Phonons, Polaritons, and Oblique Phonons in LiIO3by Raman Scattering and Infrared Reflection

Abstract

A detailed Raman-scattering investigation of LiI${\mathrm{O}}_3$, combined with the polarized infrared-reflectivity measurements, is reported. The dielectric constants determined by the Lyddane-Sachs-Teller (LST) relation (${\epsilon }_{\mathrm{xx}}=6.7,{\epsilon }_{\mathrm{zz}}=6.1\mathrm{}$) do not reveal any large contribution by the lattice dynamics to the low-frequency dielectric response. The high-temperature study of the TO Raman modes (till 220 °C) does not show any mode "softening," which could explain the critical dielectric behavior observed at very low frequencies. The study of the $A$-symmetry polariton curves has reconfirmed the LST dielectric values. The angular dispersion of the two upper oblique phonons of the $A$ and $E_1$ symmetry is compared with theory. The intensity dependence of these oblique phonons enables the determination of the second-harmonic-generation coefficient $d_{13}$ and the linear electro-optic coefficient $r_{13}$ in LiI${\mathrm{O}}_3$. These values are compared with other published data.