A measure of how ordered a system is. An order parameter of zero corresponds to complete disorder, and when the order parameter is a scalar, an order parameter of unity generally corresponds to the maximum possible order. Sometimes the order parameter is a vector. For example, if the system is a collection of atoms with spins which can point in any direction, the average spin could be used as an order parameter.

There is a fairly standard definition of the order parameter for substances such as plagioclase feldspars which become ordered by arranging two types of atom in a regular pattern. If the framework is such that equal numbers of the two types of atom can form an alternating checkerboard pattern:

A B A B
B A B A
A B A B
B A B A

then we define the order parameter Q by

Q = (No. of Al atoms on A sublattice - No. of Al atoms on B sublattice) / Total no. of Al atoms

This definition allows the order parameter to vary from 1 to -1 whatever the proportion of Al atoms is.

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