NATURE OF LONG-RANGE ORDER IN CU3+XAU1-X

The diffuse scattering and intensities of Bragg peaks from single crystals of Cu77Au23 and Cu78Au22 have been studied for various degrees of long-range order. The results indicate that the excess Cu atoms, replacing Au in Cu3Au1 distribute themselves at random for the maximum degree of long-range order. The maximum values of the order parameter indicate that the Cu sublattice is completely filled. For order less than the maximum, the results can be explained in terms of a large number of small ordered regions antiphase to the matrix; these regions are primarily two dimensional and vary in composition from point to point in the alloy. These are more Cu rich than similar regions in Cu 3Au. Evidence is presented that the difference in atomic sizes of the species varies with degree of order. It is also shown that the scattering due to differences in atomic size and thermal vibrations and that due to local order can be separated completely at one temperature by symmetry operations, as suggested by Borie and Sparks. © 1970 The American Institute of Physics.