The number of independent pair-correlation functions in multicomponent systems

An n-component substitutional solid solution in a perfect crystal can be characterized uniquely by n−1 independent composition functions. There are n(n−1)/2 distinct short-range-order parameters αij(i,j = 1,2,... n−1) corresponding to these (n−1) independent compositions. The SRO parameters are linearly but not quadratically independent: for example, in ternary systems, the following equation holds Σ[(1 − \overline c_{1}) (1 − \overline c2)α11α22 − \overline c1 \overline c2α12α21] = 0 in which \overline c1 and \overline c2 are the average compositions pertaining to atomic species 1 and 2, the summation extending over all significant correlation ranges in the crystal. This relation can be used to reduce the uncertainty relative to the interpretation of diffraction experiments realized with less than the optimum number of radiations.