Order and disorder in p-dialkylbenzene-urea inclusion compounds

X-ray diffraction measurements of a series of dialkylbenzene-urea inclusion compounds have shown complex three-dimensional ordering of the guest molec ules within the urea host. In four of the compounds large commensurate supe rstructures are observed, characterized by doubling of the a and b axes of the urea host substructure and a severalfold increase in the host c axis. L ow-temperature phase transitions have been observed in all of these compoun ds corresponding to either monoclinic or orthorhombic distortions of the ce ll from the metrically hexagonal room temperature form. Possible structural arrangements for these compounds are discussed. (C) 1998 Academic Press.