Atomic-Level Geometry in Materials

In addition to the high-symmetry cubic phases, there are a collection of bad actors which adopt structures of much lower symmetry. For example, the group 

V elements (N, P, As, Sb and Bi) adopt several orthorhombic and rhombohedral phases. Additional structural complexity is ushered in as a result of the fact that in certain cases, the basic building blocks are no longer individual atoms, but rather molecules. The most stunning example of this effect is revealed in S which has a room temperature orthorhombic crystal structure built up of 16 Ss molecules in each unit cell. 

Phase diagrams associated with systems other than intermetallics are also of interest. Our third example, shown in fig. 6.4(c), the Si-Ge system, is interesting primarily for what it is not. In particular, we note the marked lack of complexity 

From the standpoint of constructing plausible models of the type of structural complexity discussed above, we must find a way to assess the free energy of the 

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