Modeling of the Resonant X-ray Response of a Chiral Cubic Phase

Timon Grabovac,Nataša Vaupotič,Damian Pociecha,Ewa Gorecka

doi:10.3390/cryst11020214

Timon Grabovac, Nataša Vaupotič + Show 2 more

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https://doi.org/10.3390/cryst11020214

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Abstract

The structure of a continuous-grid chiral cubic phase made of achiral constituent molecules is a hot topic in the field of thermotropic liquid crystals. Several structural models have been proposed so far. Resonant X-ray scattering (RXS), which gives information on the molecular orientation in the unit cell, could be applied to select the most appropriate model. We modeled the RXS response for the recently proposed chiral cubic phase structure with an all-hexagon chiral continuous grid. A tensor form factor of a unit cell is constructed, which enables calculation of intensities of peaks for all Miller indices. We find that all the symmetry allowed peaks are resonantly enhanced, and their intensity is much stronger than the intensity of the symmetry forbidden (resonant) peaks. In particular, we predict that a strong resonant enhancement of the symmetry allowed peaks (011) and (002), not observed in a nonresonant scattering, could be observed by RXS at the carbon absorption edge. By RXS at the sulfur absorption edge, one might observe a resonant peak (113) and resonantly enhanced peak (233), and resonant enhancement of all the peaks that are observed in a nonresonant scattering, which probably hide the rest of the predicted resonant peaks.

Highlights

IntroductionOrganization of molecules in three-dimensional continuous channel network in thermotropic liquid crystals has attracted much interest from two points of view
By RXS at the sulfur absorption edge, one might observe a resonant peak (113) and resonantly enhanced peak (233), and resonant enhancement of all the peaks that are observed in a nonresonant scattering, which probably hide the rest of the predicted resonant peaks
In the case of the resonantly enhanced peaks, we focus only on the peaks that are allowed by the Im3m symmetry but are not observed in the nonresonant X-ray scattering

Summary

Introduction

Organization of molecules in three-dimensional continuous channel network in thermotropic liquid crystals has attracted much interest from two points of view The first is their complexity; these phases are interesting from a basic research point of view. In a one-continuous-grid model [7], the octagons were replaced by micelles that are enclosed by a continuous grid formed by interconnected hexagons. Neither of these models could reproduce well the intensities of the peaks observed in the X-ray scattering studies, especially regarding the intensities of the two most intensive peaks with Miller indices (123) and (004). Almost a decade after the first observation of the cubic phase with the Im3m symmetry (assigned at that time), chiral domains were observed in this phase [8]

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