Using Landau theory, we show that the red shift of the primary Bragg reflection in cholesteric blue phases and the appearance of more than one cubic structure can be understood by including harmonics of the basic spatial frequency in the order parameter. A new nonsymmorphic bcc structure, having $O^8\left(I{4}_{1}32\right)$ symmetry and no isotropic (defect) points, is found at intermediate chiralities.

• Received 5 February 1981

DOI:https://doi.org/10.1103/PhysRevA.24.635