Infrared Lattice Vibrations of KNiF3 from Reflection Data at 300° and 90°K

Abstract

Elementary group‐theoretical considerations predict three infrared‐active vibrational modes in KNiF₃ which crystallizes with the cubic perovskite structure. Reflectivity measurements at 300° and 90°K, in the spectral range extending from 80 to 600 cm⁻¹, allow the observation of the above three modes corresponding to the wavenumbers: 148, 244, 441 cm⁻¹ for T=300°K and 144, 248, 452 cm⁻¹ for T=90°K. In addition, we observe a side band near 500 cm⁻¹ at 300°K and 520 cm⁻¹ at 90°K. Assuming that the dispersion can be interpreted in terms of independent damped oscillators, the reflection data have been analyzed using both the Kramers—Kronig relationships and the classical dispersion formula. The intensity of the extra band and some feature of the vibrational absorption spectrum in the visible range suggest that this extra band may arise from a one‐phonon process. The oscillator parameters associated with each observed mode and those deduced from the two methods of analysis are in good agreement with temperature‐dependence considerations.