Showing papers on "Crystal published in 1985"

Peptide conformations. Part 31. The conformation of cyclosporin a in the crystal and in solution

Abstract: The all-light-atom X-ray crystal-structure analysis of cyclosporin A (1), a cyclic undecapeptide containing seven N-methylated amino acids, reveals a conformation very similar to that of the previously analysed iodo derivative which is characterised by a twisted β-pleated sheet involving the residues Me-Val-11, MeBmt-1, Abu-2, Sar-3, MeLeu-4, Val-5, MeLeu-6, and Ala-7. The β-bend at Sar-MeLeu-4 is of type II′, and the loop of the residual amino acids involves a cis-peptide bond between MeLeu-9 and MeLeu-10. The NH proton of D-Ala-8 closes a yi-bend with a H-bond to the MeLeu-6 CO group. The crystal was grown from acetone. A closely similar backbone conformation in apolar solvents such as CDCl3 or C6D6 has been derived from the interpretation of the NMR spectral parameters (homo- and heteronuclear NOE effects, coupling constants, chemical-shift values of the C-, H-, and N-atoms). A minor variation in the backbone conformation between crystal and solution is observed in the region of D-Ala-8, where in solution a 3-center H-bond is established between the NH of D-Ala-8 und the carbonyl O-atoms of both MeLeu-6 (yi-turn) and D-Ala-8 (C5-bend). A recently proposed technique to identify intramolecular H-bond via heteronuclear NOE from NH proton to carbonyl C-atoms is critically analysed. The main difference between crystal and solution conformations lies in the orientation of the side chains of the unusual amino acid MeBmt (χ1 = +60° in solution, -168° in the crystal) and of MeLeu-10 (X1 = −60° in solution, +60° in the crystal). The differences in crystal and solution are caused by the break of the intermolecular H-bond of the OH group of MeBmt on dissolution of the crystal. The bifurcated H-bond of D-Ala-8 twists the backbone in this region. Molecular modeling demonstrates that this is the origin of the change in the side chain conformation of MeLeu-10. The intramolecular flexibility in the crystal indicated by the thermal parameters obtained from the X-ray refinement, and in solution by an analysis of spin-lattice relaxation times in the NMR experiments, indicate a fairly rigid backbone and fixed conformations for all the side chains except for that of Abu-2 and the distal atoms of MeBmt.

The effect of the crystal field strength on the optical spectra of Cr3+ in gallium garnet laser crystals

Abstract: We have investigated the basic spectroscopic properties of Cr3+-doped gallium garnets. Weak crystal fields (Dq/B=2.39–2.55) lead to broad4 T 2→4 A 2 fluorescence in the 700–950 nm range depending on the chemical composition of the crystals. At room temperature the4 T 2→4 A 2 transition is homogeneously broadened, whereas the 4 K spectra of the2 E→4 A 2 transition show inequivalent Cr3+ sites and inhomogeneous linewidths of 8 cm−1 in these laser crystals. From 300 to 4 K, the oscillator strength of the4 T 2→4 A 2 transition is reduced by a factor of about 0.6 due to the temperature dependent electron phonon coupling. The transition probability of the2 E→4 A 2 transition correlates with the energetic position of the4 T 2 level, which is admixed into the2 E level via spin-orbit coupling. The comparison of the2 E→4 A 2 phonon sideband structure in garnets of different chemical compositions allows the assignment of some peaks to GaO4-tetrahedron modes of the garnet structure.

Phase transitions and soft modes in sodium bismuth titanate

Abstract: The results of experimental studies of the crystal and domain structures of NaO, 5BiO, 5TiO3 as well as its lattice dynamics, dielectric, optical and other physical properties are considered. Nature of peculiar temperature points is discussed.

Convergent-beam electron diffraction

Abstract: Convergent-beam electron diffraction (CBED) is a very powerful method for determing crystal symmetries, lattice defects and crystal structures of small specimen areas. We briefly describe the procedures of symmetry determination of crystals and structure refinement, and coherent effects using the CBED techniques.

Infrared spectroscopy at the surface of clusters: SF6 on Ar

Abstract: We have succeeded in obtaining infrared spectra of molecules adsorbed on the surface of clusters. The method is based on the photodissociation spectroscopy technique developed in our laboratory for the study of cluster beams and on a simple but effective way to prepare mixed clusters in which an IR chromophore is attached to the surface of a nonabsorbing host cluster. The possible extension of this technique to the study of molecular spectroscopy at the surface of clusters large enough to simulate crystal and liquid surfaces is also discussed.

Cholesterol stabilizes the crystal-liquid interface in phospholipid monolayers

Abstract: On montre que le cholesterol a une influence importante, meme aux faibles concentrations, sur la forme des cristaux bidimensionnels de dipalmitoylphosphatidylcholine, formes par compression de ce lipide a l'interface air-eau

Interface reactions between machinable bioactive glass‐ceramics and bone

Abstract: A new biomaterial for bone substitution, a "machinable bioactive glass-ceramic" has been developed. The material contains two main crystal phases, mica and apatite, and is therefore machinable and bioactive. It has the advantage to be workable by the surgeon, if necessary, during operation. The preparation method of this glass-ceramic is described. Different types of the material can be produced in dependence of the composition, nucleation, and crystallization of the basic glass. In vivo and in vitro investigations showed a characteristic solubility of the material. A Ca-phosphate-rich interface layer with apatite crystals (from the basic glass-ceramic) and a thickness of about 5-10 micrometers grows as solid-state reaction between glass-ceramic and bone. This interface reaction is interpreted as a chemical process which includes a slight solubility of the glass-ceramic and a solid state reaction between the stable apatite crystals in the glass-ceramic and the bone.

Order in metallic glasses and icosahedral crystals

Abstract: The relationship between short-range icosahedral order in metallic glasses and long-range order in icosahedral crystals is explored. Metallic-glass structure factors are assumed to describe liquids in metastable equilibrium just above the glass transition. A density-functional mean-field theory is then used to search for nearby crystalline states with a lower free energy. We find that undercooled liquids are metastable with respect to an icosahedral crystal, similar to the recently discovered icosahedral phase of Al-Mn. Conventional bcc and fcc crystals have an even lower free energy. The icosahedral phase is favored by the short-range icosahedral order present in the liquid. There is, however, an energetic cost associated with long-wavelength fluctuations forced in by the incommensurability of the icosahedral density waves. The theory predicts a rapid falloff of intensities in reciprocal space at small wave vectors, and a specific decoration of Penrose rhombahedra with atoms in real space. Our results are obtained via an exact mapping, in the density-functional framework, of the theory of icosahedral crystals onto the theory of conventional crystals in six dimensions.

Imaging of atomic clouds outside the surfaces of gold crystals by electron microscopy

Abstract: The structure of small metal crystals has been intensively examined by high-resolution electron microscopy (HREM). In particular, multiply-twinned gold and silver crystals have been characterized using the profile-imaging method1,2 at atomic resolution, and reconstructed metal surfaces observed2. Crystal structure images of large gold clusters consisting of 55 gold atoms arranged in a cubeoctahedron have been recorded3 using 2.5 A resolution, and crystal growth, row by row, on a {111} surface has been documented4 using a low-light-level silicon-intensified target television (TV) camera and video system with an on-line image processor. Direct imaging of rearrangements of atomic columns on extended gold surface5 established that profile imaging can provide information about surface self-diffusion. The motion of surface atoms, recorded with a real-time video tape-recorder (VTR) system, and the formation of surface atom steps on {100} surfaces, although not {111}, has also been reported recently6. Dynamic HREM observations at TV rate showing defect motion in gold7 and CdTe (refs. 8, 9) has given information on ‘in-lattice’ rearrangements of columns of atoms. We report here surface profile images recorded with the electron beam along the 〈110〉 direction with spatial resolution of ∼2.0 A which reveal changes in occupancy of the atom columns often within periods of less than 0.1 s. Surfaces with several adjacent atom columns involved in rapid structural changes frequently interacted with a cloud of atoms extending out to 9 A off the crystal and changes in shape and density of the clouds were recorded. Although these clouds have already been described6,10, the present work is the first to analyse these events properly and to describe them in detail. The motion of atomic columns and the existence of atom clouds revealed here may have important consequences for crystal growth, surface science and catalysis studies.

Scattering of X-rays from crystal surfaces

Abstract: X-ray measurements performed on a variety of materials demonstrate that it is possible to observe diffuse scattering that originates in the abrupt change of density at a crystal surface. Such a discontinuity gives rise, in general, to rods of scattering in reciprocal space which are most intense close to the Bragg peaks tau and are well defined for sufficiently smooth surfaces. For wave-vector transfer Q= tau +q the q-dependence of the intensity of scattering gives information on the topographic structure of the crystal surface. Experimental results on crystals of GaAs and KTaO3, with surfaces prepared in various ways, were obtained using conventional X-ray techniques with a rotating anode source and can be described by a continuum model of the surface. There are discrepancies between the predictions of the models and the experimental results and these suggest that further experiments are needed to achieve a more complete understanding.

Beam coupling in BaTiO3 at 442 nm

Abstract: Steady‐state beam coupling is used to measure the concentration of empty traps, the sign of the dominant photocarrier, and the effective electro‐optic coefficient in seven crystals of BaTiO3. The measured value of the effective electro‐optic coefficient gives the product of the fractional poling of the crystal, the relative conductivity of the crystal, and the electro‐optic coefficient. Anomalously low beam coupling gain measured in an eighth crystal suggests that this crystal is nearly perfectly compensated; i.e., the photoconductivities due to electrons and holes are nearly equal.

Evidence for solid krypton bubbles in copper, nickel and gold at 293K

Abstract: Recent work by other investigators showing solid xenon and argon precipitates in aluminium has been extended in the present Letter to an examination of the structure of krypton bubbles in copper, nickel and gold. As in previous work, the inert-gas bubbles are found in the solid face-centred cubic phase epitaxial with the host crystal. Electron microscopy diffraction patterns have been used to show the solid to gas transition during heating, and the subsequent recrystallisation on cooling. The results on the solid inert-gas bubbles are a dramatic indication of the high pressure involved; these pressures, which vary with the shear modulus of the metal, are a direct consequence of the bubble growth processes.

A NEW-TYPE ULTRAVIOLET SHG CRYSTAL——β-BaB 2 O 4

Abstract: On the basis of our work An anionic group theory of electro-optical and nonlinear optical effects of the crystals, we first proposed the possibility of searching for new-type SHG crystals among the inorganic materials possessing planar ring groups. Since then, a new-type ultraviolet StIG crystal—β-BaB2O4 has been developed, a crystal with excellent optical homogeneity, anti-deliquescence and easy-working mechanical properties. Various measurements have been made thereupon of its powder sample tests, phase diagram, structural determination and other performance tests. The following parameters of β-BaB2O4 have thus been determined: space group at room temperature R3, melting point 1095℃, phase transition temperature 925±5℃, transparent band range 190—2500mμ, phase matching angle at 539—270mμ 47°, and radiation damage threshold 2GW/em2. Its effective SHG coefficient is 3—6 times larger than that of ADP (at different wavelengths). It is expected that this new crystal will find wide and extensive applications in ultraviolet and near infrared regions.

Solution crystallization of polyethylene at high temperatures

Abstract: Single crystals of polyethylene have been grown from solution in various solvents at temperatures between 70 and 120° C. This represents an overlap in crystallization conditions with those used for melt growth, where substantial isothermal thickening is known to occur during growth. The crystal thicknesses have been measured by Raman spectroscopy. Values of the equilibrium dissolution temperature and fold surface free energy are calculated for each solvent and the results analysed using the kinetic theory. Variations in crystal properties with time of crystallization are also investigated. A specific dependence of fold length on supercooling has been found to apply over the whole temperature range, consistent with predictions by the kinetic theories of crystallization in spite of changes in morphology which are incompatible with assumptions underlying the theoretical model. No evidence for isothermal thickening has been observed, except possibly for a small marginal effect at the highest temperature of 120° C investigated, over the same temperature range where melt crystallized material shows the effect prominently. Crystals grown at all temperatures displayed a rise in dissolution temperature with time which could be associated with an increase in surface perfection. All these findings have wider implications for our picture of polymer crystallization and crystal structure which are discussed here. A further, explicit, correlation with melt crystallization is deferred to a subsequent publication.

Crystal Structure of Kaolinite: Dimethylsulfoxide Intercalate

Abstract: The crystal structure of the kaolinite : dimethylsulfoxide (DMSO) intercalate (P 1, a = 5.187(2), b = 8.964(3), c = 11.838(4) A, a = 91.53(1) ~ = 108.59(2) ~ T = 89.92(1)*) has been determined using spectroscopic and X-ray and neutron powder diffraction data. Both the X-ray and neutron powder diffraction patterns were refined. Solid-state ~3C, 298i, and 27A1 nuclear magnetic resonance data and previously collected infrared spectroscopic data provided a useful starting model for structural refinement. Due to the extreme overlap of reflections of this low-symmetry unit cell, the Rietveld method proved inadequate, and quasi-single crystal methods were employed. Each DMSO molecule was found to be triply hydrogen bonded above the octahedral vacancy in the glbbsitic sheet of the kaolinite layer. One methyl group is keyed into the ditrigonal hole in the tetrahedral sheet with the other S--C bond parallel to the sheet. The DMSO molecules are accommodated by significant horizontal displacement of individual kaolinite layers to achieve almost perfect overlap of the octahedral vacancy by the adjacent ditrigonal hole.

Epitaxial growth and characterization of CaF2 on Si

Abstract: CaF2 films have been grown epitaxially on (100) and (111) Si substrates by molecular beam epitaxy. These films have been characterized by electron microscopy, reflection high‐energy electron diffraction, Rutherford backscattering ion channeling, and back‐reflection Laue x‐ray diffraction. In addition, chemical etching has been used to reveal dislocations and to delineate cracks. Film cracking appears to be related to crystalline perfection through misfit dislocation mobility. It is possible to grow high quality, (xmin=3.0%) single‐crystal films on (111) Si which are free of cracks and atomically flat. However, the high free energy of the (100) surface in an ionic fluorite crystal prevents the growth of comparable CaF2 films on the (100) Si surface.

The pressure-induced metallic amorphous state of SnI4. I. A novel crystal-to-amorphous transition studied by X-ray scattering

Abstract: A high-pressure X-ray scattering study has revealed that the compressible molecular crystal SnI4 gradually transforms from the crystalline to the amorphous state, this process being accompanied by a remarkable decrease in electrical resistivity induced by the pressure. Taken together with the Raman scattering study described in the following paper, this indicates that the amorphous structure attained above about 20 GPa is formed by dimerised SnI4 tetrahedral molecules packing randomly and densely.