Raman spectroscopic study of barium fluorapatite

The vibrations of phosphate (PO4-3) ions in single crystals of hexagonal Ba -5(PO4)(3)F (B-FAP) have been investigated by means of polarized Raman scat tering and compared to Raman frequencies observed in fluoroapatite analogs Sr-5(PO4)(3)F (S-FAP) and Ca-5(PO4)(3)F (FAP). The Raman vibrational freque ncies decrease upon substitution of Ba2+ for Sr2+ or Ca2+ in the lattice, a symtotically approaching the Raman frequencies for vibrations of the free i on. Splitting of the vibrational Raman frequencies is also observed. The ob served splitting is in accord with predictions made by a group theory analy sis of the factor group of the unit cell. The effects of the host lattice e nvironment on the magnitude of the Davydov splittings and frequency shifts of crystals within the fluorapatite structure are demonstrated to be invers ely proportional to the unit cell Volume and proportional to the crystal fi eld in the host crystal. (C) 1999 Elsevier Science B.V. All rights reserved .