Evaporation characteristics and optical constants of LaF3 materials in 2.5-12 μm infrared band were investigated. With low refractive index LaF3 materials, high-durability antireflection (AR) coatings in 3.7-4.8 μm medium-wavelength infrared bands were designed and fabricated on Ge substrate. SEM image of the AR coatings with the LaF3 shows that the surface nanocrystals is uniform and densified. The measured optical properties by Fourier tranform infrared spectrometer indicate that the peak transmittance is as high as 99.4%, and the average transmittance increases to 98.8% from 47.7% in 3.7-4.8 μm after double-sided coating. The firmness and durability environmental tests signify that this AR coating can work in harsh environment while maintaining good optical properties.