The crystal structures of pure metals

It is surprising, in view of the many structures that are derived from either the hexagonal (ABAB) or cubic (ABCABC) closest packing (see Section 5.4.1), that so few complex arrangements occur. Cobalt is one metal that shows this behaviour. Below about 435 C the structure is a disordered random stacking of A, B and C planes of metal atoms. It can be transformed into the A3 structure by careful annealing at lower temperatures, and this transforms to the Al structure above 435 °C. 

Element Room- temperature struemre High- temperature stmcture Transition temperature/ °C 

The metals lanthanum (La), praseodymium (Pr) and neodymium (Nd) adopt mixed closest packing that has an ABAC repeat. Samarium (Sm), has a packing repeat BABCAC. 

In the semimetals antimony, arsenic and bismuth, bonding effects are more pronounced, and the structures are not related to the simple structures of most metals. Bismuth, the heaviest, is the most metallic , and phosphorus, lying above antimony in the periodic table, is not even considered to be a semimetal. 

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