  The temperature behaviour in the range 22\degree C to 500\degree C of the
dielectric permittivity in the infrared range is investigated for CaF2, BaF2
and Al2O3 through reflectivity measurements. The dielectric permittivity is
retrieved by fitting reflectivity spectra with a model taking into account
multiphonon contributions. The results extrapolated from the measurements are
applied to predict a temperature-dependent atom-surface van der Waals
interaction. We specifically consider as the atom of interest Cs (8P3/2), the
most relevant virtual couplings of which, fall in the range of thermal
radiation and are located in the vicinity of the reststrahlen band of fluoride
materials.
