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

Abstract : 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. They apply it to the phase transitions of n-nonadecane/urea and n-hexadecane/urea, while knowing that our measurements had demonstrated, via previously published diffraction experiments [2-6], that both materials undergo at least two symmetry-breaking events revealed by systematic absences of diffraction peaks. We have shown that in n-nonadecane-d 40 /urea-d 4 and n-nonadecane-h 40 /urea-h 4 , there is a complex sequence of phases that follow crystallographic symmetry conditions. The first-phase transition at T c1 is associated with the symmetry breaking from the hexagonal high-temperature phase to a second phase (phase II); a second transition to a different space group (phase III) occurs at a lower temperature T c2. These results have been discussed extensively in the literature [3,4,6-9] and were obtained using excellent spatial resolution and temperature calibration, including measurements using cold neutron scattering on triple axis spectrometers [3,4], and on a synchrotron X-ray diffractometer [7]. For n-nonadecane-h 40 / urea-h 4 , T c1 = (158.8 ± 0.1) K and T c2 = (147.0 ± 0.1) K, according to adiabatic measurements [10].
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B. Toudic, Laurent Guérin, C. Mariette, I. Frantsuzov, P. Rabiller, et al.. Comment on Couzi et al. (2018): a phenomenological model for structural transitions in incommensurate alkane/urea inclusion compounds. Royal Society Open Science, The Royal Society, 2019, 6 (8), pp.182073. ⟨10.1098/rsos.182073⟩. ⟨hal-02303232⟩

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