OBSERVATION OF TWO-MODE BEHAVIOR IN AN ALKALI HALIDE MIXED CRYSTAL SYSTEM.

This work was supported in part by the Joint Services Electronics Program (Contract DA28-043-AMC 02526(E) ) and by the U.S. Air Force (ESD Contract AF19(628)-6066). Present address of C.H. Perry: Department of Physics Northeastern University, Boston, Mass. 02115.Author Institution: Spectroscopy Laboratory and Research Laboratory of Electronics, Massachusetts Institute of TechnologyUntil recently only one mode behavior has been observed in alkali-halide mixed crystals such as KCl-KBr. However solid solutions of $(K_{x}:Rb_{1-x})I$ appear to exhibit two modes over part of the composition range. These results have been observed directly in transmission measurements of evaporated thin films and in reflection of pressed powders as a function of temperature. Several models have been applied to describe the observed frequency variation with composition, and the conditions necessary for the existence of local and gap modes will be discussed