Far-Infrared Ferroelectric Vibration Mode in SrTi${\mathrm{O}}_3$

The real and imaginary parts of the dielectric constant of SrTi${\mathrm{O}}_3$ have been obtained for the region 2.5 to 3000 ${\mathrm{cm}}^{-1\mathrm{}}$ from an analysis of the reflectivity. The imaginary part exhibits a very strong peak at 100 ${\mathrm{cm}}^{-1\mathrm{}}$ at room temperature which shifts to 40 ${\mathrm{cm}}^{-1\mathrm{}}$ at 93°K. This mode is of sufficient strength to account for more than 90% of the low-frequency dielectric constant. The accompanying dispersion of the real part of the dielectric constant is of resonant form. The connection of this mode with Cochran's theory of ferroelectricity is discussed.