Abstract
The infrared reflectivity spectra polarised in a direction perpendicular to the c axis of potassium dihydrogen phosphate were studied for both the ferro- and paraelectric phase, over a wide temperature range: 10 to 420K. The reflectivity spectra are fitted to the factorised form of the dielectric function. The temperature dependences of the optical mode parameters (frequencies, dampings and oscillator strengths) are reported for both phases. The analysis shows the activity of a low-frequency heavily damped mode in addition to the five observed lattice modes in the paraelectric phase. The parameters of the low-frequency mode, assigned to the proton collective motion, are studied in detail.