Double Close-Pack Structures

Physical properties of the element are anticipated or calculated. Sdvery metal having two aUotropic forms (i) alpha form that should have a double hexagonal closed-packed structure and (ii) a face-centered cubic type beta form density 14.78 g/cm (alpha form), and 13.25 g/cm (beta form) melting point 985°C soluble in dilute mineral acids. 

If there are sufficiently strong repulsive interactions, such as from Ihe electric double-layer lorce. then the gas bubbles at the lop of u froth collect together without bursting. Furthermore, their interfaces approach as closely as these repulsive forces allow typically on the order of 100 nm. Thus bubbles on top of a froth can pack together very closely and still allow most uf the liquid to escape downward under the influence of gravity while maintaining their spherical shape. Given sufficient liquid, such a foam can resemble the random close-packed structure formed by hard spheres. 

Fig. 5. Idealized, close-packed structures for (a) niobium and (b) rhodium penta-fluorides. Atoms in the first, second, third, and fourth layers are shown as single, double, crossed, and hatched circles, respectively. The symbols for overlapped atoms are shown dashed. The bridge bonds are shaded.

Strobel, S. A., and Doudna, J. A. (1997). RNA seeing double Close-packing of helices in RNA tertiary structure. Trends Biochem. Sci. 22, 262—266. 

An attempt to prepare FeU04 at 1000°C. and 30 kbar. proved unsuccessful, but C0UO4 and MnU04 were synthesized under similar conditions. Attempts to prepare 1 3 double oxides were unsuccessful, presumably because these compounds do not possess a close-packed structure (See discussion of MU3O10 structure). 

At a pressure of about 3.8 X 10 atm, the hexagonal close-packed structure of iron transforms to a new structure colled dhcp (double-layer hexagonal close-packed). At present, it is not clear whether the single crystal at the center of Earth possesses this structure. 

Metallic americium has a face-centered cubic structure at its melting point and a double hexagonal closed-packed structure at temperatures below its melting point. The isotope americium-241 emits a-particles and y-rays in its radioactive decay, and is a source of y-radiation, used to measure the thickness of metals, coatings, degree of soil compaction, sediment concentration, and so on. The same isotope, mixed with beryllium, is used as a neutron source in oilwell logging and other applications. Americium-241... 

Silvery metal. Two allot ropic forms a -form double close packed hexagonal structure d 13 67 transition pt 1074 P form face-centered cnbic structure, mp 1175. ... 

Metal. Two allntropic forms c -form, double hexagonal close packed structure, d (calc) 14.78 g/cc d-form, face centered cubic structure, d (calc) 13.25 g/ec. mp 986 + 25 (Fahey, foe, cit). Changes from the trivalent to the tetrava-Itnt state under the influence of oxidizing agents. In the trivalent state, its chemical properties are very close to rhose of curium. Can be separated from other transuranium dements by ion-exchange or by extraction of Bk(IV) with dioctyl-phospboric acid in heptane Haissinsky, Adloff, foe. cit. 

Silvery, hard brittle metal. Two forms reported a-form, double hexagonal, close-packed structure, d (calc i44Cm) 13.51, Cunningham Wallman, J. Inorg. JVucJ. Chem. 26, 271 (1964). /3-form, face-centered cubic structure, d (calc wCm) 19.26, Smith et alf J. Chem. Phys. 50 5066 (1969). mp 1350 + 50p. Oxidized rapidly in the presence of traces of oxygen. Chemistry of trivalent state similar to that of tri-valent lanthanides. 

Gschneidner (1990) selects 1068 K for the transformation from the double-hexagonal close-packed structure to the body-centered cubic structure and 1204 K for the melting point. There is a second-order Neel transformation with ordering on hexagonal sites at 0.039 K (Colclough, 1986). 

Fig. 2.5. The four closed-packed structures observed in the rare earth metals. A-the normal hep structure (Mg-type), B-the normal fee structure (Cu-type), C-the double c-axis hexagonal close-packed (dhep) structure (La-type), and D-the triply primative hexagonal Sm-type structure (after...

As mentioned, the rare earths exhibit five different crystal structures at room temperature. Three of these are common metallic structures, hexagonal close packed (hep), cubic close packed (cep), also called face-centered cubic fee, and body-centered cubic. The first two are pictured in Fig. 4. The bcc structure is not pictured but consists of cubes of atoms surrounding another atom inserted into the geometric center or body-center of the cube. The two structures, unique to the rare earths are the double-hexagonal close packed structure (dhep) found for La, Pr, Nd and Pm and the complex structure found for Sm. Both of these are variants of the hep structure. Their occurrence for the early members of the series can be explained by postulating that the 4/ electrons, which have relatively large radial extensions for the early elements, participate in the metallic bonding. 

UPt3 has the double hexagonal close packed structure (space group P63/mmc) with two uranium atoms per unit cell, at the positions (, , i) and (, j, ). The lattice constants... 

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