The group-theoretical analysis of the phase transitions in the ionic crystals [Ni(NH3)J (ClO&, [Mg(NH,),](ClO,),, and [Ni(NH,),](BF,), accompanied by the lowering of the space groups from Fm3m t o P2Jb is carried out. It is shown that the symmetry reduction is associated with the two-dimensional irreducible representation z10 of the star { k I 0 } , which corresponds t o the point X of the f.c.c. Brillouin zone. The analysis of the lattice normal modes in Fm3m phase allowed one to specify the structural changes created a t the phase transition point. The monoclinic crystal structure with the space group P2,/b may arise as a result of translational displacements of all ions in the plane parallel t o the unique symmetry plane. The relative orientations of anions become also fixed.