| A type of phase transition which occurs in some alloys and minerals. The simplest example is an alloy of equal quantities of two metals, called A and B, for which A-B bonds are more stable than A-A and B-B bonds. At low temperatures, the atoms form into a regular alternating pattern of A and B atoms, to minimise the energy of the alloy.
A B A B
B A B A
A B A B
B A B A
At higher temperatures, the effect of entropy becomes more important, and the atoms are mixed randomly.
Real alloys and minerals are more complicated than this simple model:
- Not all mineral lattices will allow for an alternating pattern of As and Bs - for example, there may be triangular loops of linked atoms.
- There may not be equal quantities of A and B atoms. In my PhD research, I found that if the proportion of A atoms is smaller than about 1/3 (the exact number depends on the structure of the mineral), then the ordered state will never form.
- Atoms which aren't adjacent to each other still interact, although less strongly than those which are.
- There may be more than one ordered pattern, and which one forms is dependent on the proportion of A atoms.
| 
