Raman scattering data are reported for ${\mathrm{Tl}}^{+}$ impurities in KCl, KBr, and KI. Strong lines of $E_{\mathrm{g}}$ symmetry and weaker lines of $A_{1\mathrm{g}}$, $E_{\mathrm{g}}$, and $T_{2\mathrm{g}}$ symmetry are observed. Theoretical calculations based on breathing-shell-model phonons, with no impurity-induced change in force constants, are seen to be in agreement with the data. It appears that ${\mathrm{Tl}}^{+}$ acts as an isotopic impurity and allows one to use the Raman effect as a direct probe of the pure-crystal phonons.

• Received 22 August 1969

DOI:https://doi.org/10.1103/PhysRevLett.23.922