Long Wavelength Optical Lattice Vibration of Selenium–Tellurium Mixed Crystals

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Abstract
The reflectivity of selenium–tellurium mixed crystals is studied in the far infrared spectral region. The eigenfrequencies, oscillator strengths etc. of the strongly infrared active lattice vibrations are deduced from the reflection spectra by means of a classical oscillator model. The two modes which give rise to a pronounced reststrahlen band show “one mode behaviour”. Moreover, an impurity mode is observed existing in the mixed crystals only. By means of a random‐element‐isodisplacement model (REI) adapted to the trigonal structure of these crystals, the eigenfrequencies and oscillator strengths are calculated as a function of composition and, at least, qualitative agreement with the experimental data and also some insight into the nature of the impurity mode is achieved.