“…In fact, many mono-or co-doping rare earth metal villiaumites have been reported. Tm 3+ ,Er 3+ -LiKYF 5 was irradiated by red light and emitted in blue spectrum by up-conversion mechanism [18]. Pr:LiYF 4 , Pr:LiLuF 4 [19], Er 3+ /Yb 3+ doped LiYF 4 , BaYF 5 [20], Tm 3+ /Yb 3+ doped LiYF 4 [21] and NaYF 4 :Er 3+ ,Yb 3+ [22,23] all revealed the upconversion luminescence to a certain degree.…”