Measurement of the loss tangent of single crystals of CaF2 and of LiF with a new highly sensitive loss tangent bridge

The loss tangents of single crystals of CaF₂ and of LiF were measured in the temperature range between 80 and 350 K in the audio frequency range. The lowest values of about 5×10^-6 were found at low temperature and low frequencies. These values are not measurable with standard instrumentation. In view of this a special highly sensitive ratio-transformer bridge with a sensitivity of 1×10^-8 for the loss tangent of capacitances of a few to several hundred picofarads was developed. The frequency range of the instrument is 700-15000 Hz but can be extended to 200 kHz. The high-temperature response of the two solids can be described by a relaxation effect with an activation energy of 1.40 eV for CaF₂ and of 0.60 eV for LiF. At low temperatures the response is almost independent of frequency. At higher frequencies the temperature dependence of the loss tangent disappears and extrapolation of the measurements to higher frequencies shows a tendency to a loss tangent that is almost independent of temperature and frequency