Observation of a hexagonal blue phase in systems with negative dielectric anisotropy

Abstract

The occurrence of a three dimensional hexagonal blue phase BPH (possible space groups P6₂22 and P6₄22) in materials with negative dielectric anisotropy ?_a = ?_∥ - ?_? has been detected by the investigation of Kossel diagrams. This is the first observation of a field-induced transition between different blue phase modifications for systems with ?_a < 0. BP I as well as BP II can be transformed to BPH at a sufficiently high field strength E _BP-BPH For neither transitions has any discontinuity of the wavelength of selective reflection in the field direction been observed. Investigations for field strengths E < E _BP-BPH show continuous deformation and reorientation of BP I and BP II. Decreasing (increasing) lattice constants in the field direction are found for BP I oriented with a two-fold (four-fold) axis parallel to E and for BP II oriented with either a four-fold or two-fold axis parallel to E. The preferred orientation in the field is the same as that found for ?_a > 0 (four-fold axis parallel to the field direction).