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

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 C2221(00γ)(10δ). 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 P212121(00γ) 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 P212121(00γ), it follows that the new X-ray diffraction results are in full support of our phenomenological model.

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 + 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.

Laser induced damage of fused silica polished optics due to a droplet forming organic contaminant.

We report on the effect of organic molecular contamination on single shot laser induced damage density at the wavelength of 351 nm, with a 3 ns pulse length. Specific contamination experiments were made with dioctylphthalate (DOP) in liquid or gaseous phase, on the surface of fused silica polished samples, bare or solgel coated. Systematic laser induced damage was observed only in the case of liquid phase contamination. Different chemical and morphological characterization methods were used to identify and understand the damage process. We demonstrate that the contaminant morphology, rather than its physicochemical nature, can be responsible for the decrease of laser induced damage threshold of optics.

Raman gain measurements and photo-induced transmission effects of germanium- and arsenic-based chalcogenide glasses.

The Raman gain spectra of millimeter thick As(2)S(3) and As(24)S(38)Se(38) glasses and Ge((23 - x))Ga(x)Sb(7)S((70 - y))Se(y) with x = 0 and 5 and y = 0, 2, 5 have been measured using a direct nonlinear optics technique. The pump light originated from a picosecond Nd:YAG laser operating at 1064 nm and a tunable optical parametric generator and amplifier (OPG/OPA) was used as a source for the probe light. A peak material Raman gain coefficient of (155 +/- 11) x 10(-13) m/W has been measured for the As(24)S(38)Se(38) glass. A reversible photodarkening effect which responds to picosecond pulses is also reported. Finally, surface optical damage threshold measurements were found to be less than 9 GW/cm(2) for the reported samples, values which are comparable to some TeO(2)-based glasses with lower nonlinearities.

Resolved discrepancies between visible spontaneous Raman cross-section and direct near-infrared Raman gain measurements in TeO2-based glasses.

Disagreements on the Raman gain response of different tellurite-based glasses, measured at different wavelengths, have been recently reported in the literature. In order to resolve this controversy, a multi-wavelength Raman cross-section experiment was conducted on two different TeO2-based glass samples. The estimated Raman gain response of the material shows good agreement with the directly-measured Raman gain data at 1064 nm, after correction for the dispersion and wavelength-dependence of the Raman gain process.