Compositional correlation and polymorphism in BaF2-PrF3 thin films deposited using electron-beam evaporation

Highlights: • PrF₃-BaF₂ thin films were deposited from the single sources; • BaF₂ concentrations in thin films can be corelated to those in the sources; • Addition of BaF₂ can eliminate completely PrOF in thin films. -- Abstract: Infrared low-index evaporation materials are essential in the broadband antireflection coatings to reduce Fresnel reflections on the surfaces of infrared optical components. Although the layers of praseodymium fluoride (PrF₃) show an excellent transparency and the lower refractive index n and extinction coefficient k in the spectral range of thermal infrared, the tensile stress presented in the layers prohibits PrF₃ from being used as infrared low-index coating materials. A practical solution to reduce stress is to directly evaporate the admixture of PrF₃ with alkaline fluorides, such as barium fluoride (BaF₂). However, due to the significant difference of vapor pressures between PrF₃ and BaF₂, it is commonly difficult to congruently deposit PrF₃-BaF₂ thin films utilizing evaporating directly from a single source. Moreover, more details are unknown about the phase compositions of PrF₃-BaF₂ thin films. In our investigation, BaF₂-PrF₃ thin films were deposited using electron beam evaporation from the sintered ingots of PrF₃ admixed with BaF₂. The compositions of thin films were characterized using energy dispersive X-ray spectroscopy (EDX), the crystallographic structures were explored by X-ray-diffraction (XRD). It was revealed that the concentration of BaF₂ in thin films can be correlated to that in the admixtures. Moreover, in addition to PrF₃ and BaF₂, the phase compositions in thin films include also secondary phases, such as PrOF, PrF₄, Pr₂F₂, BaPrF₆, and that of elemental Ba.