Diffuse X‐ray scattering in potassium lithium sulfate, KLiSO 4

Detailed diffuse X‐ray scattering patterns of potassium lithium sulfate, KLiSO 4 , 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. B 41 , 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 LiO 4 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.