Magnesium hexagonal crystal structure

In these equations, n is a factor sometimes needed to make the new indices into smallest integers. Thus the direction [001] always transforms to [0001]. The three equivalent directions in the basal (0001) plane of a hexagonal crystal structure such as magnesium, (Figure 2.20), are obtained by using the above transformations. The correspondence is ... 

Magnesium, density 1.7 g cm , is one of the least dense metals. It has an hexagonal crystal structure with a = 0.320 nm, c = 0.520 nm, and da = 1.624. The basal plane is close-packed and the axial ratio is only slightly greater than the theoretical value for incompressible spheres (Emley, 1966). The atomic diameter is 0.320 nm, so there is a favorable size factor with a diverse range of the solute elements Aluminum (Al), Zinc (Zn), Cerium (Ce), Yttrium (Y), Silver (Ag), Zirconium (Zr), and Thorium (Th) (Pol-mear, 1989). 

MgO has the NaCl crystal structure 23,24 each magnesium ion is coordinated by six O2 ions and each 02 by six Mg2+. The structure of a-Al203 consists of close-packed sheets of 02 ions stacked in the sequence A-B-A-B, forming a hexagonal close-packed array of anions.25 The cations are located in two-thirds of the octahedral sites. The structure of a-Al203 results in coordination numbers of 6 and 4 for the cation and the anion, respectively.25... 

With the exception of manganese and urauium, all true metals have one of the following crystal structures body-centered cubic (sodium, potassium, molybdenum), iron face-centered cubic (copper, silver, gold), iron close-packed hexagonal (beryllium, magnesium, zirconium). 

Most pure metals adopt one of three crystal structures, Al, copper structure, (cubic close-packed), A2, tungsten structure, (body-centred cubic) or A3, magnesium structure, (hexagonal close-packed), (Chapter 1). If it is assumed that the structures of metals are made up of touching spherical atoms, (the model described in the previous section), it is quite easy, knowing the structure type and the size of the unit cell, to work out their radii, which are called metallic radii. The relationships between the lattice parameters, a, for cubic crystals, a, c, for hexagonal crystals, and the radius of the component atoms, r, for the three common metallic structures, are given below. 

Figure 6.1 The crystal structures of the metallic elements. Note unit cell parameters are in nanometres. The figure given for cubic structures is Oq, A1 = copper (cubic close-packed) structure A2 = tungsten (body-centred cubic) structure A3 = magnesium (hexagonal close-packed) structure...

Figure 9.5 Crystal structure of magnesium diboride, electronic states and a vibrational mode of boron atoms, (a) Crystal structure of MgB2, hexagonal honeycomb layers of boron atoms alternate with layers of magnesium atoms, centered on the hexagons.

FIGURE 5.33 Crystal structure of magnesium phosphite (MgHPOj hHjO). Projection down [c] axis. Mg, P and H atoms lie along [c] hexagonal axes. (Adapted from D.E.C. Corbridge, AC, 9, 991,1956.)... 

Figure 12.30 Crystal structures of metals, a. Copper adopts cubic closest packing. B, Magnesium adopts hexagonal closest packing.

Hydroxyapatite (HA) is the term commonly used for calcium hydroxyapatite Caio(P04)e(OH)2, the most stable form of Ca-P under physiological pH and body temperature. With fluorapatite and chlorapatite, hydroxyapatite forms the group of apatite minerals that share a similar hexagonal monoclinic crystal structure and the general formula Aio(P04)6(OH,F,Cl)2. The A cation can be several metal ions besides calcium, such as barium, strontium, and magnesium. The phosphate anion can be substituted by a carbonate anion to a limited extent. 

Examples of metals that are cubic closest packed are aluminum, iron, copper, cobalt, and nickel. Magnesium and zinc exhibit hexagonal closest packing. Calcium and certain other metals can crystallize in either structure. 

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