Long Wavelength Optical Phonons in Mixed Alkali Halide Powder Crystals

It is well known that crystalline alkali halides have a single lattice vibrational frequency (the rests t rahlen band), which depends on the atomic masses and the interatomic forces. The vibrational behavior of a mixed alkali halide crystal depends on the concentration of the components /3/. The conditions necessary for the two types of behavior observed in mixed crystals, e.g. , one-mode or two-mode behavior, and the condition necessary for the existence of a localized and gap mode have been discussed in detail previously /4 to 8 / . In our system, there is a frequency overlap between the reststrahlen bands of the constituents, and a single lattice mode appears whose frequency lies between the vibrational frequencies of the constituents. Far -infrared techniques Measurements were carr ied out on the far-infrared asymmetric Fourier spectrometer described by Russell and Bell /9/. By replacing the fixed m i r r o r with the mixed-crystal sample, we were able directly to measure the amplitude reflectivity and the phase shift upon reflection from the sample. To cover the frequency spectrum of 30 to 300 em-', beam spli t ters of thicknesses 6.25 and 12.5 p m were used. The detector was a liquid helium cooled bolometer. Measurements were made at ambient tempera t u r e with 2 crn resolution. The average standard deviation of the interferogram points was 0.1%. -1