Symmetry hexagonal

Restructuring of a surface may occur as a phase change with a transition temperature as with the Si(OOl) surface [23]. It may occur on chemisorption, as in the case of oxygen atoms on a stepped Cu surface [24]. The reverse effect may occur The surface layer for a Pt(lOO) face is not that of a terminal (100) plane but is reconstructed to hexagonal symmetry. On CO adsorption, the reconstruction is lifted, as shown in Fig. XVI-8. 

Mechanical Properties. The hexagonal symmetry of a graphite crystal causes the elastic properties to be transversely isotropic ia the layer plane only five independent constants are necessary to define the complete set. The self-consistent set of elastic constants given ia Table 2 has been measured ia air at room temperature for highly ordered pyrolytic graphite (20). With the exception of these values are expected to be representative of... 

FIGURE 5.26 A fragment of the structure formed as described in Fig. 5.25 shows the hexagonal symmetry of the arrangement—and the origin of its name, "hexagonal close-packed."... 

The arrangement found to account for X-ray data from molybdenite is shown in Fig. 2. This arrangement is derivable from the space group Dgh as well as from the space groups D, D , Cjh, and D. Considering the atoms as points or spheres, the structure then has holohedral hexagonal symmetry. 

Fig. 18. Hexagonal symmetry projective view (above) and cut A-B (below) of a [LSl,-layer formed by a sequence of [L4S] tetrahedrons (SmSI-t3rpe). (Redrawn from C. Dagron and F. Thevet, Ann. Chim. 6, 67 (1971), Fig. 7, p. 77.)...

The adsorption of cyclohexene, cyclohexane, 1,3-cyclohexadiene, 1,4-cyclohexadiene, and benzene on Pt(l 11) was studied with STM [35, 36]. Figure 7.20a shows an STM image of 2 x 10-s Torr cyclohexene on Pt(l 11). The low-pressure structure shows a hexagonal symmetry with a periodicity of approximately 7 A that is rotated approximately 18-20° with respect to the [1 1 0] direction of the Pt crystal face. From prior... 

In view of the clear correlation of local ring geometry with methyl rotor barrier height in the S0 and D0 states, the strong effects of S, <— S0 excitation on rotor potentials seem to indicate substantial distortion of the ring away from hexagonal symmetry in the S, state as well. There is little clear evidence of this from molecular spectroscopy. We have speculated that such a n-molecular orbital orientation effect in the S j state (similar to that in the cation) might explain the observed characteristic... 

Attempts were made to include all hydrogen atoms explicitly in the simulations. This computationally demanding explicit-atom model shows (Fig. 1) that the crystal symmetry is orthorhombic, in agreement with the well-known experimental result for polyethylene single crystals, instead of the hexagonal symmetry seen in united-atom model simulations. 

Besides linear-block copolymers, also blends with miktoarm-star terpolymers have been reported [193]. Blending a PS-arm-PB-arm-P2VP miktoarm-star terpolymer showing hexagonal symmetry with a PS- -P2VP diblock... 

In most oxides, the oxygen atoms are present as close-packed layers stacked either to produce cubic symmetry or hexagonal symmetry. Some of the cubic cases have already been discussed. Now some hexagonal cases will be considered. 

Three of these compounds have cubic symmetry, while T1B2 has hexagonal symmetry. Since they are metallic, bond moduli cannot be defined for them, but valence electron densities can be. The hardnesses of the cubic titanium compounds depend linearly on their VEDs the numbers of valence electrons are (4 + 4 = 8)TiC, (4 + 3 = 7)TiN, and (4 + 2 = 6)TiO. The linear dependence is shown in Figure 11.10. A similar linear dependence on their C44s is also found (Figure 11.12). 

There are two other phases that are similar to (3- and (3"-alumina but are built from spinel blocks that are six close-packed oxygen layers thick. The material (3" -alumina is the analog of (3-alumina, with the spinel blocks related by 180° mirror-plane (hexagonal) symmetry, while the phase (3""-alumina, the analog of (3"-alumina, has blocks related by 120° rotation and rhombohedral symmetry. 

Figure 9.12 Three-dimensional map of the calculated electrostatic potential at 0.25 nm above the symmetry plane in a hexagonally ordered network of dipoles with a dipole-dipole distance of 1.61 nm and a dipole moment of 10 D. The dipoles are positioned at the minima. Note that the potential is lowered at every position on the surface. Equipotential lines for -1.05, -0.84, -0.63 and -0.42 V are indicated in the bottom plane. The contours are circular at short distances from a potassium atom, indicating that at these sites the nearest potassium atom largely dominates the potential. The equipotential tine for -0.42 V, however, has hexagonal symmetry due to the influence of the dipoles further away (from Janssens et al. [40]).

The wurtzite structure with hexagonal symmetry depicted in Figure 2.2b has a 4 4 tetrahedral coordination arising from HCP arrangement of anions with half the tetrahedral sites occupied by the cations. Examples include ZnO and BeO. 

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