The red-shift of a longitudinal optical phonon which was related to the defect structure in neutron-irradiated samples was studied by using Raman scattering, electron paramagnetic resonance, X-ray diffraction and Fourier-transform infra-red absorption methods. The defect structure was analyzed, for the cases of vacancy-interstitials and antisites, by using a simple model for the LO-transverse optical phonon frequency splitting. It was suggested that a slight reduction in such splitting arose from vacancy-interstitial clusters rather than from antisite defects. This conclusion was based upon the annealing behavior of the antisite defect concentrations, volume expansion and infra-red absorption in neutron-irradiated samples. The clusters were associated with a volume expansion of about 0.13% which was observed in neutron-irradiated samples.

Redshift of the longitudinal optical phonon in neutron-irradiated GaP K.Kuriyama, Y.Miyamoto, M.Okada: Journal of Applied Physics, 1999, 85[7], 3499-502