DIMS - A DIRECT-METHOD PROGRAM FOR INCOMMENSURATE MODULATED STRUCTURES

A direct-method program, DIMS (direct methods for incommensurate modul ated structures), has been written to solve the phase problem of incom mensurate structures with one-dimensional modulation. The program uses conventional structure factors instead of normalized structure factor s in the phase derivation. It derives phases for satellite reflections by making use of the known phases of main reflections, which can be c alculated from the known basic structure or high-resolution electron-m icroscope images of the sample, or derived by a conventional structure -analysis method involving only main reflections. Two types of phase r elationship are used in the program. The first type consists of one ma in reflection and two satellites of the same order. This type of relat ionship is used to relate phases of satellites belonging to the same o rder with phases of main reflections. The second type of relationship consists of three satellites belonging to at least two different order s. This type of relationship is used to link satellites of different o rders. X-ray as well as electron diffraction data from a dozen incomme nsurate modulated structures have been used to test the program. Two t ypical examples are described.