Hexagonal close-packed structure, high

Titanium, zirconium and hafnium in normal conditions crystallize in the hexagonal close-packed structure (a modification) with a c/a slightly smaller than the ideal one c/a = 1.587 (Ti), 1.593 (Zr) and 1.581 (Hf). At high temperature they have the bcc W-type structure ((3 modification). High-pressure transformations are known (Tables 5.21-5.23). 

According to Pearson (4), calcium exists in two crystalline modifications. The low temperature phase (a) has a face -centered - cubic (cF4) structure, as does o-Sr(cr). The high temperature phase ( B) has a body - centered - cubic (cI2) structure, as does B Sr(cr). In comparison, magnesium has a hexagonal - close - packed structure. The B-Ca(cr) region of stability is 716 3 K to 1115 2 K (5). 

The arrangement of close-packed layers in the pattern xyxy... is the hexagonal close-packed structure (hep) the pattern xyzxyz... is the cubic close-packed (cep) or face-centered cubic (fee) structure. In each of these structures, every sphere is in contact with twelve others six in its own layer, three in the layer above, and three in the layer below. The twelve coordination in these structures is shown by an exploded view in Fig. 27.3(a) and (b), and in a different view in Fig. 27.3(c) and (d). The hep and fee structures are the typical structures encountered in metals. The high coordination number (twelve) in these structures results in a crystal of comparatively high density. 

The hexagonal close-packed structure is shown in Figure 1.1(c). It consists of close-packed layers of atoms placed one above the other with the atoms of each layer following the lower layer. The conventional unit cell contains six atoms. Ideally, the coordination number is twelve and the axial ratio c/a = 2 lN3 = 1.633. In metals, there is often distortion and the ratio differs slightly from this value. For example. Mg has a value of da = 1.624 however, Zn and Cd are highly distorted with da = 1.856 and 1.886 respectively. 

Carbonyl hydrides and carbonylate anions are obtained by reducing neutral carbonyls, as mentioned above, and in addition to mononuclear metal anions, anionic species of very high nuclearity have been obtained, often by thermolysis. These are especially numerous for Rh and in certain Rh, Rh and Rhi5 anions have structures conveniently visualized either as polyhedra encapsulating further metal atoms, or alternatively as arrays of metal atoms forming portions of hexagonal close packed or body... 

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