Symmetry aspect of the phase transitions in boracites

The phase transitions in boracites are analysed by using the group-theoretical formulation of the Landau theory of phase transitions. It is shown that the orthorhombic, monoclinic and trigonal phase transitions could be induced by the same irreducible representation of the space groupT _d ⁵ with the star determined by the wave vectork=1/2(b ₁+b ₂). The corresponding free energy function is constructed and the symmetry of normal modes is discussed.The authors thank Dr. V. Janovec of the Institute of Physics for valuable remarks to this paper.