Raman Scattering and X-Ray-Scattering Studies on $\mathrm{K}{\mathrm{Br}}_{1-x}{\mathrm{I}}_x$, $\mathrm{K}{\mathrm{Cl}}_{1-x}{\mathrm{I}}_x$, and ${\mathrm{K}}_{1-x}{\mathrm{Rb}}_x\mathrm{Cl}$

Raman-scattering and x-ray-scattering experiments have been performed on the mixed-alkali-halide systems $\mathrm{K}{\mathrm{Br}}_{1-x}{\mathrm{I}}_x$, $\mathrm{K}{\mathrm{Cl}}_{1-x}{\mathrm{I}}_x$, and ${\mathrm{K}}_{1-x}{\mathrm{Rb}}_x\mathrm{Cl}$, and the results have been compared with our earlier work on the $\mathrm{K}{\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x$ system. Both powder-pattern and line-broadening studies have been carried out using x-ray scattering. Our earlier work on $\mathrm{K}{\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x$ has been reevaluated and the previous lower limit of a few hundred angstroms for cluster sizes has been raised to 4000 Å. The $\mathrm{K}{\mathrm{Br}}_{1-x}{\mathrm{I}}_x$ system has been seen to display several different microscopic structures. For $x>\mathrm{}0.7\mathrm{}$ the bulk crystals were seen to have the x-ray properties of bulk crystals. For $x<\mathrm{}0.1\mathrm{}$, the bulk crystals were seen to be composed of crystallites a few hundred angstroms in size. For $0.7\ge x\ge 0.1$, the bulk crystals were found to consist of three distinct phases, occuring in crystallites ranging in size from 300 to 1650 Å. Raman spectra for $\mathrm{K}{\mathrm{Br}}_{1-x}{\mathrm{I}}_x$ in the ranges $x<\mathrm{}0.1\mathrm{}$ and $x>\mathrm{}0.7\mathrm{}$ and for $\mathrm{K}{\mathrm{Cl}}_{1-x}{\mathrm{I}}_x$ were seen to be of $A_{1g}$ and $T_{2g}$ symmetry and showed the same systematic behavior as the $\mathrm{K}{\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x$ system reported earlier. That is, certain spectral features could apparently be identified with particular zone-boundary phonons of the mixed crystal and the variation of frequency of these phonons as a function of concentration $x$ could be studied. The variation was seen to be linear. Similar results were seen to pertain for ${\mathrm{K}}_{1-x}{\mathrm{Rb}}_x\mathrm{Cl}$, except that the mixed-crystal spectra were of $E_g$ and $T_{2g}$ symmetry. However, it is also suggested that the Raman results may merely be a reflection of the fact that the zone-boundary phonons observed also happen to correspond to the peaks in the phonon...