Diffuse X-ray scattering in potassium lithium sulfate, KLiSO₄

Detailed diffuse X-ray scattering patterns of potassium lithium sulfate, KLiSO₄, in sections normal to [00.1], have been recorded using a position-sensitive detector system. A number of distinctive diffuse features are observed including elongated diffuse peaks near Bragg-peak positions, bands of weaker intensity running normal to a*, b* etc. and darker regions relatively free from scattering in the shape of hexagons and a six-pointed star. Monte Carlo simulation of simple potential models has been carried out in an attempt to explain this scattering and also the observation from Bragg analysis [Schulz, Zucker & Frech (1985). Acta Cryst. B41, 21-26] that mean-square atomic displacements for the apical O(1) atom show a pronounced threefold symmetry. A simple potential, in which harmonic (Hookes law) springs with lengths equal to the observed average bond distances are assumed between the different types of nearest-neighbour atom pairs, is able to account qualitatively for all of the observed diffraction features but gives quite isotropic atomic displacement distributions. A second model is described, which assumes that a small number of point defects are present, in which the potassium sites are occasionally occupied by lithium. The distortion of the very flexible LiO₄ framework around such defects can account for the trigonal shape of the O(1) sites while still giving diffuse diffraction patterns similar to those from the simpler harmonic model.