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

Abstract

In a recent paper (Couzi et al. 2018 R. Soc. open sci. 5 , 180058. ( doi:10.1098/rsos.180058 )), we proposed a new phenomenological model to account for the I↔II↔“III” phase sequence in incommensurate n -alkane/urea inclusion compounds, which represents an alternative interpretation to that proposed in work of Toudic et al. In a Comment (Toudic et al. 2019 R. Soc. open sci. 6 , 182073. ( doi:10.1098/rsos.182073 )), Toudic et al. have questioned our assignment of the superspace group of phase II of n -nonadecane/urea, which they have previously assigned, based on a (3 + 2)-dimensional superspace, as C222 1 (00 γ )(10 δ ). In this Reply, we present new results from a comprehensive synchrotron single-crystal X-ray diffraction study of n -nonadecane/urea, involving measurements as a detailed function of temperature across the I↔II↔“III” phase transition sequence. Our results demonstrate conclusively that “main reflections” ( h, k, l , 0) with h+k odd are observed in phase II of n -nonadecane/urea (including temperatures in phase II that are just below the transition from phase I to phase II), in full support of our assignment of the (3+1)-dimensional superspace group P2 1 2 1 2 1 (00 γ ) to phase II. As our phenomenological model is based on phase II and phase “III” of this incommensurate material having the same (3+1)-dimensional superspace group P2 1 2 1 2 1 (00 γ ), it follows that the new X-ray diffraction results are in full support of our phenomenological model.

Phase transitions within crystals that are aperiodic by construction

“…As already discussed in the literature [40][41][42][43][44][49][50][51][52][53], this supplementary modulation doubles the unit cell within five-dimensional superspace. Two cases are encountered: a C-centered superspace group C222 1 (00γ )(10δ) and the lower symmetry P2 1 2 1 2 1 (00γ )(00δ).…”

High spatial resolution studies of phase transitions within organic aperiodic crystals

“…As mentioned in Sec. I, recently two studies argued that there is one single-phase transition in n-nonadecane/urea from P6 1 22(00γ ) to P2 1 2 1 2 1 (00γ ) [42][43][44], as initially proposed. Their model considers again the critical point M within the reciprocal (a * , b * ) plane [44].…”

“…These transition temperatures are T c1 = 158.8 K and T c2 = 147 K for the fully hydrogenated C 19 H 40 /CO(NH 2 ) 2 compound and at T D c1 = 149.4 K and T D c2 = 127.8 K for the fully deuterated C 19 D 40 /CO(ND 2 ) 2 . Very recently, some authors questioned this work [42,43]. They argue in favor of a single ferroelastic phase transition described in the (a, b) plane [26,44].…”