Far Infrared Dielectric Dispersion in KTaO3

The room-temperature reflectivity of single-crystal KTa${\mathrm{O}}_3$ has been measured from 5000 to 77 ${\mathrm{cm}}^{-1\mathrm{}}$. These data have been analyzed by a Kramers-Kronig relation and by classical dispersion theory. The three infrared-active fundamental vibrations allowed by the lattice symmetry have been observed. The strength of the lowest frequency mode accounts for the large low-frequency dielectric constant of the material. The lattice band contribution to the microwave loss tangent at 3 cm is estimated to be about 2×${10}^{-3\mathrm{}}$. Attempts to fit the reflectivity data with dispersion theory show that a frequency-dependent damping constant for the low-frequency resonance is necessary in order to obtain a good fit between the calculated curve and the data. The required variation of the damping constant is by more than an order of magnitude. The present results are compared with similar measurements reported earlier on barium and strontium titanates.