilting of the (TiAl)O6 octahedron. The tilt angle decreased with increasing temperature, reaching 0° at the Cmmm,P4/mmm transition temperature. [source] Microstructure, dielectric, and piezoelectric properties of 0.38Bi(Gax Sc1,x)O3,0.62PbTiO3 high temperature piezoelectric ceramicsPHYSICA STATUS SOLIDI - RAPID RESEARCH LETTERS, Issue 1 2008Yihang Jiang Abstract 0.38Bi(Gax Sc1,x)O3,0.62PbTiO3 (BGSPTx) ceramics have been prepared by using the conventional mixed oxide method. X-ray diffraction analysis revealed that BGSPTx has a pure perovskite structure, and the crystal symmetry of BGSPTx changed from rhombohedral to tetragonal with increasing Ga content (x). The Curie temperature (TC) of BGSPTx ceramics is in the range of 448,467 °C for different x. The ferroelectric phase transition of BGSPTx was found to be of the first order type according to the Curie,Weiss law. For x = 0.125, BGSPTx ceramics show enhanced piezoelectric properties: piezoelectric constant d33 = 420 pC/N and d31 = ,142 pC/N, planar and thickness electromechanical coupling factors kp = 56.27% and kt = 56.00%, respectively. The high- TC of BGSPTx coupled with its excellent piezoelectric properties suggests those future high-temperature applications. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Identification of van Hove singularities in the GaN dielectric function: a comparison of the cubic and hexagonal phasePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2009C. Cobet Abstract We present a detailed analysis of interband transition structures in the dielectric function of GaN. The dielectric function of the stable wurtzite and the metastable zinc blende phase were determined by means of synchrotron spectroscopic ellipsometry in the spectral range between 3 eV and 20 eV where the most significant structures of the dielectric function are located. In the hexagonal case, both the ordinary and extraordinary dielectric tensor component was measured on GaN films with M -plane/[1 00] orientation. In a comparative discussion of the two hexagonal tensor components and the zinc blende dielectric function, all prominent absorption structures were assigned to specific interband transitions at high symmetry points in the Brillouin zone. The assignment considers the individual dipole transition probabilities depending on the crystal symmetry and the geometry of the measurement. Furthermore, a detailed theoretical band-to-band analysis of dielectric function features, published by Lambrecht et al. [1], was considered. In conclusion, we suggest a new labeling of absorption structures as used in classical III,V materials like GaAs, which reflects the origin of transition structures from specific points in the respective Brillouin zones. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] X-ray atomic orbital analysis.ACTA CRYSTALLOGRAPHICA SECTION A, Issue 4 2008The scattering unit of X-ray crystal structure analysis is changed from atoms to the subshell electrons by X-ray atomic orbital analysis (XAO). All the atoms in the unit cell are divided into groups of subshell electrons in the XAO analysis. Each subshell is treated as an independent pseudo-atom, which enables the atomic orbitals (AO's) and the electron population of each AO expressed as a linear combination of s/p/d/f orbitals in each subshell to be determined. When the environmental condition of the sample is varied, the electron transfer among the AO's in the crystal can be traced with XAO. It is applicable mainly to analyses of the electron-density distribution in ionic solids including those with a nonstoichiometric structure. The expansion coefficients of each AO are calculated with the perturbation theory putting a point charge on each atom in the unit cell. This automatically makes the perturbation potential have the point-group symmetry of the atom in the crystal field. Then the coefficients of each AO are refined to fit to the observed structure factors keeping the orthonormal relationships among the AO's. Complex basis functions with , or , spin as well as real ones are employed for heavy atoms and the relationships among the coefficients for the AO's of an electron in the crystal fields of the 32 point-group symmetries are derived for p, d and f orbitals. The AO's thus derived can be applicable to an anti-symmetrized multi-electron system, although X-ray diffraction cannot specify the atomic terms occupied when the crystal symmetry permits the atom to have many terms. [source] Single-crystal X-ray and neutron powder diffraction investigation of the phase transition in tetrachlorobenzeneACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2006Charlotte K. Broder The polymorphic phase transition of 1,2,4,5-tetrachlorobenzene (TCB) has been investigated using neutron powder diffraction and single-crystal X-ray diffraction. The diffraction experiments show a reversible phase change that occurs as a function of temperature with no apparent loss of sample quality on transition between the two phases. Neutron powder diffraction gives detailed information on the molecular structural changes and lattice parameters from 2,K to room temperature. The structure of the low-temperature form has been elucidated for the first time using single-crystal X-ray diffraction. Comparison of the , and , structures show that they are both based on the same sheet motif, with the differences between the two being very subtle, except in terms of crystal symmetry. Detailed analysis of the structures revealed the changes required for inter-conversion. A computational polymorph search showed that these two sheet structures are more thermodynamically stable than alternative herringbone-type structures. [source] Twinning and structure of Eu0.6Sr0.4MnO3ACTA CRYSTALLOGRAPHICA SECTION C, Issue 1 2006Nicola Rotiroti The crystal structure of europium strontium manganese trioxide, Eu0.6Sr0.4MnO3, has been refined using a multiply twinned single crystal containing six twin components. The MnO6 octahedra show Jahn,Teller distortions with nearly fourfold symmetry, but the octahedral tilting scheme reduces the crystal symmetry to orthorhombic (space group Pbnm). The refinement of site occupancies and the analysis of difference Fourier maps show that the Eu3+ and Sr2+ cations occupy different crystallographic positions with eightfold and twelvefold coordination, respectively. [source] Integration, scaling, space-group assignment and post-refinementACTA CRYSTALLOGRAPHICA SECTION D, Issue 2 2010Wolfgang Kabsch Important steps in the processing of rotation data are described that are common to most software packages. These programs differ in the details and in the methods implemented to carry out the tasks. Here, the working principles underlying the data-reduction package XDS are explained, including the new features of automatic determination of spot size and reflecting range, recognition and assignment of crystal symmetry and a highly efficient algorithm for the determination of correction/scaling factors. [source] | ||||||||||||||||