Abstract
Using the Born-von Karman and rigid-ion models of Elcombe for the optical phonons in -quartz, we have analyzed the Raman spectra of isomorphic AlP (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 AlP Brillouin zone is halved in the [] direction compared with that of quartz, phonons characteristic of critical point in quartz are Raman active in AlP. The near equivalence of Al + P and 2Si masses and of phonon frequencies experimentally determined in Si and AlP demonstrates that a significant amount of charge compensation exists in AlP, 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 () 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 ) for each transverse/longitudinal doublet allow us to determine the difference in effective charge for Al and P ions. An estimate of is obtained from the rigid-ion expression , and values , , deduced.
- Received 4 December 1970
DOI:https://doi.org/10.1103/PhysRevB.4.1360
©1971 American Physical Society