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Crystal Lattice (crystal + lattice)
Selected AbstractsDicopper(II) Complexes with the Enantiomers of a Bidentate Chiral Reduced Schiff Base: Inclusion of Chlorinated Solvents and Chiral Recognition of1,2-Dichloroethane Rotamers in the Crystal LatticeEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 14 2006Vamsee Krishna Muppidi Abstract Bisphenoxo-bridged dicopper(II) complexes [Cu2Ln2Cl2] {1 (n = 1) and 2 (n = 2)} with the N,O-donor reduced Schiff bases N -(2-hydroxybenzyl)-(R)-,-methylbenzylamine (HL1) and N -(2-hydroxybenzyl)-(S)-,-methylbenzylamine (HL2) have been synthesised and characterised. In both 1 and 2, the bidentate chiral ligands coordinate the metal centres through the secondary amine N atom and the bridging phenolate O atom. The chloride ion occupies the fourth coordination site and completes a slightly distorted square-planar NO2Cl environment around each copper(II) centre. Magnetic susceptibility measurements in the solid state suggest a strong antiferromagnetic interaction between the metal centres in both complexes. Both 1 and 2 readily form 1:1 host-guest compounds with chlorinated solvents such as CH2Cl2, CHCl3 and Cl(CH2)2Cl. All the host-guest compounds crystallise in noncentrosymmetric space groups. 1·CH2Cl2 and 2·CH2Cl2 crystallise in the P21 space group while 1·CHCl3, 2·CHCl3, 1·Cl(CH2)2Cl and 2·Cl(CH2)2Cl crystallise in the P212121 space group. In these inclusion crystals, the C,H···Cl interactions between the guest and the host molecules are primarily responsible for enclatheration of the chloroalkane molecules. In the case of CH2Cl2, one of its Cl atoms acts as the acceptor. On the other hand, for CHCl3 and Cl(CH2)2Cl, the metal coordinated Cl atom of the host complex acts as the acceptor. The structures of 1·(P)-Cl(CH2)2Cl and 2·(M)-Cl(CH2)2Cl provide rare examples for chiral recognition of the right handed (P) and the left handed (M) gauche forms of Cl(CH2)2Cl in molecular assemblies. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] The embedded ion method: A new approach to the electrostatic description of crystal lattice effects in chemical shielding calculationsCONCEPTS IN MAGNETIC RESONANCE, Issue 5 2006Dirk Stueber Abstract The nuclear magnetic shielding anisotropy of NMR active nuclei is highly sensitive to the nuclear electronic environment. Hence, measurements of the nuclear magnetic shielding anisotropy represent a powerful tool in the elucidation of molecular structure for a wide variety of materials. Quantum mechanical ab initio nuclear magnetic shielding calculations effectively complement the experimental NMR data by revealing additional structural information. The accuracy and capacity of these calculations has been improved considerably in recent years. However, the inherent problem of the limitation in the size of the systems that may be studied due to the relatively demanding computational requirements largely remains. Accordingly, ab initio shielding calculations have been performed predominantly on isolated molecules, neglecting the molecular environment. This approach is sufficient for neutral nonpolar systems, but leads to serious errors in the shielding calculations on polar and ionic systems. Conducting ab initio shielding calculations on clusters of molecules (i.e., including the nearest neighbor interactions) has improved the accuracy of the calculations in many cases. Other methods of simulating crystal lattice effects in shielding calculations that have been developed include the electrostatic representation of the crystal lattice using point charge arrays, full ab initio methods, ab initio methods under periodic boundary conditions, and hybrid ab initio/molecular dynamics methods. The embedded ion method (EIM) discussed here follows the electrostatic approach. The method mimics the intermolecular and interionic interactions experienced by a subject molecule or cluster in a given crystal in quantum mechanical shielding calculations with a large finite, periodic, and self-consistent array of point charges. The point charge arrays in the EIM are generated using the Ewald summation method and embed the molecule or ion of interest for which the ab initio shielding calculations are performed. The accuracy with which the EIM reproduces experimental nuclear magnetic shift tensor principal values, the sensitivity of the EIM to the parameters defining the point charge arrays, as well as the strengths and limitations of the EIM in comparison with other methods that include crystal lattice effects in chemical shielding calculations, are presented. © 2006 Wiley Periodicals, Inc. Concepts Magn Reson Part A 28A: 347,368, 2006 [source] Magnonic excitations versus three-dimensional structural periodicity in magnetic compositesCRYSTAL RESEARCH AND TECHNOLOGY, Issue 6 2006M. Krawczyk Abstract The study deals with the spin wave spectrum in magnetic macrostructure (composed of two ferromagnetic materials) showing a 3D periodicity; the structure considered consists of spherical ferromagnetic grains disposed in the nodes of a simple cubic crystal lattice and embedded in a matrix with different ferromagnetic properties. It is shown that the magnonic spectrum of this composite structure exhibits frequency regions forbidden for magnon propagation, and the energy gaps are sensitive to the exchange contrast between the constituent materials as well as to the magnetization contrast. The widths of the respective magnonic gaps are studied as functions of parameters characterizing the magnetic structure. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Optical, Magnetic and Structural Properties of the Spin-Crossover Complex [Fe(btr)2(NCS)2]·H2O in the Light-Induced and Thermally Quenched Metastable StatesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 36 2007Vincent Legrand Abstract [Fe(btr)2(NCS)2]·H2O [btr = 4,4,-bis(1,2,4-triazole)] is thearchetype of highly cooperative and low-dimensional spin-crossover complexes, which exhibit low-spin (LS) to high-spin (HS) light-induced conversion at very low temperature. The structural reorganizations related to the light-induced and thermally induced LS,HS transitions were characterized by single-crystal X-ray diffraction below the relaxation temperature (T = 15 K < TLIESST) and at 130 K within the thermal hysteresis loop. We show that the LIESST and thermal spin transitions lead to the same structural variations, namely an elongation of the Fe,N bonds by 0.18 Å (Fe,NNCS) and 0.20 Å (Fe,Nbtr), on going from LS to HS, together with a reorientation of the NCS group by nearly 13°. The atomic displacement amplitudes, derived from the crystal structures, indicate lattice vibration modes of larger amplitudes and correlatively lower vibration frequencies in the HS state. The deformation of the crystal lattice as a function of temperature and laser excitation was quantitatively analyzed in terms of the HS and LS thermal-expansion (,HS and ,LS) and spin-transition spontaneous-strain (,) tensors. The eigendirections and eigenvalues of the , and , tensors correlate well with the weak and strong interactions in the solid and are responsible for the high cooperativity and low-dimensional behaviour. Magnetic and spectroscopic measurements were performed in all the different spin states and related to the structural findings. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] One-Dimensional Oxalato-Bridged Metal(II) Complexes with 4 - Amino-1,2,4-triazole as Apical LigandEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 21 2005Urko García-Couceiro Abstract The synthesis, chemical characterization, thermal behavior and magnetic properties of six new one-dimensional oxalato-bridged metal(II) complexes of formula [M(,-ox)(4atr)2]n [MII = Cu (1), Ni (2), Co (3), Zn (4), Fe(5)] and [Cd(,-ox)(4atr)2(H2O)]n (6) (ox = oxalato dianion, 4atr = 4-amino-1,2,4-triazole) are reported. The crystal structures of 1 and 6 have been solved by single-crystal X-ray diffraction, whereas the remaining compounds have been studied by means of X-ray powder diffraction methods. Compounds 1,5 are isomorphous and crystallize in the triclinic space group P1¯ with unit cell parameters for 1 of a = 5.538(1) Å, b = 7.663(1) Å, c = 7.711(2) Å, , = 62.21(1)°, , = 73.91(1)°, , = 86.11(1)°, and Z = 1. The crystal structures are comprised of one-dimensional linear chains in which the trans -[M(4atr)2]2+ units are sequentially bridged by bis(bidentate) oxalato ligands, resulting in an octahedral O4N2 donor set. Cryomagnetic susceptibility measurements show the occurrence of antiferromagnetic intrachain interactions for 2, 3, and 5, whereas compound 1 exhibits a weak ferromagnetic coupling in agreement with the out-of-plane exchange pathway involved. The magnetic behavior of 1 and 2 is analyzed and discussed in the light of the available magneto-structural data for analogous systems. CdII complex crystallizes in the monoclinic space group C2/c with unit cell parameters of a = 16.128(2) Å, b = 6.757(1) Å, c = 11.580(2) Å, , = 104.46(1)°, and Z = 4. Its crystal structure contains one-dimensional chains in which metal centers are heptacoodinated to four oxygen atoms from two symmetry-related bis(bidentate) oxalato bridges, two endocyclic nitrogen atoms of trans -coordinated triazole ligands and one water molecule, to give a CdO4OwN2 pentagonal-bipyramidal geometry. Thermoanalytical and variable-temperature X-ray powder diffraction analyzes show that compound 6 undergoes a reversible dehydration,hydration process in which the anhydrous residue crystallizes with a different crystal lattice retaining the dimensionality of the oxalato,metal framework. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] The Role of Strain in New Semiconductor DevicesADVANCED ENGINEERING MATERIALS, Issue 4 2009Alex Dommann HRXRD is a very sensitive and non destructive technique to determine the strain in thin layer materials such as electron guides or the strain induces by the second order package of SOCs. In reciprocal space mapping (RSM), it is possible to separate the elastic component of the scattered intensity from the diffuse one. As a consequence, it is possible to study diffuse scattering due to defects of the crystal lattice. As an example we show also RSM's of a high-speed SiGe pMOS structure. [source] Bentonite as a Natural Adsorbent for the Sorption of Iron from the Ground Water Exploited from Aswan Area, EgyptGROUND WATER MONITORING & REMEDIATION, Issue 1 2004Gharib M. Taha Sorption of dissolved Fe2+ on bentonite was studied using a batch technique. The distribution coefficient, Kd, was evaluated for a bentonite-iron system as a function of contact time, pH, sorbent and sorbate concentrations, and temperature. Sorption results were interpreted in terms of Freundlich's and Langmuir's equations. Thermodynamic parameters for the sorption system were determined at three temperatures: 298°, 308°, and 318°K. The values of ,H°(-4.0 kjmol,1) and ,G°(-2.46 Kjmol,1) at 298°K (25°C) suggest that sorption of iron on bentonite is an exothermic and a spontaneous process. The ,G° value became less negative at higher temperatures and, therefore, less iron was sorbed at higher temperatures. The desorption studies with 0.01M CaCl2 and deionized water at iron loading on bentonite showed that more than 90 wt% of the iron is irreversibly sorbed, probably due to the fixation of the iron by isomorphous replacement in the crystal lattice of the sorbent. [source] Metal Chalcogenide Clusters on the Border between Molecules and Materials,ADVANCED MATERIALS, Issue 18 2009John F. Corrigan Abstract The preparative and materials chemistry of high nuclearity transition metal chalcogenide nanoclusters has been in the focus of our research for many years. These polynuclear metal compounds possess rich photophysical properties and can be understood as intermediates between mononuclear complexes and binary bulk phases. Based on our previous results we discuss herein recent advances in three different areas of cluster research. In the field of copper selenide clusters we present the synthesis of monodisperse, nanostructured , -Cu2Se via the thermolysis of well-defined cluster compounds as well as our approaches in the synthesis of functionalized clusters. In case of silver chalcogenides we established a strategy to synthesis cluster compounds containing several hundreds of silver atoms with the nanoclusters arranging in a closely packed crystal lattice. Finally the presented chalcogenide clusters of the group 12 metals (Zn, Cd, Hg) can be taken as model compounds for corresponding nanoparticles as even the smallest of frameworks display a clear structural relationship to the bulk materials. [source] Nanoporous Gold Leaf: "Ancient Technology"/Advanced Material,ADVANCED MATERIALS, Issue 21 2004Y. Ding A free-standing nanoporous gold (NPG) membrane is made by dealloying commercially available white-gold leaf in nitric acid (see Figure). This porous material has an unusual combination of characteristics in that it is metallic with a continuous crystal lattice throughout the porous network, and has a pore size that is adjustable via simple room-temperature post-processing. This ultra-high-surface-area material is potentially very useful for applications such as electrocatalysis and sensing. [source] Autoindexing with outlier rejection and identification of superimposed latticesJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2010Nicholas K. Sauter Constructing a model lattice to fit the observed Bragg diffraction pattern is straightforward for perfect samples, but indexing can be challenging when artifacts are present, such as poorly shaped spots, split crystals giving multiple closely aligned lattices and outright superposition of patterns from aggregated microcrystals. To optimize the lattice model against marginal data, refinement can be performed using a subset of the observations from which the poorly fitting spots have been discarded. Outliers are identified by assuming a Gaussian error distribution for the best-fitting spots and points diverging from this distribution are culled. The set of remaining observations produces a superior lattice model, while the rejected observations can be used to identify a second crystal lattice, if one is present. The prevalence of outliers provides a potentially useful measure of sample quality. The described procedures are implemented for macromolecular crystallography within the autoindexing program labelit.index (http://cci.lbl.gov/labelit). [source] Energetic materials: variable-temperature crystal structure of ,-NTOJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2003Nadezhda B. Bolotina The crystal structure of the metastable , form of 5-nitro-2,4-dihydro-3H -1,2,4-triazol-3-one (,-NTO, monoclinic, P21/c) has been investigated at five temperatures in the range 100,298,K using single-crystal X-ray diffraction techniques. The second-rank thermal expansion tensor has been determined to describe thermal behavior of the crystal. The most significant thermal expansion is in a plane, which is almost perpendicular to the planes of all the NTO molecules. Perpendicular to the plane of maximal thermal expansion, a modest thermal contraction takes place. Both thermal expansion and contraction of the crystal lattice indicate anharmonicity of the atomic thermal motion. The experimental thermal variation of the unit-cell parameters is in qualitative agreement with that previously obtained from molecular dynamics calculations. Rigid-body analysis of the molecular thermal motion was performed using the libration and translation second-rank tensors. Although the translation part of the thermal motion is not strongly anisotropic, the largest displacements of the NTO molecules are oriented in the plane of maximal thermal expansion of the crystal and have significant anharmonic components. The libration motion is more anisotropic, and the largest libration as well as the largest translation principal axes are directed along the C5,N5 bond in each NTO molecule. [source] Highly Ordered Interstitial Water Observed in Bone by Nuclear Magnetic Resonance,JOURNAL OF BONE AND MINERAL RESEARCH, Issue 4 2005Erin E Wilson Abstract NMR was used to study the nanostructure of bone tissue. Distance measurements show that the first water layer at the surface of the mineral in cortical bone is structured. This water may serve to couple the mineral to the organic matrix and may play a role in deformation. Introduction: The unique mechanical characteristics of bone tissue have not yet been satisfactorily connected to the exact molecular architecture of this complex composite material. Recently developed solid-state nuclear magnetic resonance (NMR) techniques are applied here to the mineral component to provide new structural distance constraints at the subnanometer scale. Materials and Methods: NMR dipolar couplings between structural protons (OH, and H2O) and phosphorus (PO4) or carbon (CO3) were measured using the 2D Lee-Goldburg Cross-Polarization under Magic-Angle Spinning (2D LG-CPMAS) pulse sequence, which simultaneously suppresses the much stronger proton-proton dipolar interactions. The NMR dipolar couplings measured provide accurate distances between atoms, e.g., OH and PO4 in apatites. Excised and powdered femoral cortical bone was used for these experiments. Synthetic carbonate (,2-4 wt%)-substituted hydroxyapatite was also studied for structural comparison. Results: In synthetic apatite, the hydroxide ions are strongly hydrogen bonded to adjacent carbonate or phosphate ions, with hydrogen bond (O-H) distances of ,1.96 Å observed. The bone tissue sample, in contrast, shows little evidence of ordered hydroxide. Instead, a very ordered (structural) layer of water molecules is identified, which hydrates the small bioapatite crystallites through very close arrangements. Water protons are ,2.3-2.55 Å from surface phosphorus atoms. Conclusions: In synthetic carbonated apatite, strong hydrogen bonds were observed between the hydroxide ions and structural phosphate and carbonate units in the apatite crystal lattice. These hydrogen bonding interactions may contribute to the long-range stability of this mineral structure. The biological apatite in cortical bone tissue shows evidence of hydrogen bonding with an ordered surface water layer at the faces of the mineral particles. This structural water layer has been inferred, but direct spectroscopic evidence of this interstitial water is given here. An ordered structural water layer sandwiched between the mineral and the organic collagen fibers may affect the biomechanical properties of this complex composite material. [source] Differences in the interaction between aryl propionic acid derivatives and poly(vinylpyrrolidone) K30: A multi-methodological approachJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 11 2009Zehadin Gashi Abstract The present work aims at the application of several methods to explain differences in the physical interaction of some aryl propionic acid derivatives (ibuprofen [IBP], ketoprofen [KET], flurbiprofen [FLU], naproxen [NAP], fenbufen [FEN]) with poly(vinylpyrrolidone) (PVP) K30, stored together at 298,±,0.5 K and 22% RH. X-ray powder diffractometry and 13C-solid state NMR demonstrated that IBP was able to strongly interact with the polymer, while weak interaction was observed for KET, FLU, NAP, and the least for FEN. The interaction of comelted drug and PVP was studied by differential scanning calorimetry by applying the Gordon,Taylor equation, which revealed that small molar drug volumes may favour the drug diffusion through the PVP amorphous chains increasing the polymer free volume and decreasing the mixture Tg. The molecular docking study revealed that intermolecular energy is mainly due to the contribution of van der Waals energy component, causing the differences among the drugs, and is related to the drug,PVP surface contact area in the complex formed. Solid-state kinetic study demonstrated that IBP molecules are involved in a three-dimensional diffusion mechanism within the polymer favoured by its low molar volume that reduces molecular hindrance, and by the weakness of its crystal lattice, which facilitates crystallinity loss and stabilisation of the amorphous phase. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 98:4216,4228, 2009 [source] Extent and mechanism of solvation and partitioning of isomers of substituted benzoic acids: A thermodynamic study in the solid state and in solutionJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 9 2008German L. Perlovich Abstract Temperature dependency of saturated vapour pressure and thermochemical characteristics of fusion processes for 2-, 3- and 4-methoxybenzoic acids (anisic acids) were measured and thermodynamic functions of sublimation, fusion, and evaporation calculated. A new approach to split specific and nonspecific energetic terms in the crystal lattice was developed. For methoxybenzoic acid isomers as well as for a number of analogous molecules, a parameter describing molecular packing density by the ratio of free volume of the molecules in the crystal lattice and van der Waals molecular volume is defined. Its relationship to Gibbs energy of sublimation and to the respective melting points was analysed. Temperature dependencies of solubility in buffers with pH 2.0 and 7.4, n -octanol and n -hexane were measured. The thermodynamic functions of solubility, solvation and transfer processes were deduced. Concentration dependence of partition coefficients for the outlined isomers was measured. Specific and nonspecific solvation terms were distinguished using the transfer from the ,inert' n -hexane to the other solvents. Comparison analysis of specific and nonspecific interactions in the solid state and in solution was carried out. A diagram enabling analysis of the mechanism of the partitioning process was applied. It was found that position of substituents essentially affects the mechanism of partitioning in buffer pH 2.0, however, at pH 7.4, the mechanism is independent of the position of the substituent. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 97:3883,3896, 2008 [source] Influence of crystal structure on the tableting properties of n -alkyl 4-hydroxybenzoate esters (parabens)JOURNAL OF PHARMACEUTICAL SCIENCES, Issue 12 2007Yushi Feng Abstract Certain crystallographic features, such as the existence of slip planes, can greatly facilitate the ability of crystals to deform plastically. An investigation of the relationship between the slip planes and the tableting performance of the crystals of methyl, ethyl, n -propyl, and n -butyl 4-hydroxybenzoate (parabens) was conducted. The absence of slip planes in methyl paraben crystal structure results in significantly poorer tableting performance than the other three parabens. While slip planes are present in the crystal structures of ethyl, propyl, and butyl parabens, they exhibited different plasticity as confirmed by crystal free volume analysis, crystal nano-indentation hardness, and Heckel analysis. Sieved fraction, 150,250 µm, of each paraben powder was compressed into tablets under different conditions. Tablet tensile strength, porosity, and Indices of tableting performance (ITP) were obtained. Under the same compaction pressure, tablet tensile strength was higher for crystals with higher plasticity. Tableting performance, assessed using the ITP, also improved with increasing crystal plasticity. The results confirm that high levels of plasticity, which can result from the presence of slip planes in crystal lattice, plays a critical role in the formation of strong and intact tablets by means of powder compaction. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 96: 3324,3333, 2007 [source] Solid-state properties of warfarin sodium 2-propanol solvateJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 11 2004Agam R. Sheth Abstract The goal of the present work was to understand the effect of relative humidity (RH) and temperature on the molecular structure, crystal structure, and physical properties of warfarin sodium 2-propanol solvate (W). After previous determination of the crystal structure of W, which corresponds to a 1:1 2-propanol solvate, the present work shows that W has a critical RH (60%,<,RH0,,,68%), below which minimal uptake of water occurs, due to surface adsorption, but above which gradual and continuous uptake of water occurs, due to deliquescence. Deliquescence begins at the surface and proceeds inward into the bulk of the crystal. Single crystal X-ray diffractometry indicates no change in the crystal and molecular structure of W during the initial stages of deliquescence. Studies of the unit cell and volume computations of W show that water can neither find space to enter the crystal lattice, nor can replace 2-propanol. Thus, water does not exchange with 2-propanol within the lattice, contrary to previous reports. Storage of single crystals of W at 120°C for 23 h produces shrinkage cracks along the needle (b) axis, which are interpreted as a reduction in d -spacing of the 00l planes. Thus, under thermal stress, W crystals undergo amorphization with concurrent loss of 2-propanol, which may proceed via an intermediate crystalline phase. The phase changes of W, which depend on RH and temperature, are explained at the molecular level. © 2004 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 93:2669,2680, 2004 [source] Space-demanding intramolecular isomerizations in the solid stateJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 3 2002G. Kaupp Abstract The thermal isomerizations of meso - and rac -3,4-dibromo-1,6-diphenyl-1,6-bis(p -tolyl)-1,2,4,5-hexatetraene (1) to give stereospecifically the 3,4-bis(phenyl- p -tolylmethylene)-1,2-dibromocyclobutenes 3 and 5,+,6 were studied in the solid state using atomic force microscopy (AFM) and interpreted on the basis of known crystal structural data. These isomerizations run to completion in the bulk and include highly space-demanding internal rotations around the central bond. Far-reaching anisotropic molecular movements are detected on the major faces that align the surface features along cleavage planes in the initial phase rebuilding stage. Only one of three identified cleavage planes of meso - 1 is successful, owing to closer interactions of the bromine substituents in the non-used cleavage planes. Thus, very fine details can be correlated and predicted for the occurrence of internal rotations and molecular movements in the crystal lattice. The second stage in these intramolecular isomerizations, the phase transformation, produces very high features up to 100,nm and still parallel to the preferred cleavage plane of meso - 1 but in the,µm range without relation to the initial crystal structure in the case of rac - 1. Copyright © 2002 John Wiley & Sons, Ltd. [source] Isotope and disorder effects in the Raman spectra of LiHxD1,x crystalsJOURNAL OF RAMAN SPECTROSCOPY, Issue 8 2001V. G. Plekhanov Most of the physical properties of a solid depend on its isotopic composition in some way or another. Scientific interest, technological promise and increased availability of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies with isotopically controlled crystals. A great number of stable isotopes and well-developed methods for their separation have made it possible to grow crystals of C, LiH, ZnO, ZnSe, CuCl, GaN, GaAs, CdS, Cu2O, Si, Ge and ,-Sn with a controllable isotopic composition. Among these compounds, LiH possesses the largest value of the isotope effect. The great number of theoretical and experimental data suggest that the isotopic composition of a crystal lattice exerts some influence on the vibrational properties of crystals. These effects are fairly large and can be readily measured by modern experimental techniques (ultrasound, Brillouin and Raman scattering and neutron scattering). In addition, crystals of different isotopic compositions possess different Debye temperatures. This difference between an LiH crystal and its deuterated analogue exceeds a 100 K. Very pronounced and general effects of isotopic substitution are observed in phonon spectra. The scattering lines in isotopically mixed crystals are not only shifted (the shift of LO lines exceeds 100 cm,1) but are also broadened. This broadening is related to the isotopic disorder of the crystal lattice. It is shown that the degree of a change in the scattering potential is different for different isotopic mixed crystals. In the case of germanium and diamond crystals, phonon scattering is weak, which allows one to apply successfully the coherent potential approximation (CPA) for describing the shift and broadening of scattering lines. In the case of lithium hydride, the change in the scattering potential is so strong that it results in phonon localization, which is directly observed in experiments. The common nature of the isotopic and disorder effects in a wide range of crystals is emphasized. Copyright © 2001 John Wiley & Sons, Ltd. [source] Local structure of vanadium in doped LiFePO4JOURNAL OF SYNCHROTRON RADIATION, Issue 5 2010Ting Zhao LiFePO4 composites with 5,at.% vanadium doping are prepared by solid state reactions. X-ray absorption fine-structure spectroscopy is used as a novel technique to identify vanadium sites. Both experimental analyses and theoretical simulations show that vanadium does not enter into the LiFePO4 crystal lattice. When the vanadium concentration is lower then 1,at.%, the dopant remains insoluble. Thus, a single-phase vanadium-doped LiFePO4 cannot be formed and the improved electrochemical properties of vanadium-doped LiFePO4 previously reported cannot be associated with crystal structure changes of the LiFePO4via vanadium doping. [source] Effect of hydrogen on electronic structure of fcc iron in relation to hydrogen embrittlement of austenitic steelsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 12 2007S. M. Teus Abstract The total structural energy per primitive lattice cell, density of electron states, spatial distribution of electrons and elastic modulus in fcc Fe,H solid solutions are studied using the density functional theory and Wien2k program package. It is shown that hydrogen increases the density of electron states at the Fermi level. The density of conduction electrons is increased in the vicinity of hydrogen atoms, which suggests that the latter migrate over the crystal lattice surrounded by the clouds of conduction electrons. Calculations of elastic modulus show that hydrogen decreases the shear modulus c44. The consequences for mechanical properties of hydrogen-charged austenitic steels are analysed taking into account the stress for activation of dislocation sources, the line tension of dislocations and the distance between dislocations in pile-ups. It is concluded that hydrogen-induced brittleness of austenitic steels can be satisfactorily interpreted in terms of the hydrogen effect on the electronic structure. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Acoustic phonons in InSb probed by time-resolved X-ray diffractionPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 12 2006A. Morak Abstract Acoustical phonons in InSb were induced with femtosecond light pulses and probed by diffraction of ultrashort X-ray pulses in the crystal lattice. The time dependent transient X-ray diffraction signal due to elastic lattice deformation was measured with subpicosecond resolution. The elastic lattice deformation depends on the temporal evolution of the energy transfer from excited electrons in the semiconductor into the lattice. As already shown in previous investigation a conventional thermoelastic model is not sufficient to describe this coupling process. Here a complex simulation including a two temperature model of the electron and lattice as well as the microscopic behavior of the electron plasma is applied to explain important effects like thermal carrier diffusion and band gap deformation found in the lattice deformation experiments. When this model is used, with realistic values for both pump laser fluences and bulk material constants, excellent agreement between the experimentally observed time dependent lattice deformation and calculated values is obtained throughout the observation period. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Time-dependent evolution of crystal lattice, defects and impurities in CdIn2S4 and GaPPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2009Sergei L. Pyshkin Abstract We demonstrate that in III-V and ternary compounds the natural stimuli for ordered distribution of impurities along crystal nodes and interstitials, host atoms and inherent vacancies in right lattice positions prevail over the others which could lead to a heterogenic distribution. The result is obtained in the crystals, prepared in 1963-1977, due to each 10-15 years monitoring of their optical and mechanical properties. Here we have summarized the results of the 2005-2008 monitoring and compare them with the earlier obtained data. We show that the partly inverse spinel CdIn2S4 turns into the perfect normal spinel, while in GaP the impurity ordering leads to the formation of a new type of crystal lattices. Periodically disposed impurities and inherent structural defects modify, improve and stabilize properties of the crystals. New luminescent phenomena of these crystals, including stimulated emission and "hot" luminescence have been discovered. The existing technologies help us to reproduce these naturally ordered structures and to apply them for new generation of optoelectronic devices. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Luminescence and vibrational properties of erbium-implanted nanoporous GaNPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2008C. B. Soh Abstract Implantation of erbium (Er) into GaN is useful in creating selected areas to emit at the green, yellow and infrared wavelengths. Enhanced erbium activation is obtained when erbium is implanted into porous GaN formed by electrochemical etching than into as-grown GaN. This is due to the increase in surface areas for light extraction and the availability of more free surfaces to accommodate strain when it is annealed. Furnace annealing at 1100 °C for 30 mins in nitrogen gives rise to higher band-edge photoluminescence intensity. Apart from the host GaN phonon modes, we have also observed disorder-induced lattice vibrations at 170, 200 and 350-365 cm,1 from Er-implanted porous GaN. The E2 (high) mode of GaN also shifts towards higher energy at higher annealing temperatures, indicative of more erbium occupying the VGa site (ionic radii of Er > Ga) and hence increasing the compressive stress in the GaN crystal lattice. The prominent defect-induced local vibrational modes in Er-doped nanoporous GaN are also observed in ultraviolet resonant Raman scattering. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Nanostructured antiferromagnetic spin glass in doped Ge near the insulator,metal transitionPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2008A. G. Zabrodskii Abstract It is shown that, on the insulator side of the insulator , metal transition, the single spin density exponentially disappears as T , 0. Such spins are bound into pairs to give an antiferromagnetic (AFM) phase. Upon an increase in temperature the AFM phase is destroyed, the single-spin density, and, as a result, ESR absorption signal becomes stronger. The temperature dependencies of the densities of the pairs and single spins are typical of a chaotic distribution of neutral donors. In this case, there is no Neel temperature. In low degree of compensation, the crystal lattice of Ge with the AFM phase is actually a nanostructured system characterized by anisotropic internal stresses that are the strongest along one of the [110] directions. These stresses give rise to anisotropy of the g-factor which is responsible for experimentally observed splitting of the ESR line. The compensating impurities destroy the AFM phase and reduce this splitting. Local stresses are present in this case, too, but now they appear because of the Coulomb interaction of oppositely charged impurities and have no preferred orientation. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Silicon and carbon vacancies in silicon carbide studied by coincidence Doppler broadening spectroscopyPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007J. D. Zhang Abstract Deconvoluted Coincidence Doppler Broadening Spectroscopy (CDBS) measurements have been made on 300 keV and 1.7 MeV electron irradiated SiC. The lower energy irradiation produces only carbon vacancies while the higher energy produces both carbon and silicon vacancies. This distinction is easily seen in the high (20-35 mrad) momentum range where a clear atomic signal of Si is seen for the carbon vacancy. In addition to the higher momentum region the higher resolution of the deconvoluted CDBS spectra show structural information relating to the crystal lattice. The autocorrelation function obtained for positrons trapped at carbon vacancies is found to show a stronger lattice signal indicative of a more extended positron wave function and a less strongly bound state. Conversely that positron trapped at the silicon vacancy shows a more damped autocorrelation function characteristic of a more spatially confined positron state. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Form, symmetry and packing of biomacromolecules.ACTA CRYSTALLOGRAPHICA SECTION A, Issue 3 2010The aim of this paper is to relate morphological properties of single biomacromolecules based on molecular enclosing forms indexed by an appropriate form lattice to the symmetry of the crystal where the molecules are periodically packed. Similar to the way in which the `molécule intégrante' of Haüy permitted a molecular interpretation of the law of rational indices of crystal growth forms, alternative molecular enclosing forms, indexed by a so-called packing lattice, allow one to bridge the gap between form and crystal lattices. In this first part, selected tutorial examples illustrate the validity of the approach and the crystallographic compatibility between molecular and crystal structures. In particular, integral molecular lattices are shown to imply the observed axial ratios between crystal lattice parameters, leading sometimes to surprising results, like a cubic crystal lattice with a unit cell having a trigonal molecular filling with hexagonal enclosing form. [source] Concise intensity statistics of Friedel opposites and classification of the reflectionsACTA CRYSTALLOGRAPHICA SECTION A, Issue 4 2009U. Shmueli A previous extensive analysis of the mean-square intensity difference of Friedel opposites [Shmueli et al. (2008). Acta Cryst. A64, 476,483] is here concisely re-examined and confirmed by purely statistical methods. The analysis applies to noncentrosymmetric crystals only. For special reflections and centered lattices both mean-square intensity difference and average intensity of Friedel opposites depend on the centering factor of the crystal lattice and/or on the isotropy subgroup of the reflection. A complete classification of the reflections, based on the above intensity statistics, is presented. It is also shown that the experimentally important Bijvoet ratio is found to depend only on the chemical composition of the unit-cell content and the wavelength of the radiation. [source] Atomic-resolution crystal structure of Borrelia burgdorferi outer surface protein A via surface engineeringPROTEIN SCIENCE, Issue 8 2006Koki Makabe Abstract Outer surface protein A (OspA) from Borrelia burgdorferi has an unusual dumbbell-shaped structure in which two globular domains are connected with a "single-layer" ,-sheet (SLB). The protein is highly soluble, and it has been recalcitrant to crystallization. Only OspA complexes with Fab fragments have been successfully crystallized. OspA contains a large number of Lys and Glu residues, and these "high entropy" residues may disfavor crystal packing because some of them would need to be immobilized in forming a crystal lattice. We rationally designed a total of 13 surface mutations in which Lys and Glu residues were replaced with Ala or Ser. We successfully crystallized the mutant OspA without a bound Fab fragment and extended structure analysis to a 1.15 Å resolution. The new high-resolution structure revealed a unique backbone hydration pattern of the SLB segment in which water molecules fill the "weak spots" on both faces of the antiparallel ,-sheet. These well-defined water molecules provide additional structural links between adjacent ,-strands, and thus they may be important for maintaining the rigidity of the SLB that inherently lacks tight packing afforded by a hydrophobic core. The structure also revealed new information on the side-chain dynamics and on a solvent-accessible cavity in the core of the C-terminal globular domain. This work demonstrates the utility of extensive surface mutation in crystallizing recalcitrant proteins and dramatically improving the resolution of crystal structures, and provides new insights into the stabilization mechanism of OspA. [source] The role of side chain conformational flexibility in surface recognition by Tenebrio molitor antifreeze proteinPROTEIN SCIENCE, Issue 7 2003Margaret E. Daley AFP, antifreeze protein; DQF-COSY, double quantum filtered correlated spectroscopy; NMR, nuclear magnetic resonance; NOE, nuclear Overhauser enhancement; NOESY, nuclear Overhauser effect spectroscopy; 3J,,, 3-bond scalar coupling constant between spins H, and H, Abstract Two-dimensional nuclear magnetic resonance spectroscopy was used to investigate the flexibility of the threonine side chains in the ,-helical Tenebrio molitor antifreeze protein (TmAFP) at low temperatures. From measurement of the 3J,,1H- 1H scalar coupling constants, the ,1 angles and preferred rotamer populations can be calculated. It was determined that the threonines on the ice-binding face of the protein adopt a preferred rotameric conformation at near freezing temperatures, whereas the threonines not on the ice-binding face sample many rotameric states. This suggests that TmAFP maintains a preformed ice-binding conformation in solution, wherein the rigid array of threonines that form the AFP-ice interface matches the ice crystal lattice. A key factor in binding to the ice surface and inhibition of ice crystal growth appears to be the close surface-to-surface complementarity between the AFP and crystalline ice, and the lack of an entropic penalty associated with freezing out motions in a flexible ligand. [source] Geometrical theory of triple junctions of CSL boundariesACTA CRYSTALLOGRAPHICA SECTION A, Issue 4 2001V. Y. Gertsman When three grain boundaries having misorientations generating coincidence site lattices (CSLs) meet at a triple junction, a common (triple-junction) CSL is formed. A theory is developed as a set of theorems establishing the relationships between the geometrical parameters of the grain-boundary and triple-junction CSLs. Application of the theory is demonstrated in detail for the case of the cubic crystal system. It is also shown how the theory can be extended to an arbitrary crystal lattice. [source] | ||||||||||||||||||||||||||||||||||