Quantum Rotation of Ammonia Molecules in Disordered Ca(NH3)6
Calcium hexaammine is a low electron density, metallic compound. In this expanded metal, the ammonia molecules show important rotational and reorientational motions down to 20 K as deduced from the study of the quasielastic neutron scattering ( LECLERCQ et al./1/ ). A complete understanding of the inelastic scattering needs a simultaneous determination of the structure in the whole temperature range. Since the calcium compound crystallizes in a high symmetry group (bcc) and since no phase transition occurs as the temperature is decreased, the ammonia molecules appear to be in a frozen-in orientationally disordered state at low temperature. For temperature lower than 10 K, the Inelastic Neutron Scattering spectrum shows a very asymmetric line shape with a maximum near the energy of uniaxial rotation of the NH3 / ND3 groups. This is interpreted by quantum rotation in a (random) distribution of low hindering potentials. It is shown that these properties are very sensitive to non-stoichiometry effects.
KeywordsInelastic Scattering Ammonia Molecule Rotational Excitation Spectrometer Resolution Reorientational Motion
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