FAR INFRARED IMPURITY INDUCED ABSORPTION IN $ABF_{3}$ SINGLE CRYSTALS.

The Research Laboratory of Electronics is supported in part by the Joint Services Electronics Program (Contract DA28-043-AMC-2536(E)). $^{1}$ P.L.Richards, J. Appl, Phys. 34, 1237 (1963).Author Institution: Spectroscopy Laboratory and Research Laboratory of Electronics, Massachusetts Institute of TechnologyThe perovskite fluoride crystals $K(Mg_{x}:Ni_{1-x})F_{3}, KCoF_{3}, KMnF_{3}$ and $RbMnF_{3}$ have been observed in transmission at low temperatures. Several bands of varying strengths were found at liquid helium temperature, and these have been attributed to impurities present in the samples. The stronger bands were also observed at liquid nitrogen temperature. These sharpen and shift to lower frequencies as the temperature is decreased. In $KNiF_{3}$ the band attributed to antiferromagnetic $resonance,^{1}$ at $49 cm^{-1}$, has been found to be incorrectly assigned. This band is also present in $KMgF_{3}$ at $53 cm^{-1}$ and shifts continuously at intermediate concentrations in the mixed crystals. The Spectroscopy Laboratory is supported in part by the National Science Foundation Grant No. GP-4923