The optical properties of calcium fluoride (fluorspar, ${\mathrm{CaF}}_2$) are mainly determined by a strongly temperature-dependent giant infrared (IR) peak and a series of nearly temperature-independent ultraviolet (UV) peaks. We find that the temperature dependence of the IR peak can be modeled, to good accuracy, by a radiation-reaction improved coupled-oscillator model (RRCO model), with temperature-dependent parameters. For the UV peaks, we find a convenient functional form which covers both the real as well as the imaginary parts of the dielectric function and provides a comparison to first-principles calculations based on time-dependent density-functional theory. The result is a compact functional form for the dielectric function of undoped ${\mathrm{CaF}}_2$ applicable to wide frequency and temperature ranges $(0<\hslash \omega <60\mathrm{eV},$ $22{}^{\circ }\mathrm{C}<T<500{}^{\circ }\mathrm{C})$. With the help of the temperature-dependent dielectric function, we obtain temperature-dependent values of the short-range and long-range asymptotics of atom-surface interactions with ${\mathrm{CaF}}_2$, for hydrogen, as well as ground-state and metastable helium. The giant IR absorption peak of ${\mathrm{CaF}}_2$ is shown to lead to a delayed onset of the fully retarded Casimir-Polder limit in the long-range interaction regime. We present arguments supporting a more general applicability of the RRCO model to materials of general interest.

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• Dielectric properties
• Electric polarization
• Dielectrics
• Electronic structure
• Density functional calculations
• Density functional theory