Origin of the 330 nm absorption band and effect of doping Yb in LiYF 4 crystals

Abstract

The electronic structures of LiYF 4 (YLF) crystals containing F color center (YLF-F) and Yb doped YLF crystals (Yb 3+:YLF, Yb 2+:YLF) are systematically studied within the framework of the density functional theory. The calculated results indicate that the 330 nm absorption band originates from the F center in YLF crystals. Thus the doping of Yb 3+ can weaken the 330 nm absorption band by competing with F vacancies in capturing free electrons arising after γ-irradiation and change to Yb 2. By analyzing the lattice relaxation and the electronic structure of YLF containing Yb 2+, we can reasonably believe that once Yb 2+ is formed in YLF crystal, its compensating hole will turn out to be shared by two F − nearest to Yb 2+ forming a diatomic fluoride molecular ion ( F 2 − ) perturbed by Yb 2+, or to say V F color center. According to the molecular–orbital linear combination of atomic orbital (MO-LCAOs) theory, compared to the alkali halides, e.g. LiF, the F 2 − in V F center in LiYF 4 peaks at about 340 nm, which is in agreement with the experimental results.