Local structure of the B4 phase studied by second harmonic generation and X-ray diffraction measurements

Abstract

Texture observations, second harmonic generation (SHG) and X-ray diffraction measurements have been carried out in the B₄ phase of two classical bent-core liquid crystals. Several sample geometries have been used for the different techniques, including a well-aligned cell for the optical studies. An indexation of the whole diffraction diagram was performed on the basis of a three-dimensional lattice. A quantitative analysis of the X-ray data is also reported. The results are compatible with the well-known twist grain boundary (TGB) structure for the B₄ phase. However, within the smectic blocks the structure is monoclinic P2, implying an antiferroelectric in-layer arrangement of the molecules instead of the usually accepted ferroelectric. This antiferroelectric packing also explains the low efficiency of the B₄ phase for SHG.