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

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Abstract
The occurrence of a three dimensional hexagonal blue phase BPH (possible space groups P6222 and P6422) in materials with negative dielectric anisotropy ϵa = ϵ∥ - ϵ⊊ 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 ϵa < 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 ϵa > 0 (four-fold axis parallel to the field direction).