It has been established that the intrinsic radiation defects F3 in lithium fluoride crystals and nanocrystals exist in three different configurations (isomers). It is found that at influence on a sample by a radiation with a wavelength that falls in the absorption band of one of the isomers this isomer is transformed to another one. After treatment cessation the equilibrium ratio of concentrations for these isomers is restored. The value of the energy barrier separating equilibrium states of isomers and the contribution of entropy in this barrier are defined.