Diffuse absences due to the atomic size effect

Diffuse intensity absences, on reciprocal planes that pass through the Bragg positions, have been observed in diffraction patterns of Zr_0.61Y_0.39O_1.81 and (Ta₂O₅).7%(WO₃). These absences, occurring on planes normal to near-neighbour directions and high-lighted by sheets of diffuse intensity on either side, are shown to result naturally from atomic size effect distortions. The origins of these diffuse features are explained, using both real-space and modulation-wave approaches, by concentrating on a Monte Carlo simulated two-dimensional binary alloy. These absences, which should be characteristic of systems that distort via the size effect, result from intensity components that do not depend on scattering-factor differences and so will be most prominent when the atomic species have similar scattering powers, thus suppressing the normal size effect intensity component [Warren, Averbach & Roberts (1951). J. Appl. Phys. 22, 1493-1496].