LATTICE-DYNAMICS OF FLUORIDE SCHEELITES .1. RAMAN AND INFRARED STUDY OF LIYF4 AND LILNF(4) (LN=HO, ER, TM AND YB)

IR and Raman active modes have been measured in several fluoride schee lites, namely the laser matrix LiYF4 (studied as a reference) and four LiLnF(4) compounds (Ln = Ho, Er, Tm and Yb). The observed phonons hav e been assigned in accordance with the various irreducible representat ions of the point group of the crystal. In LiYF4 three modes never obs erved before have been measured. The polarized experimental IR reflect ion spectra have been analysed through a 'four-parameters' model to gi ve the mode frequencies and dampings. For the whole set of the compoun ds, the frequency dependence of the dielectric function, optical indic es and absorption coefficient have been deduced from the IR study. The electronic Raman effect has been shown in the low-temperature Raman s pectra for Ln = Tm, Ho and Yb. The corresponding measured wavenumbers are compared with the calculated energies between crystal-field levels of LSJ states, each being associated to an irreducible representation of the ion-site group S-4.