Transformation of the microstructure and luminescence characteristics of LiF films during annealing

The microstructure of lithium fluoride thin films deposited on a glass substrate by thermal vacuum evaporation was investigated. An additional annealing of the films in air at temperatures of 400, 500, and 600$^\circ$C resulted in an increase in the size of grains forming the film, as well as in the transformation of luminescence spectra of the initial and annealed samples before and after their X-ray irradiation used to generate in the films color centers that played the role of luminescent probes. It was found that an increase in the annealing temperature with an increase in the size of grains leads to a decrease in the intensity of the characteristic photoluminescence bands of color centers in lithium fluoride and to the appearance of new luminescence bands that are not characteristic of this material. Based on the results of the X-ray powder diffraction analysis, it was concluded that the increase in the grain size is caused by the change in the chemical composition of the films primarily due to the interaction of lithium fluoride with the substrate material.