Structural phase transitions in crystals ofR3¯csymmetry

A systematic classification of phase transitions corresponding to a variety of points in the Brillouin zone is made for the $R\overline{3}c\left(D_{3d}^6\right)$ structure. The Landau group-theoretical description of continuous transitions as formulated by Birman and Jarić is the framework for the classification. Only order parameters corresponding to the $\overrightarrow{\Gamma }$ and $\overrightarrow{\mathrm{F}}$ points satisfy the necessary conditions. The resulting subgroups are indicated together with their translational subgroups as expressed in terms of the original trigonal primitive translations. Nonprimitive as well as primitive subgroups are included.