Phase transition and rotational excitations in Ni(NH₃) ₆I₂

Neutron scattering techniques were used to investigate both the crystal structures and the rotational dynamics of the ammonia groups in Ni(NH ₃)₆I₂ above and below the phase transition near 20 K. The phase transition involves orientational ordering of the ammonia groups and is accompanied by a distortion of the crystal lattice from cubic (space group Fm 3m) to trigonal symmetry along with a doubling of the primitive cell. A deuterated sample was employed for the structural studies. Its transition temperature was found to be 26.5±0.3 K. Extensive studies of the rotational dynamics of the NH₃ groups above the phase transition showed that their motion is best described as one-dimensional rotational diffusion about their trigonal axis. Diffusion constants derived from this analysis as a function of temperature exhibit increasing orientational correlations as the transition temperature is approached. At low temperatures the NH₃ groups perform one-dimensional tunneling motions. The resulting splitting of the librational ground state was determined to be 0.0634±.00M meV in good agreement with the value derived from earlier specific heat measurements. A further excitation at 9 meV is interpreted as a transition to the first excited librational state.