We present a detailed study of the zone-center optical modes of LiI${\mathrm{O}}_3$ subjected to uniaxial stress directed both along and perpendicular to the hexagonal axis. Representative lines in the Raman spectrum where chosen from three frequency regions: low-frequency ($\omega <200\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$) translational modes, high-frequency ($\omega >740\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$) internal vibrations of the I${\mathrm{O}}_3^{-}$ ion, and intermediate-frequency ($200 {\mathrm{cm}}^{-1\mathrm{}}<\omega <500\mathrm{} {\mathrm{cm}}^{-1\mathrm{}}$) modes of mixed character. The translational modes exhibit large changes in frequency ($\frac{d\omega }{\mathrm{dP}}\sim 0.4$ ${\mathrm{cm}}^{-1\mathrm{}}$/kbar) as a function of stress, while higher-frequency modes are much less affected by the external force. These changes are well described by a linear deformation-potential theory used previously in a large number of cases. Deformation-potential constants and mode Grüneisen parameters are obtained for all modes studied. The latter are in excellent agreement with independent hydrostatic pressure measurements. The deformation potential $b^{\prime }$, which describes frequency shifts when the force is directed along the hexagonal axis, was obtained at two temperatures: $T=77\mathrm{} \mathrm{and} 300$ K. No significant difference was observed in the values of this parameter for these temperatures.

• Received 24 August 1981

DOI:https://doi.org/10.1103/PhysRevB.26.720