Raman Spectra and Lattice Dynamics of α-Berlinite (AlPO₄)

Using the Born-von Karman and rigid-ion models of Elcombe for the optical phonons in α-quartz, we have analyzed the Raman spectra of isomorphic AlPO₄ (berlinite). The resulting analysis yields several physical parameters which are not accessible to any other experimental measurement; for example, the dispersion of phonon branches in the high-frequency region. Since the AlPO₄ Brillouin zone is halved in the [00ζ] direction compared with that of quartz, phonons characteristic of critical point A in quartz are Raman active in AlPO₄. The near equivalence of Al + P and 2Si masses and of phonon frequencies experimentally determined in SiO₂ and AlPO₄ demonstrates that a significant amount of charge compensation exists in AlPO₄, such that Al and P each have a nominal valence of ∼ +4, rather than +3 and +5. This is confirmed by the measured LO/TO splitting of modes which would be described as having wave vector (0,0,π/c) in quartz: If Al and P charges and masses were identical, these degeneracies would remain unsplit by the lattice Coulomb field. The small splittings we observe (7-19 cm^-1) for each transverse/longitudinal doublet allow us to determine the difference in effective charge for Al and P ions. An estimate of Z(P)-Z(Al)≈0.2e is obtained from the rigid-ion expression 4π²Σj(ω_LO,j²-ω_TO,j²)=(4π/V)ΣkZ_k²/m_k, and values Z(P)=2.4e, Z(Al)=2.2e, Z(O)=-1.15e deduced.