Dielectric Properties and Optical Phonons in LiNb${\mathrm{O}}_3$

The optical phonons and dielectric properties of LiNb${\mathrm{O}}_3$ have been studied by measurements of the infrared reflection spectra in the range 70 to 10 000 ${\mathrm{cm}}^{-1\mathrm{}}$ and of Raman spectra for shifts in the range 10 to 900 ${\mathrm{cm}}^{-1\mathrm{}}$. Transmission measurements have also been made in the range 1000 to 10 000 ${\mathrm{cm}}^{-1\mathrm{}}$. The infrared data have been analyzed by oscillator fits to the reflectivity. These lead to values for the frequency, line-width, and strength of each optical phonon. The optical-phonon frequencies are confirmed by the Raman measurements, which also give information on the variation of phonon frequency with direction of propagation. All except two of the infrared and Raman phonons predicted by group theory have been identified. The index of refraction in the transparent region from 900 to 24 000 ${\mathrm{cm}}^{-1\mathrm{}}$ has been fitted analytically within the limits of experimental accuracy, by assuming the presence of a single ultraviolet oscillator in addition to the infrared phonon oscillators whose frequencies and strengths have been determined. The nature of the ferroelectric transition has been studied by means of group theory and by an investigation of lattice phonon-mode instabilities and Lorentz factors. The latter suggest that the transition in LiNb${\mathrm{O}}_3$ is similar to the transitions in the ferroelectric perovskites. In LiNb${\mathrm{O}}_3$, a lattice-vibration mode has been identified having the correct symmetry to cause the ferroelectric transition. This mode has a significant temperature dependence and a large Lorentz factor, favoring instability.