Raman Scattering and Small-Angle X-Ray Scattering inKCl1−xBrx

Abstract

The Raman spectra of the mixed halide $\mathrm{K}{\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x$ were studied as a function of concentration. First-order Raman-active phonons of $A_{1g}$ symmetry were observed at approximately 120 and 145 ${\mathrm{cm}}^{-1\mathrm{}}$ at $x<\mathrm{}0.\mathrm{}3$. Above this concentration, the 120-${\mathrm{cm}}^{-1\mathrm{}}$ mode disappeared, leaving the higher-frequency band only in the first-order spectrum. This band was found to shift linearly with concentration and the extrapolation of the straight-line fit intersects the pure-crystal axes at values close to that of the pure-crystal $\mathrm{TO}\mathrm{}\left(X\right)\mathrm{}$ phonon. At the KBr end of the concentration range, the first-order spectrum resembles the pure-crystal density of states. The first-order spectra of the $T_{2g}$ symmetry were also studied. These reflect the pure-crystal density of states at both ends of the concentration range. The crystals were also investigated by small-angle x-ray scattering for evidence of clustering. No clusters smaller than ${\mathrm{}\left(30\right)\mathrm{}}^3$ unit cells were found. Because of the comparable intensities of the first- and second-order spectra, it was not possible to arrive at perfectly conclusive results.