LATTICE-DYNAMICS OF FLUORIDE SCHEELITES .2. INELASTIC NEUTRON-SCATTERING IN LIYF4 AND MODELIZATION

A general view of the fluoride scheelite LiYF4 Lattice dynamical prope rties is proposed. The symmetry properties of the vibrations and the s ymmetry-adapted eigenvectors of the scheelite structure have been dete rmined for the most important directions of the Brillouin zone, namely (xi 00), (xi xi 0) and (00 xi). The low-frequency phonon spectrum alo ng these three directions has been measured by inelastic neutron scatt ering. The corresponding data, together with the measured sound veloci ties and the Raman-and infrared-active phonon frequencies, are describ ed in the framework of a rigid ion model. The average error between ex perimental results and calculated frequencies is less than 5%. The mos t important normal modes of vibration and the one-phonon density of st ates are deduced.