INELASTIC NEUTRON-SCATTERING STUDY OF PROTON DYNAMICS IN CA(OH)(2) AT20 K

Inelastic neutron scattering (INS) spectra of Ca(OH)(2) at 20 K are pr esented from 30 to 4000 cm(-1) for a powder sample, from 30 to 12000 c m(-1) for an oriented single-crystal. INS band splitting due to the la ttice density-of-states is observed. Polarization effects reveal the o rientation of atomic displacements for each mode and a new band assign ment scheme is proposed. For the single-crystal, the nu OH mode and ov ertones are observed. This oscillator shows only moderate anharmonicit y. Spectrum simulation reveals that INS spectral intensities are not c onsistent with simple normal coordinates and harmonic force-fields for Ca(OH)(2) entities. A new dynamical model is proposed, including anha rmonic terms mixing the vibrational states of the OH and lattice modes . Previous assignment schemes proposed on the basis of infrared and Ra man data are reconsidered. The structure of the OH stretching mode obs erved in the infrared is related to the lattice modes density-of-state s.