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Undoped and Eu, Na co-doped LiCaAlF6 scintillation crystals: paramagnetic centers, charge trapping and energy transfer properties

Single crystals of LiCaAlF6 undoped and Eu, Na co-doped were studied by electron paramagnetic resonance, radioluminescence and thermally stimulated luminescence techniques applied in a correlated manner. The undoped samples exposed to X-ray irradiation exhibited two hole-like charge trapping centers creation, the molecular ions of the form: ClF- and F2^- - F2^- dimer. Their trap depths and frequency factors were determined as follows: Et1=1.7 eV and Et2=1.1. eV for trap depths and f ~ 10^13 s-1 for frequency factor, respectively. It was found that the europium preferable charge state is 2+ in the LiCaAlF6:Eu,Na samples, however, some amount of the Eu3+ is also present. Moreover, there were two Eu2+ centers: the dominating Eu2+(Ca) and the low-content Eu2+(Li). The amount of the latter is easily governed by the sodium admixture while the former is insensitive to the Na co-doping. Eu and Na co-doping affected the defects distribution and incorporation in the LiCaAlF6 host.