Search results for "Lattice"
showing 10 items of 3278 documents
Structural Evolution and Vanadium Distribution in the Preparation of V4+-ZrSiO4 Solid Solutions from Gels
2004
Vanadium-containing ZrSiO4-gel precursors with nominal compositions Vx-ZrSiO4 with x= 0.0, 0.002, 0.004, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.1, and 0.2 were prepared using a previously reported procedure and thermally treated over a range of temperature up to the formation of the V4+-ZrSiO4 solid solution. The structural evolution and the V4+ location and its homogenous distribution were followed using powder X-ray diffractometry and electron spin resonance spectroscopy (ESR). Our experiments showed that a tetragonal form of V4+-ZrO2 was the first crystalline phase obtained on heating the gels. On further heating, a phase transformation to the monoclinic form of V4+-ZrO2 took place. Final…
Low-temperature methyl group dynamics of hexamethylbenzene in crystalline and glassy matrices as studied by 2H NMR
1991
Abstract 2 H NMR spectra of hexamethylbenzene (HMB) in protonated crystalline and amorphous matrices at low temperatures are presented. All spectra reveal lineshape changes which can be attributed to methyl group tunnelling. Compared to neat HMB, a drastic increase of the tunnelling frequency is found for all systems. This indicates that the hindering potential originates predominantly from intermolecular forces. We studied the temperature dependence of these spectra and the spin-lattice relaxation in order to exclude a distribution of motional correlation times describing a thermally activated process. In addition, we find a distortion of the methyl tetrahedron.
Mn3+ in garnets III. Absence of Jahn-Teller distorition in synthetic Mn3+-bearing garnet
1985
The crystal structure of a ternary Mn3+-bearing garnet, close to the composition Gross34 Spess26 CaMnGt40 has been refined to a weighted R-value of 0.051 for 440 unique reflections. The lattice constant is 11.867(1) A, fractional atomic parameters for oxygen are x=0.0374 (2), y=0.0470 (2), and z=0.6532 (2). Refinement of site occupancies gave 0.81 (3) Ca+0.19 (3) Mn2+ in the eight coordinated site 24c and 0.64 (2) Al+0.36 (2) Mn3+ in the octahedral site 16a. No deviation from the cubic space group Ia3d has been observed, the Mn3+-bearing YO6 octahedron is nearly regular. Thus, the presence of the 3d 4-configurated ion Mn3+ does not cause Jahn-Teller distortion in this garnet structure.
Improved embedded molecular cluster model
2002
We demonstrate that boundary effects (i.e., displacements of the cluster boundary atoms from their lattice sites and the difference between effective charges of the perfect crystal atoms and those of the cluster atoms in the case of a cluster with no point defect in it) in an embedded molecular cluster (EMC) model can be radically reduced. A new embedding scheme is proposed. It includes search for the structural elements (SE) of which perfect crystal is composed, use of corresponding to these SE expression for the total energy, and application of the degree of localization of equations consistent with the wave functions of the cluster. To get equations for the cluster wave functions, the pr…
An Inorganic Double Helix Sheathing Alkali Metal Cations: ANb2P2S12(A=K, Rb, Cs), A Series of Thiophosphates Close to the Metal-Nonmetal Boundary-Cha…
2004
The new quaternary niobium thiophosphates ANb 2 P 2 S 1 2 (A=K, Rb, Cs) have been prepared and characterized. The title compounds were synthesized by reacting Nb metal, A 2 S, P 2 S 5 , and S at 600-700°C in evacuated silica tubes. They crystallize as "stuffed" variants of the tetragonal TaPS 6 structure type in the tetragonal space group I42d with eight formula units per unit cell and lattice constants a=15.923(2) and c=13.238(3) A for CsNb 2 P 2 S 1 2 , a= 15.887(3) and c=13.132(3) A for RbNb 2 P 2 S 1 2 , and a=15.850(2) and c= 13.119(3) A for KNb 2 P 2 S 1 2 . Their structures are based on double helices formed from interpenetrating, noninteracting spiral chains of binuclear [Nb 2 S 1 2…
Site-Specific Information on Membrane Protein Folding by Electron Spin Echo Envelope Modulation Spectroscopy
2010
Compared to folding of soluble proteins, folding of membrane proteins is complicated by the fact that it requires an amphiphilic environment. Few existing techniques can provide structurally resolved information on folding kinetics. For the major plant light harvesting complex LHCII, it is demonstrated that changes in water accessibility of a particular amino acid residue can be followed during folding by measuring the hyperfine interaction of spin labels with deuterium nuclei of heavy water. The incorporation of residue 196 into the hydrophobic core of a detergent micelle was investigated. The technique provides a time constant that is similar to the one found with fluorescence spectroscop…
Phase transition kinetics and reorientational dynamics of the plastic crystal meta-carborane studied by deuteron NMR
2002
Meta-carborane shows an orientationally disordered, orthorhombic phase which transforms into an ordered monoclinic phase near 170 K and into a quasi-isotropic cubic phase near 280 K. From deuteron magnetization measurements both transitions are found to exhibit a thermal hysteresis and slow transformation kinetics. Furthermore, solid-echo NMR spectra and spin-lattice relaxation times are reported. These results indicate that in the orthorhombic phase the meta-carborane molecules perform a strongly anisotropic motion which is characterized by a temperature dependent amplitude.
Rietveld Refinement of Tetragonal V4+–ZrO2 Solid Solutions Obtained from Gels by X-ray Powder Diffraction
2002
Abstract The crystal structure of three tetragonal V x Zr 1− x O 2 solid solutions, with x =0.025, 0.05, and 0.075, prepared by heating dried gel precursors at 450°C in air atmosphere, have been determined by Rietveld refinement on the basis of powder X-ray powder diffractometer data. They contain V 4+ cations surrounded by eight oxygens, four at a distance between 2.079 and 2.093 A and another four at longer distances between 2.369 and 2.348 A. The estimation of the crystal average oxygen position from the X-ray lattice parameter of V x Zr 1− x O 2 conform with the relationship proposed by Howard et al. ( J. Am. Ceram. Soc. 81, 241 (1998)).
Fast microscopic method for large scale determination of structure, morphology, and quality of thin colloidal crystals.
2006
We present a novel fast microscopic method to analyze the crystal structures of air-dried or suspended colloidal multilayer systems. Once typical lattice spacings of such films are in the range of visible light, characteristic Bragg scattering patterns are observed. If in microscopic observations these are excluded from image construction, a unique color coding for regions of different structures, morphologies, and layer numbers results. Incoherently scattering defect structures, however, may not be excluded from image construction and thus remain visible with high resolution.
A Wavy Two-Dimensional Covalent Organic Framework from Core- Twisted Polycyclic Aromatic Hydrocarbons
2019
A high degree of crystallinity is an essential aspect in two-dimensional covalent organic frameworks, as many properties depend strongly on the structural arrangement of the different layers and their constituents. We introduce herein a new design strategy based on core-twisted polycyclic aromatic hydrocarbon as rigid nodes that give rise to a two-dimensional covalent organic framework with a wavy honeycomb (chairlike) lattice. The concave–convex self-complementarity of the wavy two-dimensional lattice guides the stacking of framework layers into a highly stable and ordered covalent organic framework that allows a full 3D analysis by transmission electron microscopy revealing its chairlike …