Long-Wavelength Optical Lattice Vibrations in Mixed KMgF3-KNiF3Crystals

Infrared reflection and transmission data are analyzed to give the transverse and longitudinal phonons in mixed KNi${\mathrm{F}}_3$-KMg${\mathrm{F}}_3$ crystals for seven concentrations ranging from 0 to 100% Ni. Both one- and two-mode behavior are observed. A point-ion model is developed patterned on earlier work on simpler mixed-crystal systems. The model assumes randomly distributed impurity ions and nearest-neighbor forces. The model gives good agreement with the observed mode frequencies, and in particular it fits the observed splitting of one of the reststrahlen bands at intermediate compositions.