Simulation of X-ray powder diffraction patterns for low-ordered materials

A new program for simulation of X-ray diffraction patterns of polycrystalli ne materials with different kinds of imperfections has been developed. Thes e calculations are performed on the base of the model of one-dimensional (1 D) disordered crystal being the statistical sequence of the biperiodic laye rs. Each layer is characterized by its structure, thickness and probability of occurrence. The sequence of layers is specified with the use of order-d isorder parameters and some probability coefficients. Such defects as small sizes of coherently scattering domains (CSD), microstrains and stacking fa ults are taken into account. Our program simulates two kinds of layers: iso tropic (circle) and anisotropic (rectangle). Along with mean sizes of CSD, the variances of normal or lognormal size distributions of CSD can be taken into account. Also, the program makes it possible to specify whether the f luctuations in the layer position are correlated or not. The plane groups o f symmetry are introduced to reduce the run time. The background is approxi mated by using the smoothing spline-functions. The correction for instrumen tal line broadening is calculated with a standard. Simulated diffraction pa ttern is compared and fitted for best correspondence with the experimental one. Real structures of some specimens have been investigated with this program. These are metastable Ni-In alloys prepared by mechanochemical synthesis, f ilamentary carbons and metallic Co with microdomain structure. (C) 2000 Els evier Science B.V. All rights reserved.