Phase Transitions in [M(NH₃)₆] (XY₄)₂ Compounds

Abstract

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.

Structure, molecular motion, and phase transition of a highly disordered crystal [Co(NH3)6](ClO4)3

“…5,6 Interestingly, [M(NH 3 ) 6 ] 3+ complexes with trivalent metals crystallize in the same space group and have comparable unit cell parameters to the analogical [M(NH 3 ) 6 ] 2+ complexes with divalent metals, [7][8][9] but surprisingly their phase behavior is significantly different. 10,11 Although the thermal behavior of both types of the compounds has been widely characterized in the literature through the decades, there is still not enough information about the molecular dynamics and the nature of the hydrogen bond like interactions occurring in these systems, depending on the temperature. One of the reasons can be the general difficulty in investigating the highly disordered materials, and the other a necessity to use complementary physico-chemical methods for their proper examination.…”