A selective decrease in the spectral transmittance of cleaved LiF crystals at wavelengths shorter than 1600 Å has been observed when they are exposed to the atmosphere. The decrease is attributed to a surface layer produced on the crystals by reaction with moisture. It can be substantially inhibited by immediate post-cleavage storage in dry gas or in vacuum. Optical-quality thin plates of currently available, commercial hard-to-cleave LiF that is softer than LiF previously manufactured can be obtained by first hardening the crystal with x rays, cleaving the plates, and then thermally annealing out the optical effects of the x rays.
D. A. PATTERSON and W. H. VAUGHAN, "Influence of Crystal Surface on the Optical Transmission of Lithium Fluoride in the Vacuum-Ultraviolet Spectrum," J. Opt. Soc. Am. 53, 851-854 (1963)