The fundamental vibrations of the (PO3 anions in the AgGd(PO3)4 crystal are predicted by employing the
correlation method based on the group theory. Nine external vibrations are allowed with no coincidence ac
tivity between infrared and Raman spectra. The free PO3 ion with C3v molecular symmetry has six normal
vibrations. Due to the crystalfield splitting effect, the six normal modes of vibrations are split into 96 in
tramolecular vibrations. Symmetric and asymmetric stretching and bending modes are identified with deter
mining of their spectral activity.