Unit cell hexagonal closest-packed

There are four spheres, four octahedral interstices and eight tetrahedral interstices per unit cell. Therefore, their numerical relations are the same as for hexagonal closest-packing, as well as for any other stacking variant of closest-packings one octahedral and two tetrahedral interstices per sphere. Moreover, the sizes of these interstices are the same in all closest-packings of spheres. 

According to the discussion in Section 17.3, many structures can be derived from the hexagonal closest-packing of spheres by occupying a fraction of the octahedral interstices with other atoms. If the X atoms of a compound MX form the packing of spheres, then the fraction 1 fn of the octahedral interstices must be occupied. The unit cell of the... 

Section of the hexagonal closest-packing. Gray area Unit cell, space group 2/m l/c. 

A summary of stacking patterns, coordination numbers, amount of space used, and unit cells for the four kinds of packing of spheres is given in Table 10.10. Hexagonal closest-packing is the only one of the four that has a noncubic unit cell. 

Mg crystallizes in the hexagonal closest packed structure and has a density 1.74 g/cm3. (a) What is the volume of the unit cell (b) What is the distance between nearest neighbors (c) How many nearest neighbors does each atom have ... 

The aba arrangement has the hexagonal unit cell shown in Fig. 16.14, and the resulting structure is called the hexagonal closest packed (hep) structure. The abc arrangement has a face-centered cubic unit cell, as shown in Fig. 16.15 on page 779, and the resulting structure is called the cubic closest packed (ccp) structure. Note that in the hep structure the spheres in every other layer occupy the same vertical position (ababab. . . ), while in the ccp structure the spheres in every fourth layer occupy the same vertical position (abcabca. . . ). A characteristic of both structures is that each sphere has 12 equivalent nearest... 

Use the diagram of the unit cell for the hexagonal closest packed structure in Fig. 16.14 to determine the net number of atoms in the hep unit cell. 

Hess s law in going from a particular set of reactants to a particular set of products, the enthalpy change is the same whether the reaction takes place in one step or in a series of steps in summary, enthalpy is a state function. (9.5) Heterogeneous equilibrium an equilibrium involving reactants and/or products in more than one phase. (6.5) Hexagonal closest packed (hep) structure a structure composed of closest packed spheres with an ababab arrangement of layers the unit cell is hexagonal. (16.4)... 

The sequence AB AB. .. is referable to a hexagonal unit cell (Fig. 4.13) and is called hexagonal closest packing (h.c.p.). In this arrangement, also illustrated in Fig. 4.14(d), the twelve neighbours of an atom are situated at the vertices of the coordination polyhedron of Fig. 4.5(a) ( twinned cuboctahedron ). 

Examples of metals that form cubic closest packed solids are aluminum, iron, copper, cobalt, and nickel. Magnesium and zinc are hexagonal closest packed. Calcium and certain other metals can crystallize in either of these structures. Some metals, however, assume structures that are not closest packed. For example, the alkali metals have structures characterized by a body-centered cubic (bcc) unit cell (see Fig. 10.9), where the spheres touch along the body diagonal of the cube. In this structure, each sphere has 8 nearest neighbors (count the number of atoms around the atom at the center of the unit cell), as compared with 12 in the closest packed structures. Why a particular metal adopts the structure it does is not well understood. 

What is closest packing What is the difference between hexagonal closest packing and cubic closest packing What is the unit cell for each closest packing ... 

Hexagonal closest packed (hep) structure a structure composed of closest packed spheres with an ababab arrangement of layers the unit cell is hexagonal. (10.4)... 

The use of the polyhedral view of a structure becomes more and more useful as the structure becomes more complex. For example, rutile (Ti02> can be thought of as hexagonal closest packing of oxide ions with TT+ ions in half of the octahedral sites. The views in Figure 53 along the three axes show that the unit cell is definitely not cubic. 

The zinc blende lattice Is named after its parent compound, ZnS. Zinc sulfide also exists in a different structure known as the wurtzite lattice. Molecules that can exist in more than one type of crystalline form exhibit polymorphism. The wurtzite lattice is comprised of one type of ion forming a hexagonal closest-packed unit cell, with the other type of ion occupying half of the tetrahedral holes. The following molecules can assume the wurtzite lattice ZnO, ZnS, ZnSe, ZnTe, BeO, Agl, CdS, MnS, SiC, AIN, and NH4F. Both types of lattices consist of corner-shared tetrahedrons, but the tetrahedrons in wurtzite are canted in alternating layers. 

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