Far-infrared evidence for glass-like properties exhibited by crystals of CaF2 doped with YF3

Low temperature far-infrared transmission measurements between 3 cm(-1) and 18 cm(-1) have been performed on crystals of CaF2 doped with YF3 for molar concentrations ranging from 1% to 22%. These single crystal materials posses a broad distribution of two-level systems typical of amorphous materials. The distribution is similar in magnitude and frequency dependence to that of a standard soda-lime silica glass. In contrast to other fluorite mixed crystal systems the magnitude of the two-level system distribution for YF3 doped samples can be reversibly altered by high temperature cycling between 600 and 900 degrees C.