Volume effects on the Raman frequencies of phosphate ions in fluorapatite crystals

Abstract The vibrations of phosphate (PO4−3) ions in single crystals of hexagonal Ba5(PO4)3F (B-FAP) have been investigated by means of polarized Raman scattering and compared to Raman frequencies observed in fluorapatite analogs Sr5(PO4)3F (S-FAP) and Ca5(PO4)3F (FAP). The Raman vibrational frequencies decrease upon substitution of Ba2+ for Sr2+ or Ca2+ in the lattice, asymtotically approaching the Raman frequencies for vibrations of the free ion. The observed splitting is in accord with predictions made by a group theory analysis of the factor group of the unit cell. The effects of the host lattice environment on the magnitude of the Davydov splittings and frequency shifts of phosphate ions within different crystals with the fluorapatite structure are demonstrated to be inversely proportional to the unit cell volume.