Powder X-ray diffraction studies of a low-temperature phase transition in the n-hexadecane/urea inclusion compound

Abstract

Powder X-ray diffraction studies of the n-hexadecane/urea inclusion compound have been carried out between 296 and 46 K to investigate structural aspects of the host (urea) structure, particularly in relation to a low-temperature phase transition in this compound. Our results show that, at ca. 150 ± 1 K, the symmetry of the host structure changes from hexagonal (high-temperature phase; space group P6₁22) to orthorhombic (low-temperature phase; probable space group P2₁2₁2₁). This assessment of the transition temperature is in good agreement with previous measurements using other experimental approaches, and our interpretation of the structural change associated with the transition confirms earlier speculations based upon single-crystal X-ray diffraction investigations. Although the transition involves a major change of crystal symmetry, there is only a relatively minor change in the actual distribution of scattering matter within the crystal, and we outline, in general terms, a systematic approach towards rationalizing changes in a powder diffractogram arising from such a transition. No satisfactory determination of the host structure within the low-temperature phase has been reported previously, and since multiple crystal twinning is thought to accompany the phase transition on passing from the high-temperature phase to the low-temperature phase, powder X-ray diffraction represents a more viable route to this information than single-crystal X-ray diffraction. Structure refinement from the powder X-ray diffraction data collected at 120 K has been attempted using the Rietveld profile refinement method.