“…Therefore, one can conclude, that doping by Ln 3+ ions results in changing only extinction coefficients due to the changing probability of symmetry forbidden 6 H 5/2 → 4 K 11/2 transition. Luminescence intensity enhancement resulted from co-doping of Eu 3+ -containing materials by non-luminescent ions such as Bi 3+ , Gd 3+ , alkali, and alkali earth metal ions was reported previously [ 11 , 26 , 27 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 ]. The observed effect was explained by structure distortion due to the difference between radii of substituted and doping ions resulting in the increase in the emission and absorption probabilities.…”