Far‐Infrared Dielectric Measurements on Potassium Dihydrogen Phosphate, Triglycine Sulfate, and Rutile

Polarized reflection spectra have been made of single crystal KH2PO4, TGS, TiO2, and KD2PO4. KH2PO4 and TiO2 were studied in the range 2.5 to 1500 cm—1, TGS and KD2PO4 over a somewhat smaller range. The reflectivity data were analyzed using dispersion theory and classical‐oscillator fits to obtain the optical constants. The hydrogen bonded ferroelectrics show a behavior quite different from the perovskite ferroelectrics studied earlier. A highly overdamped ferroelectric mode is found in KH2PO4 near 50 cm—1 at room temperature. The close similarities of certain modes in KH2PO4 and KD2PO4 do not support previous speculations on proton tunneling. An analysis of the mode strengths in TGS shows that the major dispersion which contributes to the low‐frequency dielectric constant takes place below 14 cm—1. TiO2 exhibits strong modes near 190 cm—1 which are connected with its large value of low‐frequency dielectric constant, and are quite similar to the low‐frequency mode in SrTiO3.