A phenomenological model for structural phase transitions in incommensurate alkane/urea inclusion compounds.

n-Alkane/urea inclusion compounds are crystalline materials in which n-alkane 'guest' molecules are located within parallel one-dimensional 'host' tunnels formed by a helical hydrogen-bonded arrangement of urea molecules. The periodic repeat distance of the guest molecules along the host tunnels is incommensurate with the periodic repeat distance of the host substructure. The structural properties of the high-temperature phase of these materials (phase I), which exist at ambient temperature, are described by a (3 + l)-dimensional superspace. Recent publications have suggested that, in the prototypical incommensurate composite systems, n-nonadecane/urea and n-hexadecane/urea, two low-temperature phases II and 'III' exist and that one or both of these phases are described by a (3 + 2)dimensional superspace. We present a phenomenological model based on symmetry considerations and developed in the frame of a pseudo-spin-phonon coupling mechanism, which accounts for the mechanisms responsible for the I <-> II <-> 'III' phase sequence. With reference to published experimental data, we demonstrate that, in all phases of these incommensurate materials, the structural properties are described by (3 + l)-dimensional superspace groups. Around the temperature of the II <-> 'III' transition, the macroscopic properties of the material are not actually associated with a phase transition, but instead represent a 'crossover' between two regimes involving different couplings between relevant order parameters.