Showing papers on "Crystal published in 1977"
TL;DR: In this paper, the Raman-active lattice vibrations of Bi2Se3, Bi2Te3, Sb2T3, and SbT3 were investigated by Raman scattering and three of the four expected Raman modes, Eg and A1g, could be determined.
Abstract: The Raman-active lattice vibrations of Bi2Se3, Bi2Te3, Sb2Te3, and their solid slutions, whose symmetries correspond to the R3m space group, are investigated by Raman scattering. Three of the four expected Raman modes, Eg and A1g, could be determined. The FIR optical properties of Bi2Te3 crystal surfaces of improved quality (E ∥ c and E ⟂ c) and Bi2Se3 (E ⟂ c) are re-examined near helium and room temperature with a Fourier spectrometer, allowing a determination of the infrared-active mode frequencies with higher accuracy. The results for Bi2Te3 are compared to the predicted frequencies from the lattice dynamical model, given by Jenkins et al. The frequncy shifts of the Raman-active modes in the mixed crystals show single-mode and two-mode behaviour, which is in agreement with simple models for the substitution of antimony and selenium atoms for bismuth and tellurium, respectively, in Bi2Te3.
473 citations
TL;DR: In this paper, three single crystal and two polycrystalline Pt surfaces were studied, one single crystal was cut to expose the smooth, hexagonally close-packed plane of the fee Pt crystal (the (111) surface).
283 citations
Book•
01 Jan 1977
TL;DR: In this article, a preliminary examination of crystal geometry by optical and X-ray methods is presented, showing that the intensity of scattering of XRays by Crystals is relatively low.
Abstract: Crystal Geometry I. Crystal Geometry II. Preliminary Examination of Crystals by Optical and XRay Methods. Intensity of Scattering of XRays by Crystals. Methods in XRay Structure Analysis I. Methods in XRay Structure Analysis II. Direct Methods and Refinement. Index.
275 citations
TL;DR: In this article, the authors explored the possibility of expressing the specific volume of organic substances in the crystal state as the sum of increments, including corrective terms, and the unitary volumes listed herein allow an estimate of density with a confidence limit of 2-3%.
Abstract: The possibility is explored of expressing the specific volume of organic substances in the crystal state as the sum of increments, including corrective terms. The unitary volumes listed herein allow an estimate of density with a confidence limit of 2-3%.
232 citations
TL;DR: The surface structures of twenty-two high Miller index crystal faces of platinum were studied in ultrahigh vacuum (uhv) when clean and in the presence of a monolayer of chemisorbed oxygen or carbon by low-energy electron diffraction (LEED).
210 citations
TL;DR: In this paper, a molecular mechanism for the photoelectrochemical oxidation of water on MoS2 was elaborated and its conclusions tested by investigating the oxidative behaviour of various additional layer-type transition metal compounds.
Abstract: Abstract Kinetical, energetical and solid state considerations were elaborated in a search for suitable electrodes for the oxidation of water with visible light. They led to layer type transition metal dichalcogenides and to the photo-electrochemical utilization of optical d-d-transition which do not break chemical bonds. MoS2 was selected as a promising compound and it was actually shown to react with water on illumination with light between 400 and 715 nm. At low electrode potentials the liberation of small quantities of molecular oxygen was traced with polarographic techniques. The main portion of the oxidation products of water was, however, found to be lost for the oxydation of crystal bound sulfur to sulfate. An improvement of this situation in favour of an increased rate of oxygen evolution has been accomplished by means of a redox-catalyst, tris (2,2'-bipyridine) -ruthenium (II), which is oxidized through photochemically generated holes from the MoS2 4 dz²- valence band and channels them into oxygen-evolution through nucleo-philic addition of hydroxide. A molecular mechanism was elaborated for the photoelectrochemical oxidation of water on MoS2 and its conclusions tested by investigating the oxidative behaviour of various additional layer-type transition metal compounds. It involves a hole mediated valence change of molybdenum with an intermediate formation of a 5-valent molybdenum hydroxide. A basic kinetical and energetical similarity is suggested with the function of manganese reaction centers in photosynthesis.
208 citations
TL;DR: The first member of a new family of materials for nonlinear optics, methyl-2,4-dinitrophenyl) amino-2 propanoate (MAP), has been prepared and its linear and nonlinear optical properties have been investigated as discussed by the authors.
Abstract: The first member of a new family of materials for nonlinear optics, methyl‐ (2,4‐dinitrophenyl) ‐amino‐2 propanoate (MAP), has been prepared and its linear and nonlinear optical properties have been investigated. High nonlinear coefficients result from the favorable electronic properties of the MAP molecule, as well as its chirality, which ensures a noncentrosymmetrical crystal structure. This material is phase matchable over its entire transparency range, and its figure of merit d2/n3 for parametric interactions is 15 times larger than LiNbO3. The optical damage threshold is higher than 1 GW/cm2 for 10−8‐sec pulses at 1.06 μm. The 30% second‐harmonic conversion efficiency has been observed in a 1‐mm‐thick crystal. In addition, we present an extension of the analysis of Maker fringes to monoclinic crystals of point group 2.
187 citations
TL;DR: The regrowth behavior of ion-implanted amorphous Ge is similar to that of implanted amorphus Si, with an activation energy of 2.0 eV and a rate of 100A/min on Ge at 350°C as discussed by the authors.
165 citations
TL;DR: The surface structures of the growing organic crystals do not corresond to planes in their reported bulk crystal structures and are evidence for epitaxial growth of pseudomorphic crystal forms.
Abstract: The surfaces of the normal paraffins (C3–C8) and cyclohexane have been studied using low‐energy electron diffraction (LEED). The samples were prepared by vapor deposition on the (111) face of a platinum single crystal in ultrahigh vacuum, and were studied both as thick films and as adsorbed monolayers. These molecules form ordered monolayers on the clean metal surface in the temperature range 100–220 K and at a vapor flux corresponding to 10−7 Torr. In the adsorbed monolayers of the normal paraffins (C4–C8), the molecules lie with their chain axes parallel to the Pt surface and Pt[110]. The paraffin monolayer structures undergo order–disorder transitions as a function of temperature. Multilayers condensed upon the ordered monolayers maintained the same orientation and packing as found in the monolayers. The surface structures of the growing organic crystals do not corresond to planes in their reported bulk crystal structures and are evidence for epitaxial growth of pseudomorphic crystal forms. Multilayer...
155 citations
152 citations
TL;DR: In this article, the shape stability of growing polyhderal crystal is discussed, and the curve of stability limit, σ ∗∗∞ versus L, is obtained by numerical calculations.
TL;DR: In this article, the crystal sturcture of the high temperature modifications have been determined be elastic neutron scattering experiments on powdered samples, which can be summarized as follows: on the basis of these structural data, the different behavior of the ionic conductivity of CuI and CuBr at high temperature can be understood.
TL;DR: In this article, microstructures for this composition are essentially single phase, consiting of non-faceted, sub-micron, β′ grains with a grain-boundary segregate layer of glass-forming silicate composition, containing impurity and additive metal ions.
Abstract: Si-Al-O-N ceramics have been prepared by hot-pressing mixtures of Si3N4, AIN and SiO2 (with an addition of 1% MgO) having varying ratios of AIN/SiO2. Microstructural analysis by transmission electron microscopy and Auger electron spectroscopy has demonstrated the progressive increase in grain-boundary silicate glass in pressings prepared from compositions with excess SiO2 to compositions given by the formula Si6−zAlzOzN8−z. This formula represents the simple substitution of Al for Si atoms and O for N atoms in the hexagonalβSi3N4 crystal. Microstructures for this “balanced” composition are essentially single phase, consiting of non-faceted, sub-micron,β′ grains with a grain-boundary segregate layer of glass-forming silicate composition, containing impurity and additive metal ions, which may be detected only by Auger spectroscopy. This microstructure is in contrast with “unbalanced” compositions which contain facetedβ′ grains joined by a glassy silicate phase which is easily detected by electron microscopy. Final microstructural analysis combined with observations of density and phase content with progress of hot-pressing has confirmed the important role of liquid silicate formation and a “solution-reprecipitation” mechanism for densification. The presence of a 1% MgO additive is shown to accelerate this process, forming a low melting point silicate by reaction with SiO2, assisting the early solution of AIN and the reprecipitation ofβ′ substituted crystals.
TL;DR: Monte Carlo calculations of crystal growth rates are presented for a number of different situations, including two-dimensional nucleation with mobile adatoms, spiral growth, growth and evaporation of a crystal containing a small columnar hole, growth with impurities, and growth on three crystallographic faces of the face-centered cubic crystal.
01 Jul 1977
TL;DR: In this article, a general procedure for building static models of interfaces, which involves a change in phase-specific construction rules at the boundary plane, is outlined, and detailed topologies, bond angle distortions and radial distribution functions for each of the four layers are reported.
Abstract: Abstract A general procedure for building static models of interfaces, which involves a change in phase-specific construction rules at the boundary plane, is outlined. Its application to tetrahedrally coordinated materials shows that an amorphous-crystalline interface model can be created by replacing the ‘chair’-type sixfold rings (typical of the crystal) by a mixture of different ones (typical of the amorphous phase). The interface consists of two crystalline and two amorphous layers. The detailed topologies, bond angle distortions and radial distribution functions for each of the four layers are reported. The surface tension has a large energetic component due to the excess strain energy in both the amorphous and crystalline interface layers. For Ge the estimated surface tension is 0.23 J m−2. This is used to show that the model, which contains no dangling bonds, represents a state of minimum energy. Application to the problem of creating a model for amorphous Ge by connecting randomly oriented crystallites with a random network matrix shows that such a model consists of more than 80% random network. Finally it is pointed out that the interface model is a starting point for a detailed description of the crystallization process.
TL;DR: Ferricenium tetrachlorobismuthate, (C5H5)2FeBiC1 4, crystallizes in the monoclinic space group P21/c with a = 10.998(5)A, b = 17.449(4)
Abstract: Ferricenium tetrachlorobismuthate, (C5H5)2FeBiC1 4, crystallizes in the monoclinic space group P21/c with a = 10.998(5)A, b = 17.449(4)~, c = 7.569(4)A, B = 98.46(5)~. There are four molecules in the unit cell. Reflection intensities were measured by the 8 28 scan method with a Picker FACS-1 automated diffractometer, MoKa radiation, and a graphite . 2 ( 2) monochromator. For 1374 data with F >3o F ; R1 = 0.030, and R2 = 0.037. The Bi atom is coordinated by 6 chloride ions in an irregular octahedral array; two pairs of Cl ions form halogen bridges with neighboring Bi ions resulting in an infinite chain of edge sharing octahedra. There are 6 independent Bi-Cl bond distances: 2.50(1)~ and 2.52(1)~, to nonbridging chlorines, and 2.70(1)~, 2.75(l)A, 2.95(l)A and 3.10(l)A to bridging chlorines. The bridging Cl-Bi-Cl bond angles are 80.3° and 83.9°; the ten non-bridging Cl-Bi-Cl bond angles range from 85.9° to 99.3°. The axial Cl-Bi-Cl bond angles are 166.0°, 174.5°, and 175.9°.
TL;DR: In this article, the electronic structures of water and ice are investigated by means of XPS, UPS, and VUV absorption spectroscopy, and it is suggested that ice is essentially a kind of molecular crystal.
Abstract: Electronic structures of water and ice are investigated by means of XPS, UPS and VUV absorption spectroscopy. All the bands found in the XPS spectra of ice show a fairly good agreement with the electronic band structure theoretically calculated for cubic ice using molecular tight binding approximation. From the UPS spectra the vacuum level of ice is found at 10.5 eV above the top of the valence band, and a conduction band level of high density of states at 3.5 eV above the vacuum level. In VUV spectra D 2 O ice film shows a well separated absorption band at 8.75 eV, whose shape is almost independent of temperature, and it is suggested that ice is essentially a kind of molecular crystal. The absorption coefficients in the tail regions of the fundamental absorption bands of water and ice show nearly exponential dependence on the photon energy, and they seem to be well expressed as the tail of a Gaussian band.
TL;DR: In this paper, the surface peak of the W(100) surface has been measured as a function of the He/sup +/ε energy on a clean W (100) crystal.
Abstract: The surface peak, observed in backscattering-channeling experiments, has been measured as a function of projectile (He/sup +/) energy on a clean W(100) crystal. We show that the intensity of the surface peak is in good agreement with classical models of channeling. Furthermore, we deduce an upper limit of 6% for the relaxation of the W(100) surface.
Patent•
17 May 1977
TL;DR: In this article, a solution of polyethylene or polypropylene in a vessel containing a spinning rotor is used to obtain a slightly roughened surface, which is then removed at a rate equal to the crystal growth rate.
Abstract: Filament-like polymer crystal fibers are prepared from a solution of a crystallizable polymer, such as polyethylene or polypropylene, in a vessel containing a spinning rotor, preferably having a slightly roughened surface, according to the disclosed invention. The fiber thus formed is taken up and removed at a rate equal to the crystal growth rate. The longitudinal crystal growth rate is of sufficient speed for commercial application while at the same time yielding fibers of outstanding mechanical properties.
TL;DR: In this article, the authors examined the thermal strains existing in crystals during and after pulling and the strains which can be built-in by the facet effect, and they showed that during the growth of a crystal with a radius R, there is a maximum acceptable axial gradient roughly proportional to R−1.5 and that to prevent cracking after growth, there was a maximum rate of cooling proportional to r−2.
TL;DR: In this article, a theory relating crystal radius to the time-variation of the force exerted on the pull-rod by the weight of the growing crystal and surface tension forces is developed.
TL;DR: In this article, it is suggested that hydrogen present in various forms in the oxide film plays a crucial and complex role in the irradiation behavior of Si/SiO2 interface structures.
Abstract: Similarly to vitreous silica, irradiation of Sio2 films on silicon releases bond strain by creating network defects and a small increase in density and a decrease in polarizability. In contrast, the density of quartz crystal decreases and its polarizability increases during irradiation. These effects are due to the basic trend of maximizing ?-bonding and minimizing bond strain in the Si-O network. From the irradiation-generated electron-hole pairs, the holes are trapped in narrow and localized ?-bands at ~0.4 eV above the SiO2 valence band while the electrons move rather freely. This hole trapping is an intrinsic property of the Si-O bond. Hole trapping also occurs at the Si/SiO2 interface where interface states are generated. It is suggested that this process involves breaking surface Si-H bonds. Results obtained with various analytical techniques demonstrate that hydrogen present in various forms in the oxide film plays a crucial and complex role in the irradiation behavior of Si/SiO2 interface structures.
TL;DR: In this article, the Edwards-Anderson mean field theory for a spin glass is extended to the case of a general S Ising model with random exchange and uniform but variable crystal field energy.
Abstract: The Edwards-Anderson mean-field theory for a spin glass is extended to the case of a general S Ising model with random exchange and uniform but variable crystal field energy. The model exhibits anisotropic behaviour of the static susceptibility.
TL;DR: In this article, it was shown that crystal water not only provides a high ionic conductivity which is conditional for a fast electrochromic reaction but also stabilises electrocatalytically active surface sites for fast hydrogen or Li exchange with the adjacent electrolyte.
Abstract: WO3 films prepared under different conditions (evaporation, reactive sputtering and spraying of aqueous solutions of metatungstic acid) differ by orders of magnitude in their electrochromic sensitivity. Diffuse X-ray studies show the evaporated and sputtered films to be amorphous and to consists of a disordered network of corner sharing WO6 octahedra. Sprayed films have different degrees of crystallinity depending on spraying conditions. From differential scanning calorimetry we conclude that the crystal water present in most films strongly affects the local order of the corner sharing octahedra. We find that crystal water not only provides a high ionic conductivity which is conditional for a fast electrochromic reaction but also stabilises electrocatalytically active surface sites for fast hydrogen or Li exchange with the adjacent electrolyte.
TL;DR: In this paper, the intensity profiles of low-energy electron diffraction beams backscattered from the ordered (2×2) overlayers of acetylene and ethylene on the platinum (111) crystal face are measured by a photographic technique.
Abstract: The intensities of the low‐energy electron diffraction beams back‐scattered from the ordered (2×2) overlayers of acetylene and ethylene on the platinum (111) crystal face are measured by a photographic technique. The intensities are presented in an appendix in the form of normalized diffraction beam intensity versus incident electron energy (I‐V curves). Acetylene spontaneously forms the ordered (2×2) adsorbate on platinum at 300 K and low exposure under ultrahigh vacuum conditions. The intensity profiles reveal that this structure is metastable; and upon heating to 350–400 K for 1 hr it undergoes a transformation to a stable structure with the same (2×2) unit cell size. The acetylene transformation is thought to involve a strengthening of the carbon–platinum bond accompanied by an expansion of the carbon–carbon bond length. Ethylene adsorbes on the platinum (111) surface and with electron beam exposure forms an ordered (2×2) surface structure that is identical to the stable acetylene structure as shown b...
TL;DR: In this article, the work function and its temperature dependence have been measured for the 〈100〉, 》110〉 and 14 other Cu crystal planes, and the results were in good agreement with recent photoelectric and contact potential difference results reported on ion−sputter−cleaned surfaces.
Abstract: The work function and its temperature dependence has been measured for the 〈100〉, 〈110〉, 〈111〉, 〈112〉 as well as 13 other Cu crystal planes. The results, obtained with two types of incident electron beam techniques in this experiment, are in good agreement with recent photoelectric and contact potential difference results reported on ion‐sputter‐cleaned surfaces. Variation in secondary emission for different crystal faces was found to be determined primarily by the work function changes of the different crystals and not so much by the bulk lattice orientation. It was also observed that the secondary electrons are not as dependent on the velocity constraints imposed by the Bragg condition as are the electrons having thermal energies.
TL;DR: In this paper, the absorption spectra of the noble metal atoms Cu, Ag, and Au isolated in Ar, Kr, and Xe matrices have been measured as a function of matrix temperature in the range between 3 K and about 0.4 of the respective matrix triple point temperature.
Abstract: The absorption spectra of the noble metal atoms Cu, Ag, and Au isolated in Ar, Kr, and Xe matrices have been measured as a function of matrix temperature in the range between 3 K and about 0.4 of the respective matrix triple point temperature. The spectra show a pronounced energy shift of the absorption peaks with temperature which is entirely reversible in well annealed matrices in the above‐given temperature range. These spectra were evaluated by crystal field arguments to obtain more direct information on the matrix influence, which is discussed in terms of difference in level shifts, change in spin–orbit splitting, and crystal field splitting. The results are then explained by a model in which the matrix atoms are treated as clouds of negative charge which introduce a repulsive potential in the region of the metal electrons when the metal atom is squeezed into the matrix cage. The change with temperature in the interaction between metal atom and matrix is discussed in detail. In addition, an unambiguo...