Experiments for the production of vacuum evaporated films of thorium oxyfluoride showed that these films cannot be produced by direct evaporation of thorium oxyfluoride, because this material decomposes at approximately 1000°C into ThF4 and ThO2. ThF4 evaporates at approximately 900°C and yields very stable films. The production of ThF4 films is described and the dispersion of the refractive index, the dielectric constant, and the useful range of high transparency are reported. Precautions for the handling of ThF4, because of its radioactivity, are given.
W. Heitmann and E. Ritter, "Production and Properties of Vacuum Evaporated Films of Thorium Fluoride," Appl. Opt. 7, 307-309 (1968)