Far-Infrared Optical Constants of KI

Abstract

The far-infrared optical constants of KI were studied in the vicinity of the $\mathrm{TO}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}=0)$ phonon frequency ${\ensuremath{\nu}}_{0}=101$ ${\mathrm{cm}}^{\ensuremath{-}1}$ (in wave-number units). Experimental values of the optical constants at $T=300$\ifmmode^\circ\else\textdegree\fi{}K were determined from transmission and reflection measurements employing a Michelson interferometer operated in the asymmetric mode. Theoretical values were calculated from a theory which utilizes thermodynamic Green's functions to include the effects of the interaction of the optically active phonon with other phonons because of cubic anharmonicity. Phonon-dispersion data for the calculation were generated from a shell model, with parameters selected by Dolling to give the phonon frequencies as determined from neutron-diffraction experiments. Good agreement for the optical constants in the region approximately 20-175 ${\mathrm{cm}}^{\ensuremath{-}1}$ was obtained by using a nearest-neighbor central-force model and adjusting the third derivative of the ${\mathrm{K}}^{+}$-${\mathrm{I}}^{\ensuremath{-}}$ bond potential to ${\ensuremath{\Phi}}^{\ensuremath{'}\ensuremath{'}\ensuremath{'}}({r}_{0})=\ensuremath{-}3.6\ifmmode\times\else\texttimes\fi{}{10}^{12}$ ${\mathrm{e}\mathrm{r}\mathrm{g}/\mathrm{c}\mathrm{m}}^{3}$.