Hexagonal dose-packed

When Li metal is cold-worked it transforms from body-centred cubic to cubic close-packed in which each atom is surrounded by 12 others in twinned cuboctahedral coordination below 78 K the stable crystalline modification is hexagonal dose-packed in which each lithium atom has 12 nearest neighbours in the form of a cuboctahedron. This very high coordination... 

Figure 3. Schematic drawing of the crystal structure of f-MnOj. The manganese atoms are randomly distributed in the octahedral voids of the hexagonal dose packing of oxygen atoms (adapted from [47]).

The most important metals for catalysis are those of the groups VIII and I-B of the periodic system. Three crystal structures are important, face-centered cubic (fee Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au), hexagonally dose-packed (hep Co, Ru, Os) and body-centered cubic (bcc Fe). Figure 5.1 shows the unit cell for each of these structures. Note that the unit cells contain 4, 2, and 6 atoms for the fee, bcc, and hep structure, respectively. Many other structures, however, exist when considering more complex materials such as oxides, sulfides etc, which we shall not treat here. Before discussing the surfaces that the metals expose, we mention a few general properties. 

Draw the simple (111), (100), and (110) surfaces of face-centered cubic metals, the (110) and (100) surfaces of body-centered cubic metals, and the (001) surface of an hexagonally-dosed packed surface. 

Fig. 29 Cartoon demonstrating arrangement of dendrons in hexagonally dose-packed cylinder morphology (left) or in lamellar morphology (right). From [93]. Copyright 2002 American Chemical Society...

Fig. 6-1. TVo-dimensional atomic structure on the (100) plane of platinum crystals (1x1) = cubic close-packed surface plane identical with the (100) plane (5 x 20) = hexagonal dose-packed surface plane reconstructed finm the original (100) plane. [From Kolb, 1993.]...

Fig. 6.8. Hexagonal dose-packed surface, (a) Real. space, (b) Reciprocal space. The length of the primitive reciprocal lattice vector is b = (4Tr/y 3 a).

Bismuth Triiodide. Bismuth(III) iodide is a greenish black crystalline powder. The iodines are in a hexagonal dose-packed array with each bismuth having six nearest-neighbor iodines at 0.32 nm (21). This suggests that the lone pair on bismuth is stereochemically inactive and that the compound is largely ionic in character. 

Titanium Diiodide. Titanium diiodide is a black solid (p = 499(0) kg/m3 ) that has the cadmium iodide structure. Titaniums occupy octahedral sites in hexagonally dose-packed iodine layers, where a = 411 pm and c = 682 pm (144). Magnetic studies indicate extensive Ti—Ti bonding. Til2 reacts rapidly with water to form a solution of titanous iodide, Til3. 

Figure 6.2 (a) The hexagonal dose-packed structure of a-alumina. (b) Two important slip systems, basal (0001) [0001] and prismatic (0110)[2110], in a hexagonal structure. 

Figure 4. Idealized hexagonally dose-packed RhF tetramer. (The atoms are numbered to correspond with Figure 1 and the fluorine-bridge bonds of the tetramer are heavily shaded. The closest F-atom layer is represented by the biggest open circles and the farthest by the smallest.)...

The Cu occupy one set of tet interstices in y-CuI and, on heating, Cul undergoes a phase transition to the P phase at 642 K. The structure of P-Cul can be described as an hexagonal dose-packed (hep) sublattice of anions, with the Cu distributed in the approximate ratio 5 1 over two sets of tet interstices within space group Pi ml [20]. 

Silvery-white metal hexagonal dose-packed structure-Slowty oxidizes in moist air. Available as bars, ribbons, wire and powder, mp 651°. bp 1100°. dM 1,738. Sp heat (20°) 0.245 cal/g. Heat of fusion 88 cal/g. Electrical resistivity 4.46 fiohm-cm, E° (aq) MgJ + /Mg —2.37 V. Reads very slowly with water at ordinary temp, less slowly at 100°. Reacts readily with dil adds with liberation of hydrogen reacts with aq solns of ammonium salts, forming a double salt. Reduces carbon monoxide, carbon dioxide, sulfur dioxide, nitric oxide, and nitrous oxide at a red heat. Burns in air continues to bum in a current of steam. Combines directly with nitrogen, sulfur, the halogens, phosphorus, and arsenic, Reacts with methyl alcohol at 200° giving magnesium methylate. 

STM images showed that CO forms a hexagonal dosed-packed (hep) (2x2)-3 CO adlattice with a coverage, Oqo, of 0.75 on Pt(lll) in 0.1 M HCIO4 at potentials just positive of 0.0 Vrhe (where the subscript RHE indicates that the voltage is reported with respect to the reversible hydrogen electrode reference). 

Figure 3-27. The slightly distorted hexagonal dose packed AU39 core of the [Au39(PPh3) 4Cls] cation.

FIGURE 4 Crystal structures exhibited by the rare earth elements, (a) Hexagonal dose packed (hep), (b) cubic dose packed (cep), (c) double hexagonal close packed (dhep), and (d) the complex structure of Sm. [From Gschneider, K. A., Jr. (1961). Crystallography of the rare-earth metals. In The Rare Earths (F. H. Spedding and A. H. Daane,... 

The hexagonal dose-packed (hep) lattice is a crystal lattice geometry based on a hexagonal unit cell, and it is capable of the same packing efficiency as the fee lattice. The body-centered cubic (bcc) lattice is based on the body-centered cubic unit cell, and it gives a packing efficiency sUghtly lower than the fee or hep lattices. 

Note hep = hexagonal dose-packed, cep = cubic close-packed, bcc = body-centered cubic, sc = simple cubic, tet = tetragonal, rh = rhombohedral, d = diamond, ortho = orthorhombic, mono = monoclinic. 

---