Symmetry reduction in the phase transitions of alkali cyanides

For the Landau type phase transitions of alkali cyanides the symmetry of the order parameter and general relevant structural changes including orientational distribution of the CN molecules have been found. All phase transitions are driven by the representation of either the elastic dipol or the elastic quadrupol of the CN molecule and this leads to either ordered or disordered head-tail distribution at CN molecules. It is shown that the monoclinic phaseAa would be the result of condensation of at least two irreducible representations ofFm3m space group and that the CN molecules in this phase can be still disordered with respect to head-tail orientation.