Vibrational spectra of monazite-type rare-earth orthophosphates

The room-temperature vibrational spectra of the monazite-type rare-earth orthophosphates (RPO4, R = La, Ce, Pr, Nd, Sm, Eu and Gd) have been investigated using polarized Raman and infrared spectroscopies. As in the case of the previously reported Raman results, we also observe 22 peaks, while only 18 Ag/Bg vibrations are predicted by group theoretical analysis. Through a careful review of the combined Raman–IR modes, in conjunction with the measured generalized phonon density of states of LaPO4 by neutron spectroscopy and a comparison with the corresponding data for the xenotime RPO4 members, we have obtained a self-consistent assignment of the Raman and IR modes. We also discuss on the possible origins of the previously unidentified “extra” peaks.