INELASTIC NEUTRON-SCATTERING STUDY OF PROTON DYNAMICS IN CA(OH)(2) AT 20 K

Inelastic neutron scattering (INS) spectra of Ca(OH)(2) at 20 K are presented from 30 to 4000 cm(-1) for a powder sample, from 30 to 12000 cm(-1) for an oriented single-crystal. INS band splitting due to the lattice density-of-states is observed. Polarization effects reveal the orientation of atomic displacements for each mode and a new band assignment scheme is proposed. For the single-crystal, the nu OH mode and overtones are observed. This oscillator shows only moderate anharmonicity. Spectrum simulation reveals that INS spectral intensities are not consistent with simple normal coordinates and harmonic force-fields for Ca(OH)(2) entities. A new dynamical model is proposed, including anharmonic terms mixing the vibrational states of the OH and lattice modes. Previous assignment schemes proposed on the basis of infrared and Raman data are reconsidered. The structure of the OH stretching mode observed in the infrared is related to the lattice modes density-of-states.