Dielectric characterization of single-crystal LiF, CaF2, MgF2, BaF2, and SrF2 at microwave frequencies

Lithium, calcium, magnesium, and barium fluorides have widespread use in UV, IR, and scintillation applications. Although their optical properties have been well-studied, little data exist on their dielectric properties at microwave frequencies. Microwave dielectric properties of single-crystal LiF, CaF2, MgF2, BaF2, and SrF2, synthesized by Stockbarger melt-growth techniques, are measured using cylindrical specimens as TE01delta dielectric resonators enclosed in a cylindrical cavity. Single-crystal permittivity and dielectric loss tangent were evaluated at fixed frequencies between 7 and 9 GHz and over a temperature range from -75 to 150 degreesC. The real permittivities of the measured fluorides increase quasi-linearly with temperature, permitting evaluation of the thermal coefficients of permittivity. The dielectric loss tangents increase approximately linearly with frequency, so that Qf (GHz) products at room temperature for BaF2, SrF2, CaF2, LiF, and MgF2 (normal to c-axis) are 57600, 73000, 92000, 192400, and 458600, respectively. The dielectric data support existing ion polarizabilities that are used with molar volumes and molecular additivity rules to estimate the permittivities of more complex fluorides whose values have not been experimentally determined.