Neicheva S. Еnergy transformation in wide-gap LiCaAlF6 and LiSrAlF6 crystals

This work presents a complex spectroscopic study of absorption, luminescent, scintillation and thermoluminescent properties of pure and rare-earth doped LiCaAlF6 and LiSrAlF6 crystals in a wide range of excitation energy, registration time and temperature. It shows how the composition, defect structure and activator valency influence energy transfer and energy storage. The main characteristics of four-component LiCaAlF6 and LiSrAlF6 dielectrics as transformers of ionizing radiation have been presented for the fist time. The main energy parameters of LiCaAlF6 and LiSrAlF6 crystals: band gap, absorption and emission energy of self-trapped excitons have been determined and their energy zone schemes have been estimated for the first time as well. The energy level position of rare-earth ions in the band gap has been also estimated and new models of luminescence centers and traps have been proposed. The radiative and nonradiative relaxation mechanisms in cerium and europium doped crystals under ionizing irradiation have been determined, and the main mechanisms of energy transfer and radiation damage in crystals containing Eu- and Ce has been explained. It has been found that the low light yield of LiCaAlF6:Ce and LiSrAlF6:Ce is due to a considerable migration losses of energy and the energy storage efficiency depends on radiation stimulated redox reactions of Ce3+ ions as well as the presence and topology of compensative defects. These crystals doped with isovalent Eu2+ ions result in effective energy transfer and radiation stability.