DIFFUSE X-RAY SCATTERING AND PHASE TRANSITIONS OF UREA INCLUSION COMPOUNDS. S.C. Mayo & T.R. Welberry, Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia.

Diffuse X-ray scattering studies have been carried out on the long chain alkane/urea inclusion compounds Hexadecane-Urea (HD-Urea) and 1,10-Dibromo-n-decane-Urea (DBrD-Urea). In both examples the guest species are thought to occupy the hexagonal tunnels in the host urea structure with the alkane chains able to rotate about their long axis and switch between six energetically favourable orientations. At room temperature there is thought to be very little correlation between the orientation of molecules in adjacent tunnels or (in the case of HD-urea) in their relative positions along the channels. The guests thus form a 1D crystal and this gives rise to planes of diffuse scattering normal to the tunnel direction. In the case of HD-Urea the 1D repeat is almost commensurate with the c-axis repeat of the urea framework, but for DBrD-Urea the diffuse planes occur with a repeat distance incommensurate with that of the urea.

As well as these sharp planes of scattering other distinctive diffuse scattering features are observed which are associated with the orientational ordering of the alkanes and the flexing of the urea framework. Although the diffuse scattering in these materials has been noted by other workers (see for example Harris & Thomas, 1990), the resolution of our position-sensitive detector (PSD) X-ray system has allowed us to collect high quality diffuse diffraction data at a range of different temperatures, and investigate the changes that occur as the crystals undergo a series of phase transitions at temperatures below 150K, which are thought to correspond to the orientational ordering of the alkane chains (see for example Harris, 1993). The highly structured diffuse scattering patterns have been modelled using Monte Carlo methods which simulate both the orientational ordering of the alkane chains and the flexing of the urea tunnels about them.

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