Hexagonally close-packed layer

The hardest of the transition-metal borides are the diborides. Their characteristic crystal structure (Figure 10.6) consists of plane layers of close-packed metal atoms separated by plane openly-patterned layers of boron atoms ( chicken-wire pattern). If the metal atoms in the hexagonal close-packed layer have a spacing, d, then the boron atoms have a spacing of d/V3. 

The structure of boehmite contains double sheets of octahedra with A1 ions at their centers, and the sheets themselves are composed of chains of octahedra. In diaspor the oxygens are in a hexagonal close packed layer those within the double octahedral layers in boehmite are in a cubic pacldng relationship. Luminescence center Cr characterizes steady-state spectra (Solomonov et al. 1994 Shoval et al. 1999). The natural boehmite and diaspor in our study consisted of twelve samples. The laser-induced time-resolved technique enables us to detect Cr emission centers (Figs. 4.62-4.63). 

At low adsorbate coverages the surface structure of the deposited metal is determined by the substrate periodicity. Thus, under these conditions the adsorbate-substrate interaction is predominant. At higher coverages the adsorbate may continue to follow the substrate periodicity or form coincidence structures with new periodicities that are unrelated to the substrate periodicity. The ordering geometry of large-radius metallic adatoms (especially K, Rb and Cs) shows relatively little dependence on the substrate lattice they tend to form hexagonal close-packed layers on any metal... 

Although the (111) face of fee metals is of the lowest surface free energy — a fact which may explain the reconstructions of the (100) and (110) faces —, the (111) face itself may also reconstruct Au(l 11) is normally reconstructed with a structure that may nevertheless still involve the hexagonally close-packed layer geometry (since extra sites of hexagonally arranged spots appear in LEED), but with a lattice constant different from that of the bulk . ... 

A type of reconstruction that one might expect to occur but that has not been observed is related to the relatively easy phase transition between hep and fee metals this involves only the shifting of hexagonally close-packed layers of atoms, from the. .. ABABAB. .. to the. . . ABC ABC. . . stacking arrangement. Such a shift could easily occur for the topmost atomic layer of hcp(0001) or fcc(l 11) surfaces. Interestingly, it does not seem to take place in reality on the five hcp(OOOl) and nine fcc l 11) surfaces analyzed so far this includes the case of Co on both sides of its hep-fee phase transition. 

Adsorption in many different adsorption sites simultaneously is expected for overlayers with an incommensurate lattice (cf. Sect. III). This has been confirmed by LEED intensity analyses for the case of an incommensurate overlayer of Xe on Ag(l 11), where both the substrate and the overlayer consist of hexagonally close-packed layers (with unrelated unit cells) parallel to the surface. 

Given the radius of an adsorbed H2 molecule is 1.2 x 10 10 m and H2 adsorbs on Ti in a hexagonally close-packed layer, calculate the length of time that will result in the coverage of 946 cm2 of freshly deposited Ti at 300 K with half a monolayer of H2 if a TSP is working at 10-7mbar at 300 K. 

Structure Cubic close packed layer structure Hexagonal close packed layer structure ... 

Hexagonal close-packed layers. The third layer is exactly over the first layer in this ABA pattern. Octahedral holes are aligned exactly over each other, one set between the first two layers A and B, the other between the second and third layers, B and A. 

Figure 6.36 Shear rate at which the shear viscosity suddenly jumps dis-continuously upward as a function of the volume fraction of poly(vinyI chloride) (PVC) spheres of various diameters in dioctyl phthalate. Point A denotes the volume fraction at which two-dimensional hexagonal close-packed layers of spheres first touch. Point B is the volume fraction for three-dimensional simple cubic packing. (From Hoffman, reprinted from Trans. Soc. Rheol. 16 155, Copyright 1972 American Institute of Physics.)...

To address this, Liem, Brown, and Clarke ° simulated in excess of 40,000 particles interacting via a Weeks-Chandler-Andersen (WCA) potential. While the x and z directions were treated normally, the y direction was divided into three regions two atomistic walls separated by a fluid region. The walls consisted of three hexagonally close-packed layers of particles. The wall atoms interacted with the fluid particles and with each other through the same WCA potential used for the fluid-fluid interactions. Additionally, each wall particle felt a harmonic potential centered at its triangular lattice site. This setup allowed heat transfer from the fluid to the wall while allowing the wall to remain crystalline. The momenta of the wall particles were rescaled to keep the total... 

Green Rusts are not oxides or hydroxides in a strict sense but contain anions as an essential structural component. They consist of hexagonally close-packed layers of O and OH of the Fe(OH)2-structure type with Fe° and Fe in the interstices (Feitknecht and Keller, 1950). Fe gives the layer a positive charge which is balanced by intercalation of anions, such as chloride, sulfate, carbonate etc., between the layers. The sulfate and chlor-... 

Dent and Kokes consider that wurtzite derives from isotropically expanded, hexagonal close-packed layers of oxide ions, with correspondingly expanded zinc layers in which zinc ions occupy one half of the tetrahedral holes between oxide layers. This expansion increases the radius of the trigonal holes in the oxide layers such that, at 0.058 nm, they can almost accommodate a zinc ion. The structure is quite open and consists of straight channels of octahedral sites, each 0.20 nm in diameter, separated by trigonal squeeze points , 0.12 nm in diameter. 

BS forms an enantiotropic smectic B (Sm-B) phase between 15 and 26 C, in which the individual constituent molecules preferentially adopt fully extended conformations in hexagonally close-packed layers, with their long molecular axes perpendicular to the layer planes. NMR and IR evidence indicates that the constituent molecules rotate rapidly about their long molecular axes in this Sm-B phase, and methylene groups near the middle of the stearyl chains are somewhat more-ordered than those near the ends (35,37-38). This material forms an isotropic liquid phase above 26 C, and a progression of differently structured solid phases below 15°C. 

Iron Core Only a small fraction of the iron atoms in ferritin bind directly to the protein. The core contains the bulk of the iron in a polynuclear aggregate with properties similar to ferrihydrite, a mineral found in nature and formed experimentally by heating neutral aqueous solutions of Fe(III)(N03)3. X-ray diffraction data from ferritin cores are best fit by a model with hexagonal close-packed layers of oxygen that are interrupted by irregularly incomplete layers of octahedrally coordinated Fe(III) atoms. The octahedral coordination is confirmed by Mossbauer spectroscopy and by EXAFS, which also shows that the average Fe(III) atom is surrounded by six oxygen atoms at a distance of 1.95 A and six iron atoms at distances of 3.0 to 3.3 A. 

Many metal surfaces also reconstruct. For example, at the (1(X)) surfaces of Ir [7-11], Pt [10-13], and Au [10, 11, 14-16], the interatomic distance in the topmost layer shrinks by a few percent, parallel to the surface. It then becomes more favorable for this square unit cell to collapse into a hexagonally close-packed layer rather... 

Symbols of structure types which may be considered as sequences of 2-dimensional hexagonal nets can be partly derived from those of cubic structure types, if the nets are arranged perpendicular [111]. According to Hermaim a hexagonal closed packed layer may be symbolised by H, a graphite layer by G, and a Kagome net by N. 

Figure 25. Crystal structure of -Fe2N. The iron atoms form hexagonal close-packed layers which extend along the yz-plane of this orthorhombic structure (space group Pbcn). In the x direction these planes have the stacking sequence AB, AB. Hence, the iron atoms form a distorted hexagonal close-packed arrangement where the nitrogen atoms fill half of the octahedral voids.

Figure 3,4 The creation of (a and b) octahedral interstices and (c and d) trigonal prismatic interstices by stacking of hexagonal close-packed layers of anions...

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