Diffuse and satellite scattering in urea inclusion compounds with various alkane molecules
Zeitschrift Fur Kristallographie 211(4): 238-246
X-ray diffuse scattering phenomena and satellite reflections in urea inclusion compounds with guest molecules of varying lengths (tridecane, tetradecane, pentadecane, heptadecane and a mixture of pentadecane and hexadecane) were investigated between room temperature and 30 K. It was found that diffuse s-layers, which are due to weak lateral correlations of the guest molecules, are not sharp parallel C* at room temperature as assumed previously, but show a broadening of their widths with increasing order. With decreasing temperature the widths of the s-layers decrease and become sharp within the resolution limit at very low temperature, i.e. there is an ordering of the guest molecules along the urea host tunnels. The modulations of the s-layers point towards a lateral correlation of the guest molecules at room temperature which can be described by a box-shaped distribution function. A model is described which explains the existence of (numerous) satellite reflections and satellite s-layers at low temperatures. Some of the satellites are also observable at room temperature in inclusion compounds with guest molecules longer than tetradecane. They can be assigned to a non-negligible interaction between host and guest. Possible commensurations of these satellite reflections with main (guest-) reflections have been a source of misinterpretations in the literature.