Phase transition and rotational excitations in Ni(NH/sub 3/)/sub 6/I/sub 2/
Neutron scattering techniques were used to investigate both the crystal structures and the rotational dynamics of the ammonia groups in Ni(NH/sub 3/)/sub 6/I/sub 2/ 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 NH/sub 3/ 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 NH/sub 3/ 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.
- Research Organization:
- Brookhaven National Laboratory, Upton, New York 11973
- DOE Contract Number:
- EY-76-C-02-0016
- OSTI ID:
- 5270733
- Journal Information:
- J. Chem. Phys.; (United States), Journal Name: J. Chem. Phys.; (United States) Vol. 73:1; ISSN JCPSA
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
360602* -- Other Materials-- Structure & Phase Studies
AMMINES
COHERENT SCATTERING
COMPLEXES
CRYSTAL STRUCTURE
CRYSTAL-PHASE TRANSFORMATIONS
DIFFRACTION
HALIDES
HALOGEN COMPOUNDS
IODIDES
IODINE COMPOUNDS
MOTION
NEUTRON DIFFRACTION
NICKEL COMPOUNDS
PHASE TRANSFORMATIONS
ROTATION
SCATTERING
TRANSITION ELEMENT COMPOUNDS