Search results for "Crystallographic"
showing 10 items of 202 documents
Ab initiocalculations of theHcenters in MgF2crystals
2012
MgF2 with rutile structure is important wide-gap optical material with numerous applications. We present and discuss the results of calculations for basic hole defects - interstitial F atoms (called also the colour H centres). This study is based on the large scale ab initio DFT calculations using hybrid B3PW exchange-correlation functional as implemented into CRYSTAL computer code. The electronic structure, atomic geometry, charge density distribution are calculated and discussed.
First‐principles modeling of the H color centers in MgF 2 crystals
2012
MgF2 with a rutile structure is important wide-gap optical material with numerous applications. We present and discuss the results of calculations for basic hole defects – interstitial F atoms (called also the colour H centers). This study is based on the large scale ab initio DFT calculations using hybrid B3PW exchange-correlation functional as implemented into CRYSTAL computer code. The electronic structure, atomic geometry, charge density distribution are calculated and compared with similar defects in CaF2 fluorite. It is shown that the H centers oriented nearly parallel to the (110) axis are energetically more favourable than those oriented along the (001) axis, in agreement with exper…
Average Structure vs. Real Structure: Molecular Dynamics Studies of Silica
2003
The microscopic structure of a crystal and thermal fluctuations of the atoms constituting the crystal are intimately connected with the macroscopic elastic properties including mechanical stability. In some cases, however, the picture is more complex than that which is drawn in text books on solid state physics. (i) The instantaneous microscopic structure can deviate in a non-Gaussian way from the average structure even when domain disorder and/or crystal defects are absent. Quasi harmonic approximations may then turn out to be meaningless. (ii) The crystal is subject to external pressures that are sufficiently large in order to render the definition of elastic constants non unique. These t…
Decacyclene as complexation manifold: synthesis, structure and properties of its Fe2 and Fe4 slipped triple-decker complexes.
2006
Reaction of [(eta(5)-Me4EtC5)Fe(II)Cl(tmeda)] (tmeda = N,N,N'N'-tetramethylethylenediamine) with a polyanion solution of decacyclene (1) results in the formation of the triple-deckers [{(eta(5)-Me4EtC5)Fe}2-mu2-(eta(6):eta(6)-decacyclene)] (3) and [{(eta(5)-Me4EtC5)Fe}4-mu4-(eta(6):eta(6):eta(6):eta(6)-decacyclene)] (4). Metal complexation in 3 and 4 occurs on opposite faces of the pi perimeter in an alternating mode. The decacyclene ring adopts a gently twisted molecular propeller geometry with twofold crystallographic symmetry (C2). Complex 4 crystallizes in the chiral space group C222(1); the investigated crystal only contains decacyclene rings with M chirality. The handedness can be ass…
Symmetry and Stability of the Rutile-Based TiO2 Nanowires: Models and Comparative LCAO-Plane Wave DFT Calculations
2012
The rod symmetry groups for monoperiodic (1D) nanostructures have been applied for construction of models for bulk-like titania nanowires (NWs) cut from a rutile-based 3D crystal along the direction of a chosen crystallographic symmetry axis (in this study we consider only Ti atom-centered axes). The most stable [001]-oriented TiO2 NWs with rhombic cross sections are found to display the energetically preferable {110} facets only, while the nanowires with quasi-square sections across the [110] axis are formed by the alternating {110} and {001} facets. For simulations on rutile-based nanowires possessing four different diameters for each NW type, we have performed comparative large-scale ab …
Morphotropic ceramic solid solutions of the Pb(B3+½Nb½)O3- PbTiO3binary system
2000
Abstract The structure, dielectric, ferroelectric and electromechanical properties of lead lutecium niobate-titanate (PLuNT) and lead erbium niobate-titanate (PErNT) binary systems are reported. The data of phase diagram, crystallographic symmetry and morphotropic phase boundaries (MPB) are provided. High values of the electromechanical coupling coefficients kp = 0.66, kt = 0.48, k31 = 0.36 of (1−x)PLuN − xPT ceramics are attained in compositions near the MPB at x≈0.41.
Coin metal adsorption on defective MgO(001) surface: ab initio study
2005
First principles slab calculations have been performed for Ag and Cu adsorption on periodically distributed point defects (a single O2– or Mg2+ vacancy per 2×2 surface supercell) on the non-polar MgO(001) substrate. Using the procedure implemented in the CRYSTAL-03 code, both Fs and Vs centers were modeled by retaining in the vacancy the basis set of the missing O2– and Mg2+ ions, respectively, with the local relaxation of the nearest substrate ions. Adsorption of metal atom over the Fs center (2.4 eV vs. 2.1 eV per Cu and Ag adatoms, respectively) is much stronger as compared with regular O2− sites (0.6 eV vs. 0.4 eV, respectively). The Fs center donates a substantial charge towards Ag and…
An investigation of the spot profiles in transmission electron diffraction from Langmuir-Blodgett films of aliphatic chain compounds
1990
The profiles of the transmission diffraction spots from a Langmuir-Blodgett film of aliphatic chain compound are derived from three plausible models of molecular organisation : a polycrystalline phase with long-range orientational order; a paracrystal possessing a density of point defects which do not interrupt the lattice rows ; and a Nelson and Halperin hexatic smectic phase in the «Debye-Huckel» limit of weakly-interacting dislocations. The three resulting predictions are distinctly different. Experimental results are presented for the room-temperature diffraction patterns from Langmuir-Blodgett films of a lipid, DMPE, and the cadmium soap of a fatty acid, 22-tricosenoic. Only the Nelson…
Using Electron Diffraction to Solve the Crystal Structure of a Laked Azo Pigment
2009
The structure of the ζ phase of Pigment Red 53:2 was solved using electron diffraction data. The unit cell determination appeared to be more complicated due to twinning of the crystals. A series of...
Effective hydrostatic limits of pressure media for high‐pressure crystallographic studies
2007
The behavior of a number of commonly used pressure media, including nitrogen, argon, 2-propanol, a 4:1 methanol–ethanol mixture, glycerol and various grades of silicone oil, has been examined by measuring the X-ray diffraction maxima from quartz single crystals loaded in a diamond-anvil cell with each of these pressure media in turn. In all cases, the onset of non-hydrostatic stresses within the medium is detectable as the broadening of the rocking curves of X-ray diffraction peaks from the single crystals. The onset of broadening of the rocking curves of quartz is detected at ∼9.8 GPa in a 4:1 mixture of methanol and ethanol and at ∼4.2 GPa in 2-propanol, essentially at the same pressures …