Thorium fluoride (ThF
4) is an infrared coating material with excellent optical and mechanical properties. However, the natural radioactivity of thorium leads to forbidding its usage. The continuing search for alternate non-radioactive substitutes is a main topic among the coating materials scientists worldwide. Investigation into the admixture of rare earth fluorides with other fluorides will be an important solution.
It was disclosed that the admixture of four rare earth fluorides, LaF3, PrF3, ErF3 and SmF3 with BaF2, has the optimal optical and mechanical properties. Moreover, it was also observed that the intrinsic stress in the mixed fluoride films is not reduced with the increasing ionic radius of the admixed fluorides, as proposed by S. F. Pellicori. An explanation for the stress behavior was suggested that alkaline earth fluorides on the grain boundaries of rare earth fluorides have a lower surface energy than rare earth fluorides, hence, the inter-grain forces of rare earth fluorides are reduced, according to the grain boundries model.