Spectres de transmission et de reflexion de I'iodure de potassium, pur et dopé, aux basses températures

The far ir reflectivity of a pure single crystal of KI at liquid helium, liquid nitrogen, and room temperature has been measured from 10 micro to 700 micro. Kramers-Kronig analysis, and a two-oscillators Lorentz analysis lead to close results concerning the refractive index n, and the absorption index k in the whole ir spectrum, with maximum values of n and k ranging from 4 to 8. The strongest Lorentz oscillator has a frequency at 80 degrees K (nu(1))= 110 cm(-1))) close to the transverse optic frequency recently determined from neutron scattering at 95 degrees K (TO = 107 +/- 1.3 cm(-1)). The strength of the second Lorentz oscillator decreases rapidly when the temperature is lowered. Its frequency (nu(2) = 150 cm(-1) at 80 degrees K) is such that it can be considered as the TO + TA combination at point L of the first Brillouin zone, which is ir active, and expected at 152.7 +/- 3 cm(-1) from neutron scattering. Absorption bands,locaized in the gap determined by neutron scattering as extending from 104-143 micro (96-70 cm(-1)), are induced by sodium, cesium, and chlorine ions impurities. A peak of absorption in the acoustic phonons spectrum is induced by sodium ion impurities at 63 cm(-1)) close to a maximum in the frequency distribution function for the normal modes of vibration of KI computed recently by Cowley et al. Bromine ions, on the other hand, lead to a general background absorption.