The evolution of hardness and transmittance of irradiated LiF single crystals at elevated temperatures has been investigated. The Vickers hardness is proportional to φ1/3 where φ is the dosage of γ-rays. The thermal annealing of hardness which is analyzed according to a first-order kinetic process has one stage for φ = 100 KGy and two stages for φ ≥ 250 KGy. For a given dosage, the activation energy of stage I is larger than that of stage II. The transmittance of irradiated LiF crystals is attributed to F, M, R1, and R2 centers. The transmittance decreases with increasing time at the brief durations, then oscillates, and finally increases monotonically. The region in which the transmittance increases monotonically with time is simulated by a second-order kinetic process of the color center. The activation energy of optical absorption was determined. Comparing both activation energies of hardness and optical absorption, we find that the mechanism of the kinetic process for hardness differs from that for optical absorption.
Coherent control of thermal phonon transport in van der Waals superlattices