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(NH3)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 Fm3m) 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 +- 0.0014 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