A new order parameter model for the improper ferroelastic phase transitions in KMnF single crystal.

This paper proposes a new order parameter model which satisfactorily explains complicated symmetry changes, the temperature-pressure (T-P) phase diagram and elastic anomalies observed experimentally with the improper ferroelastic phase transitions in multiferroic KMnF single crystal. First, it is shown that the order parameter model is transformed according to the four-dimensional reducible representation of the wavevector star channel group. Second, based on the order parameter model and the singularity theory, the sixth-order structurally stable Landau potential model is constructed.

The wavevector star channel and symmetry group.

The concepts of `wavevector star channel' and `wavevector star channel group' are newly defined, which allow the effective study of phase transitions considering directly the translational symmetry breaking in crystals. A method is suggested by which the wavevector star channels can be found using the image of the representation of the translational group. According to this method, the wavevector star channels are found for the 80 Lifschitz stars in the reciprocal lattice.