RAMAN SPECTRA OF ALKALI HALIDES AND SOME OF THEIR MIXED CRYSTALS.

Abstract

This work was supported in part by the NASA grant NGR 22 011-051 at Northeastern University and the Joint Services Electronics Program (Contract DA28-043-AMC 02536 (E) at M.I.T. Jeanne H. Fertel is a Visiting Research Associate, Department of Physics, Northeastern University.Author Institution: Research Laboratory of Electronics, Massachusetts Institute of Technology; Department of Physics, Northeastern UniversityThe Raman spectra of room temperature KI RbI, and $Kcl_{1-x}Br_{x}$ (with $x=0,.08,.25,.5,.75,.92$, and 1.0) single crystals have been observed from $30-400 cm^{-1}$ frequency shift from the exciting line. These measurements were made with a Spex double monochromator using He-Ne excitation. No first order spectrum is expected for these crystals and we have assigned the observed peaks to two-phonon combinations and overtones at various symmetry points in the Brillouin zone. Experimental values for the critical point frequencies are available only for KBr. For KCl and KI we have used the frequencies calculated by Karo and Hardy (but these values are expected to be accurate only to about 10 per cent for the acoustic modes and 40 per cent for the optic modes). The assignments for the mixed crystal spectra were made by comparison, i.e., it was expected that the peaks in the KCl spectrum would vary smoothly into the KBr peaks as the mole fraction of KBr was increased. No frequencies, either calculated or experimental, are available for RbI, but we have been able to assign the RbI spectrum by comparison with the KI spectrum which it closely resembles