Kossel Diagrams Show Electric-Field-Induced Cubic-Tetragonal Structural Transition in Frustrated Liquid-Crystal Blue Phases

Convergent beam diffraction patterns from a bcc blue-phase (I) monocrystal in an electric field applied in the [110] direction show this twofold axis transforming through an orthorhombic distortion to become, at a critical field, the fourfold axis of a tetragonal crystal. The implication on structures of blue phase I is important. Although the transition is weakly first order because the two order parameters describing it are coupled, the diffraction pattern changes continuously through the transition.