Mn⁴⁺ Activated New Red Phosphors Based On Oxide Of LiMgGe

Nowadays,solid state light based on the phosphor-converted light emitting diodes（pc-LEDs）became the mainstream technology for light application,in which,illumination based on white pc-LEDs and plant photosynthesis based on blue-red pc-LEDs are the two active fields.Thus,red light component from red phosphors is very important for pc-LED technology.However,the property of red phosphors is not satisfied yet,mainly due to the thermal quenching of luminescence,environment unstable,harsh synthesis condition,and reabsorption in green light region.Hence,the research on red phosphors is being kept on.This work focuses on the development of a new series of red phosphors based on Mn⁴⁺activated Li Mg Ge oxides(LMGO:Mn⁴⁺).Detail work is summarized as follows:Firstly,a series of red phosphors of Mn⁴⁺-activated LMGO without or with excessive MgO were synthesized using high-temperature solid-phase reaction in air.It was found that the crystal structure of the phosphor matrices consists of[Li O₄],[MgO₄]and[Ge O₄]tetrahedral units,contrast to the often reported octahedral structure in red phosphors.All phosphors exhibit intense and sharp red light emission in the range of 600-700 nm under blue light excitation.The luminescence properties of the phosphors with excessive MgO were found significantly improved.The sample（LMGO 2.2）with2.2 folds MgO（relative to Ge O in mole ratio）exhibited the strongest emission with a photoluminescence quantum yield（PLQY）of 46%,which is 9 times enhancement compared to that of sample with stoichiometric ratio of Li₂MgGeO₄.Thermal stability investigation showed that the luminescent intensity was maintained at 83%and 85%for Li₂MgGeO₄:Mn⁴⁺and LMGO 2.2:Mn⁴⁺,respectively,when the temperature increases from room temperature to 200 ^oC.The activation energy for luminescence quenching were calculated to be 0.256 e V and 0.194 e V,respectively,all of which indicate high luminescence thermal stability of the phosphors.Secondly,a series of new phosphors were prepared via doping LMGO 2.0:Mn⁴⁺phosphor with magnesium/lithium fluoride.These phosphors exhibit strong red luminescence（peaked around 656nm）under UV or blue light excitation.For Mg F₂ doping,the sample with MgO:Mg F₂=3.5:0.5(LM_2.0GOF_3.5:0.1%Mn⁴⁺)has the best luminescence performance;in the case of Li F doping,sample with Li₂CO₃:Li F=0.75:0.5(LM_2.0GOF_0.5:0.1%Mn⁴⁺)is the best;and their PLQY is 73%and 71%,respectively,which are 1.82 times and 1.78 times higher than that of parent LMGO 2.0:0.1%Mn⁴⁺.By thermal stability investigation,it was found that the luminescent intensity of the LM_2.0GOF_3.5:0.1%Mn⁴⁺still remained at 85%when the temperature increases from room temperature to 200 ^oC,with activation energy for luminescence quenching of 0.124 e V.These results show the good thermal stability of this phosphor.