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Metallic Crystals - The Structures of Pure Metals

Summary: Structures of Pure Metals

Tab.1
PackingCoordination numberPacking efficiencyRelative frequencyStructure type
Hexagonal close12 (at a distance of 1.0)74 %42 % Magnesium structure A3
Cubic close 12 (at a distance of 1.0)74 %19 % Gold structure A1
Body-centered cubic8 (at a distance of 1.0)(+ 6 neighbors at a distance of 1.51)) 68 %22 % Tungsten structure A2

Of the 72 metals with known structures, 30 crystallize in a hexagonal closest packed structure, 14 in a cubic closest packed structure, and 16 in body-centered cubic. Twelve have more complicated structures. Some metals, such as iron (α-Fe body-centered cubic, γ-Fe face-centered cubic), can occur as different allotropes, or structures, depending on the method of preparation.

Fig.1
Hint
No relationships between the structures of metals and their location in the periodic table have been found. The reasons for which individual metals prefer certain structures are relatively complicated and not fully known.

Comparison of the macroscopic properties of metals that crystalize in the gold and magnesium structures shows that the former tend to be soft and malleable, while the latter are harder and brittle. One possible explanation suggests that the cubic closest-packed structure has four systems of closest-packed lattice planes perpendicular to the body diagonal of the unit cells, whereas the hexagonal closest-packed structure only has one such system in the stacking direction shown. If plastic deformation of the metal is viewed as the sliding of closest-packed planes relative to each other, this should be easier for the cubic closest-packed structure because it has four times as many systems of planes. Typical representatives of the body-centered cubic structure, such as chromium, molybdenum, and tungsten, are hard and difficult to deform, which can also be explained by the small number of glide planes. However, this approach can only be used as a rough rule of thumb because the mechanical behavior of metals depends greatly on their purity. This means that highly pure metals can behave very differently than pure metals containing a few tenths or hundredths of a percent of impurities.

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