Optical Phonons in $\mathrm{K}{\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x$ and ${\mathrm{K}}_{1-x}{\mathrm{Rb}}_x\mathrm{I}$ Mixed Crystals

The infrared lattice-vibrational spectra of mixed crystals of $\mathrm{K}{\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x$ and ${\mathrm{K}}_{1-x}{\mathrm{Rb}}_x\mathrm{I}$ have been observed at temperatures down to 80°K. The results were obtained from thin-film transmission measurements and Kramers-Kronig analyses of reflection data. A classical oscillator model was used to fit the reflectivity curves for the pure end members. The $\mathrm{K}{\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x$ system shows the normal "one-mode" behavior. Two infrared-active phonons occur in the ${\mathrm{K}}_{1-x}{\mathrm{Rb}}_x\mathrm{I}$ system, and this is the first observation of this type of behavior in an alkali halide mixed crystal. The applicability of several theoretical models is discussed, and the various criteria for one- and two-mode behavior are evaluated.