Search results for "Crystallographic"
showing 10 items of 202 documents
Synthesis and X-ray crystal structure of manganese(II) pyridine-2-thiolate
1997
2:1 complexes of pyridine-2(1H)-thione (LH) with manganese(II) were prepared: (LH)2MnCl2, (LH)2MnBr2 and [L2Mn]n. The polymeric chain structure of [L2Mn]n contains a distorted octahedrally coordinated manganese (MnN2S4) with chelating ligands, the N-atoms are in cis positions and the S-atoms of the pyridine-2-thiolates bridging; crystallographic data for [C10H8MnN2S2]n: space group C2/c (monoclinic), and R1 = 0.0571 for 1326 reflexes with Fo4σ(Fo).
About the mechanism of the solid-way synthesis of barium metatitanate. Industrial consequences
1990
Abstract Barium metatitanate BaTiO 3 is the main component of the dielectric material of class 2 ceramic capacitors. The solid state reaction between BaCO 3 and TiO 2 is still one of the main industrial ways for the BaTiO 3 preparation. The aim of the paper is to propose a coherent set of interfacial reactions in order to explain all the experimental results previously published about this well known solid state reaction. The paper gives at first a summary of the main experimental results. Then all the interfacial reactions at the various stages of the advancement are proposed. All these reactions involve the same main point defects in BaTiO 3 which are barium and oxygen vacancies. It has b…
First‐principles modelling of defects in advanced nuclear fuels
2007
In this paper we present and discuss the results of first first-principle modelling of point defects in nitride nuclear fuels. Calculations have been performed using the VASP computer code combined with supercells containing up to 250 atoms. The effective atomic charges, the electronic density redistribution, atomic displacements around U and N vacancies and their formation energies are discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Hydrogen-related conversion processes of Ge-related point defects in silica triggered by UV laser irradiation
2005
The conversion processes of Ge-related point defects triggered in amorphous SiO2 by 4.7eV laser exposure were investigated. Our study has focused on the interplay between the (=Ge•-H) H(II) center and the twofold coordinated Ge defect (=Ge••). The former is generated in the post-irradiation stage, while the latter decays both during and after exposure. The post-irradiation decay kinetics of =Ge•• is isolated and found to be anti-correlated to the growth of H(II), at least at short times. From this finding it is suggested that both processes are due to trapping of radiolytic H0 at the diamagnetic defect site. Furthermore, the anti-correlated behavior is preserve…
UV-Photoinduced Defects In Ge-Doped Optical Fibers
2005
We investigated the effect of continuous-wave (cw) UV laser radiation on single-mode Ge-doped H2- loaded optical fibers. An innovative technique was developed to measure the optical absorption (OA) induced in the samples by irradiation, and to study its dependence from laser fluence. The combined use of the electron spin resonance (ESR) technique allowed the structural identification of several radiation-induced point defects, among which the Ge(1) (GeO4 -) is found to be responsible of induced OA in the investigated spectral region.
LCAO calculation of neutral defects in GaN
2005
Four well known HF, LDA, GGA and B3LYP Hamiltonians in LCAO approximation have been used in band structure calculations to obtain the main properties of the perfect GaN crystal with hexagonal lattice (C space group). Calculated lattice parameters, elastic constants and the band gap have been compared with the experimental data and the results of other calculations. As a consequence, the GGA Hamiltonian has been chosen, giving the lattice parameters a = 3.20 A, c = 5.20 A, u = 0.377, the bulk modulus B = 206 GPa and the energy gap Eg = 2.7 eV. These results reasonably reproduce the experimental data. For the point defects calculation (VGa, VN, MgGa, ZnGa, CN, and SiN) the supercell model was…
Calculations of radiation-induced point defects, polarons and excitons in ferroelectric perovskites
2002
Abstract We review results of our recent large-scale computer simulations of radiation-induced point defects, excitons and polarons in ABO3 perovskite crystals, focusing mostly on KNbO3 and KTaO3 as representative examples. We have calculated the atomic and electronic structure of defects, their optical absorption, defect-induced electron density redistribution, and activation energies for defect migration. The majority of our results were obtained using the quantum-chemical method of the intermediate neglect of differential overlap (INDO) based on the Hartree–Fock formalism, as well as the shell model (SM). The main findings are compared with those obtained by means of ab initio density fu…
Large scale computer modelling of point defects in ABO 3 perovskites
2005
We present results for basic intrinsic defects: F-type electron centers, free and bound electron and hole polarons in ABO3 perovskites. Both one-site (atomic) and two-site (molecular) hole polarons are expected to coexist, characterized by close absorption energies. Shell Model (SM) and intermediate neglect of differential overlap (INDO) calculations of the F center diffusion indicate that the relevant activation energy is quite low, ca. 0.8 eV. Further INDO calculations support the existence of self-trapped electron polarons in PbTiO3, BaTiO3, KNbO3, and KTaO3 crystals. The relevant lattice relaxation energies are typically 0.2 eV, whereas the optical absorption energies are around 0.8 eV.…
Computer modelling of radiation damage in cation sublattice of corundum
1998
Results of quantum chemical computer simulations of close Frenkel defects in corundum crystals are presented and discussed. The conclusion is drawn that the energy barrier for a back recombination up to fourth nearest neighbours is less than 0.3 eV, i.e. such pairs should be unstable at temperatures above 40 K.
Regular Horizontal Patterning on Colloidal Crystals Produced by Vertical Deposition
2009
Colloidal particles have proved to be a suitable precursor to the formation of nanoscaled materials. More explicitly, they are a suitable way to create photonic band gap materials in 3D. Several methods have been developed to assemble colloidal multilayer systems, and have yielded various levels of success. The vertical deposition method has shown itself to be one of the best in terms of time, control of the final product, crystal size and homogeneity. Despite this, the resulting crystals often present point defects, dislocations, cracks and polycrystallinity, as well as a horizontal modulation of film thickness. These defects compromise the possible utilities of the crystals. The study of …