Hexagonal dose packed structures

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. 

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. 

TVansition Phases of Metastable Beta Alloys. The decomposition of metastable beta into a Type 1 or Type 2 alpha may be preceded by the formation of an isothermal omega ((Oigo) phase or the splitting of the beta phase into solute-rich and solute-lean regions. Occurrence of these transition products indicates the difficulty of the direct beta-to-alpha decomposition, possibly because the hexagonal-dose-packed structure of the alpha phase... 

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. 

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.]...

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. 

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]. 

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,... 

Brookite is also based upon a double hexagonal close-packed arrangement of the oxygen ions. The dimensions of the unit of structure differ so much on account of distortion from those for the ideal arrangement, however, that the existence of dose-packing was recognized only after the structure had been determined. 

A convenient description of these structures utilizes the symbols A, By and C for the three layers of (dose-packed spheres differing from one another in position. Hexagonal closest packing corresponds to the... 

In pure titanium, the crystal structure is dose-packed hexagonal (a) up to 882°C and body-centered cubic (p) to the melting point. The addition of alloying dements alters the a—p transformation temperature. Elements that raise the transformation temperature are called a-stabilizers those that depress the transformation temperature, p-stabilizers the latter are divided into p-isomorphous and p-eutectoid types. The p-isomorphous elements have limited a-solubility and increasing additions of these dements progressively depresses the transformation temperature. The p-eutectoid elements have restricted p-solubility and form intermetallic compounds by eutectoid decomposition of the p-phase. The binary phase diagram illustrating these three types of alloy... 

Crystal Structure. In the solution treated and aged condition, the microstructure consists of a + P phases. The lattice parameters of the dose-packed hexagonal a phase are a = 2.9287 Aandc = 4.6606 A, whereas the lattice parameter of the body-centered cubic phase is a = 3.2040A. 

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