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

Abstract

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 + 1)-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 + 1)-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.

Comment on Couzi et al . (2018): a phenomenological model for structural transitions in incommensurate alkane/urea inclusion compounds

“…Inexplicably, Couzi et al . do not address this fundamental criticism of their data in the present article [1]. They have not presented any data on pure phase II on which to base their assertion that phase II is the same space group as phase III.…”

“…However, in their present article [1], Couzi et al make the following surprising statement (where references [12,28,30] and [31] are references [5,3,4] and [11], respectively, in this Comment):Given that the (3 + 2)-dimensional superspace groups proposed previously [12,28,30] for n -nonadecane/urea and n -hexadecane/urea have been shown [31] to be incorrect,……”

“…In their recent article [1], M. Couzi et al . develop a standard phenomenological model of coupled order parameters, which generates one single symmetry-breaking phase transition.…”

“…2(a) of [7], (ref. 32 in [1] and ref. 11 in [11]), which was acquired at 154.5 K, in the middle of phase II of n -nonadecane-h 40 /urea-h 4 .…”

“…We maintain that the data collected by Couzi et al . [11] were on the wrong phase, and as a consequence, their phenomenological description of the phase behaviour in n -nonadecane/urea [1] is contrary to reliable experimental measurements and does not apply to the isotopologues of n -nonadecane/urea. As already extensively argued in our previous Comment [2], we maintain also that two different symmetry-breaking events are indeed present in n -hexadecane/urea.…”