A Raman study of the orientational phase transitions of hexammine nickel (II) nitrate

Abstract

The Raman spectrum of hexammine nickel (II) nitrate has been investigated as a function of temperature. Two orientational phase transitions have been observed; at Tc=245+or-5K there is a transition associated with orderings of O3 groups in the nitrate anion, and at Tc=90+or-5K there is a transition associated with orderings of H3 groups within the hexammine cluster. Both transitions are signalled by critical changes in light scattering intensity of internal modes and by the critical growth of the torsional modes of the O3 and H3 groups respectively as Tc is approached from above. At the higher critical temperature, there is evidence that a small structural distortion occurs.