Dielectric properties of single crystal fluorides at microwave frequencies and cryogenic temperatures

Whispering gallery mode method was used for very accurate permittivity, and the dielectric loss measurements of LiF, and CaF₂ over the temperature range of 4 K-300 K. This method is the most accurate for determining the loss tangent of very low loss materials. The absolute uncertainty in the real part of permittivity was estimated to be less than 0.1% and it was limited principally by uncertainty in dimensions of the samples. Dielectric loss tangents were measured with uncertainties of about 10% limited by accuracy of Q-factor measurements of whispering gallery modes. For the measured materials dielectric losses varied as a function temperature by a few orders of magnitude, exhibiting dielectric losses of 3×10^-6 at 4 K. As expected, at temperatures below 10 K turning points were observed in the frequency-temperature dependence of modes probably due to paramagnetic impurity ions, leftover from the manufacturing process. CaF₂ exhibited a turning point at 7.4 K in a whispering gallery mode at 17.5 GHz and a Q-factor of 3.45×10⁶. LiF exhibited a turning point at 5.3 K in a whispering gallery mode at 13.5 GHz and a Q-factor of 2.55×10⁶.