Determination of Order Parameters from the X-Ray Diffraction Effect

SINCE Bragg and Williams' well-known paper 1 on order phenomena in certain alloys (? brass, CuAu, AuCu 3 , etc.) the so-called order parameters have played an important part in theoretical considerations. Bragg and Williams themselves defined the state of order with the help of s , the long-distance order, which measures the preference of the lattice sites for occupation by the 'right' sort of atoms. Bethe 2 introduced?, the neighbour order, as a quantity more intimately related to the ordering process itself. He assumes that the energy of the crystal is to a sufficient approximation defined by the interaction energy of all pairs of nearest neighbours only ; the energy is then a simple function of?, which measures the preference of neighbour lattice sites for an occupation consistent with order.