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Crystallographic Positions (crystallographic + position)
Selected AbstractsHydrogen bonds and local symmetry in the crystal structure of gibbsiteMAGNETIC RESONANCE IN CHEMISTRY, Issue 11 2010Anastasia Vyalikh Abstract First-principles quantum mechanical calculations of NMR chemical shifts and quadrupolar parameters have been carried out to assign the 27Al MAS NMR resonances in gibbsite. The 27Al NMR spectrum shows two signals for octahedral aluminum revealing two aluminum sites coordinated by six hydroxyl groups each, although the crystallographic positions of the two Al sites show little difference. The presence of two distinguished 27Al NMR resonances characterized by rather similar chemical shifts but quadrupolar coupling constants differing by roughly a factor of two is explained by different character of the hydrogen bonds, in which the hydroxyls forming the corresponding octahedron around each aluminum site, are involved. The Al-I site characterized by a CQ = 4.6 MHz is surrounded by OHgroups participating in four intralayer and two interlayer hydrogen bonds, while the Al-II site with the smaller quadrupolar constant (2.2 MHz) is coordinated by hydroxides, of which two point toward the intralayer cavities and four OH-bonds are aligned toward the interlayer gallery. In high-resolution solid-state 1H CRAMPS (combination of rotation and multiple-pulse spectroscopy) four signals with an intensity ratio of 1:2:2:1 are resolved which allow to distinguish six nonequivalent hydrogen sites reported in the gibbsite crystal structure and to ascribe them to two types of structural OH groups associated with intralayer and interlayer hydrogen bonds. This study can be applied to characterize the gibbsite-like layer,intergallery interactions associated with hydrogen bonding in the more complex systems, such as synthetic aluminum layered double hydroxides. Copyright © 2010 John Wiley & Sons, Ltd. [source] The oxygen vacancy in Ga2O3: a double resonance investigation,MAGNETIC RESONANCE IN CHEMISTRY, Issue S1 2005H. J. Kümmerer Abstract When produced under reducing conditions, ,-Ga2O3 is transformed into an n -type semiconductor with delocalized conduction electrons that exhibit a very strong electron spin resonance (ESR) and a strong hyperfine coupling to the gallium nuclei of the host lattice. We apply the Overhauser-shift technique to investigate single crystals of this compound. With extension to the high magnetic field of a W-band spectrometer, we were able to resolve all spectral lines that were recorded and to assign them to their corresponding electronic and nuclear states. This separate analysis was the basis to access additional sample characteristics: the hyperfine coupling that is actually averaged out in the ESR signal, as well as the nuclear relaxation rates could be analyzed. Systematic measurements by varying the microwave power revealed the Overhauser shift in thermal equilibrium. The signal could be tracked to very small microwave saturation parameters, at which the deviation from the usual linear relation between power and shift becomes evident and the shift clearly approaches a constant value. This value in equilibrium was determined directly from a fit to a sequence of measurements, whereas standard X-band experiments only provided indirect conclusions. The probability densities of the electrons at the nuclei in the two nonequivalent crystallographic positions,the lattice sites with octahedral and tetrahedral coordination,could also be determined directly. The enhanced resolution revealed an otherwise hidden substructure in the nuclear resonance signals. On the basis of a microscopic model, this structure could be used to probe the environment of the oxygen vacancy more precisely and to determine the extension of the electronic wave function of the donor electrons. Copyright © 2005 John Wiley & Sons, Ltd. [source] Fully relativistic analysis of the absorption spectra of Ca3Sc2Ge3O12:Ni2+PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 12 2006M. G. Brik Abstract Systematic analysis of the energy level schemes, ground state absorption (GSA) and covalency effects for the Ni2+ ion in Ca3Sc2Ge3O12 was performed. The recently developed first-principles approach to the analysis of the absorption spectra of impurity ions in crystals based on the discrete variational multi-electron method (DV-ME) [K. Ogasawara et al., Phys. Rev. B 64, 115413 (2001)] was used in the calculations. As a result, complete energy level schemes of Ni2+ and its absorption spectra at both possible crystallographic positions (distorted octahedral Sc3+ and tetrahedral Ge4+ positions) were calculated, assigned and compared with experimental data. Energies of the charge transfer (CT) transitions for both positions are estimated. Numerical contributions of all possible electron configurations into the calculated energy states were determined. By performing analysis of the molecular orbitals (MO) population, it was shown that the covalency of the chemical bonds between the Ni2+ and O2, ions increases in passing from the hexa- to the tetra-coordinated complex. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electronic structure of the uranium monostannide USnPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003A. Szajek Abstract The uranium monostannide USn crystallizes in the orthorhombic ThIn-type structure (Pbcm space group) in which the uranium and tin atoms each occupy three crystallographic positions. The electronic structure of USn compound has been studied using spin-polarized tight binding version of the linear muffin-tin orbital method in the atomic sphere approximation (TB LMTO ASA). The calculations showed that all three types of uranium atoms are magnetically ordered and their moments depend on local environments. [source] Cluster approach to the Ti2Ni structure typeACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2006D. Rodi It has been established that the 16(c) first coordination clusters in the Ti2Ni structure type (space group Fd3m) follow icosahedral-face orientational ordering along regular tetrahedron edge directions. The actual crystal structure appears due to the prevalence of the face-centred cubic translational ordering over the cluster assembling. This way, the competition of the `regular' crystal phase and its icosahedral analogue is reconstructed at the atomic level. The model accounts for the markedly different electronic characteristics at the different crystallographic positions obtained by hyperfine interaction measurements, and other curious experimental facts help to create an exact physical definition of the first coordination in the solid state and to distinguish between various structure types on fundamental principles. [source] Two different one-dimensional structural motifs in [catena -{Cu(tacn)}2Pd(CN)4]Br2·[catena -Cu(tacn)Pd(CN)4]2·H2O (tacn is 1,4,7-triazacyclononane)ACTA CRYSTALLOGRAPHICA SECTION C, Issue 7 2009Juraj Kuchár The title compound, catena -poly[[bis[(triazacyclononane-,3N,N,,N,,)copper(II)]-di-,-cyanido-,4N:C -palladate(II)-di-,-cyanido-,4C:N] dibromide bis[[(triazacyclononane-,3N,N,,N,,)copper(II)]-,-cyanido-,2N:C -[dicyanidopalladate(II)]-,-cyanido-,2C:N] monohydrate], {[Cu2Pd(CN)4(C6H15N3)2]Br2·[Cu2Pd2(CN)8(C6H15N3)2]·H2O}n, (I), was isolated from an aqueous solution containing tacn·3HBr (tacn is 1,4,7-triazacyclononane), Cu2+ and tetracyanidopalladate(2,) anions. The crystal structure of (I) is essentially ionic and built up of 2,2-electroneutral chains, viz. [Cu(tacn)(NC),Pd(CN)2,(CN),], positively charged 2,4-ribbons exhibiting the composition {[Cu(tacn)(NC)2,Pd(CN)2,Cu(tacn)]2n+}n, bromide anions and one disordered water molecule of crystallization. The O atom of the water molecule occupies two unique crystallographic positions, one on a centre of symmetry, which is half occupied, and the other in a general position with one-quarter occupancy. One of the tacn ligands also exhibits disorder. The formation of two different types of one-dimensional structural motif within the same structure is a unique feature of this compound. [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] | ||||||||||