The dispersion curves along the directions [100] and [001] of have been measured by inelastic neutron scattering. The results are analysed with different rigid-ion models: Born - von Karman and valence force field models, which give a good agreement with experiments. Additional measurements of the acoustical branches allow us to determine elastic stiffness constants which are () , , , , , . The compressibility calculated from these constants is in accordance with the experimental value.
