Study On The Preparation And Properties Of Yb³⁺/Ln³⁺ Doped La₂MgTiO₆ Double Perovskite Phosphors

Rare earth ion-doped upconversion luminescence materials are widely used in temperature sensing and anti-counterfeiting due to their sharp spectral lines,tunable color and no background fluorescence.However,the currently researched upconversion luminescence materials greatly limit their practical applications due to their low luminescence efficiency,low temperature sensing sensitivity,and low level of anti-counterfeiting.Among them,La₂Mg Ti O₆（LMT）double perovskite is an oxide with stable physicochemical properties and low phonon energy,which can provide a suitable crystal field environment for doping ions and is a promising upconversion luminescent material.Er³⁺,Ho³⁺and Tm³⁺have a large number of metastable energy levels of stepped structure,which can not only absorb multiple photons to realize the upconversion process,but also reduce the non-radiative relaxation process and realize efficient multicolor upconversion luminescence.Yb³⁺has a high absorption coefficient at 980nm and can be used as a sensitizer.Therefore,in this paper,the excellent upconversion luminescence properties of Yb³⁺/Ln³⁺（Ln=Er,Ho,Tm）co-doped LMT double perovskites are investigated in detail for the first time,and based on the thermal coupling level of Er³⁺and the non-thermal coupling level of Ho³⁺/Tm³⁺,the temperature properties of Yb³⁺/Ln³⁺double-doped LMT phosphors were investigated.The introduction of Mn⁴⁺not only improves the temperature sensing performance of the phosphor,but also realizes dual-mode luminescence with Ln³⁺,which is used in the field of anti-counterfeiting.The main research work and results of this paper are as follows:（1）A series of Yb³⁺/Ln³⁺double-doped LMT phosphors were prepared by a high-temperature solid-phase method.Under the excitation of 980 nm,the phosphor has bright green and blue light emission,which originate from the energy level transitions of ⁵S₂→⁵I₈of Ho³⁺,⁴S_3/2→⁴I_15/2of Er³⁺and ¹G₄→³H₆of Tm³⁺,respectively.By adjusting the concentrations of Yb³⁺and Ln³⁺,the up-conversion luminescence intensity of the phosphor can be optimized.In addition,due to the increased content of Yb³⁺and Ln³⁺,the distance between Yb³⁺-Ln³⁺and Ln³⁺-Ln³⁺becomes smaller,leading to back-energy-transfer（BET）and cross-relaxation between ions（CR）process is intensified,so that the fluorescence intensity ratio of the phosphors changes significantly.The temperature sensing performance of Yb³⁺/Ln³⁺double-doped LMT phosphors are investigated using fluorescence intensity ratio technology.By calculating the relative sensitivity（S_R）and absolute sensitivity（S_A）of the LMT:Yb³⁺/Er³⁺phosphors,the S_Rand S_Abased on the ⁴S_3/2/²H_11/2energy level gradually decreases with the increase of temperature,the maximum values?are83.5×10^-4K^-1and 74.5×10^-4K^-1at 301 K.The LMT:Yb³⁺/Ho³⁺phosphors based on⁵F₄/⁵F₅has maximum S_Aand S_Rvalues of 70.1×10^-4K^-1（297 K）and 48×10^-4K^-1（548K）.Based on ³H₄/¹G₄energy level,the maximum S_Aand S_Rvalue of LMT:Yb³⁺/Tm³⁺phosphor are 931×10^-4K^-1（548 K）and 155×10^-4K^-1（293 K）.（2）Based on the optimal concentration of Yb³⁺/Ln³⁺doped LMT phosphors,a series of LMT:Yb³⁺/Ln³⁺/Mn⁴⁺double perovskite phosphors were synthesized by doping Mn⁴⁺.Among them,the introduction of Mn⁴⁺does not change the crystal structure of the phosphor,and the energy transfer process with Ln³⁺can also be used to effectively improve the intensity ratio of LMT:Yb³⁺/Ln³⁺phosphor.The existence of the energy transfer process between Ln³⁺and Mn⁴⁺is proved by the analysis of the fluorescence lifetime and pump dependence.Then,based on ⁴S_3/2/²H_11/2energy level of Er³⁺,the maximum S_Aand S_Rof the LMT:Yb³⁺/Er³⁺/Mn⁴⁺phosphor are 103.8×10^-4and 95×10^-4K^-1;Based on ⁵F₄/⁵F₅energy level of Ho³⁺,the maximum S_Aand S_Rof LMT:Yb³⁺/Ho³⁺/Mn⁴⁺phosphor are 86×10^-4and 61×10^-4K^-1;Based on ³H₄/¹G₄energy level of Tm³⁺,the maximum S_Aand S_Rof LMT:Yb³⁺/Tm³⁺/Mn⁴⁺phosphor are1152×10^-4K^-1（548 K）and 184×10^-4K^-1（300 K）.By comparison,it is found that the introduction of Mn⁴⁺can greatly improve the S_Avalues,whether based on the thermally coupled or the non-thermally coupled energy level.In addition,based on the two-photon centers of Ln³⁺and Mn⁴⁺,the maximum S_Rof triple-doped double perovskite phosphors can reach 161×10^-4and 185×10^-4K^-1.As a down-conversion activator,Mn⁴⁺can combine with the up-conversion ion Ln³⁺to realize multi-mode luminescence excited by different wavelengths.Under 980 nm excitation,the LMT:Yb³⁺/Ln³⁺/Mn⁴⁺samples emit green light(from Er³⁺and Ho³⁺)and blue light(from Tm³⁺);under 365 nm excitation,they both emit red light(from Mn⁴⁺).