CONCENTRATION DEPENDENCE OF LONG WAVELENGTH OPTIC PHONON FREQUENCIES IN MIXED CRYSTALS

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1968

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Ohio State University

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Infrared-reflection spectra were measured from $160 cm^{-1}$ to $660 cm^{-1}$ at temperatures ranging from $130^{\circ}K$ to $958^{\circ}K$ for NaF and from $130^{\circ}K$ to $1002^{\circ}K$ for $CaF_{2}$. The reflectivity data were analyzed by Kramers-Kronig relations and by two-resonance damped oscillator models. Some data were also obtained from evaporated thin films. The transverse and logitudinal optic mode frequencies and calculated optical properties will be presented as functions of temperature. The maximum reflectivity varies almost linearly with temperature and begins to saturate at low temperatures (below $300^{\circ}K$). The total frequency shift of NaF spectra is found to be $33 cm^{-1}$ for the temperature range considered. The total frequency shift of $CaF_{2}$ spectra is not clear due to the fact that the multi-phonon structures sustain while the main band diminishes at high temperatures. The temperature dependence of shift and half-width (damping constant) will be discussed in the light of current lattice dynamical theories incorporating anharmonic forces.

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Present address of I. F. Chang: Components Division, IBM, Fishkill, New York.
Author Institution: Department of Electrical Engineering, University of Rhode Island

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