Study On The Upconversion Luminescence And Temperature Sensing Behaviors Of Er³⁺/Tm³⁺/Yb³⁺doped Double Perovskite Gd₂ZnTiO₆ Phosphors

Rare earth doped oxide upconversion（UC）phosphors exhibit broad prospect in temperature sensing application owing to their excellent physicochemical properties.However,UC phosphors reported currently suffer from low sensitivity and poor thermal stability effects and thereby are restricted extremely,which suggest that the production of UC phosphors with good stability,excellent emissions,and color-tunable property play a key role in practical application.The novel double perovskite Gd₂ZnTiO₆（GZT）host is a kind of good oxide material with well physicochemical properties and gratifying thermal stability.And more importantly,it is a good choice for UC luminescence based on its low phonon energy.Both Er³⁺and Tm³⁺are typical activators and the two ions are investigated commonly for their emissions of green,red,blue and near-infrared,that are derived from abundant metalstable levels.On the other hand,The sensetizer Yb³⁺is usually utilized because it can serve as a brige between excitation light source and activators and play a well energy transfer function among them.As mentioned above,UC phosphors with with good stability,excellent emissions,and color-tunable property may be obtained in Er³⁺single-,Er³⁺/Yb³⁺and Tm³⁺/Yb³⁺co-,and Er³⁺/Tm³⁺/Yb³⁺tri-doped GZT UC phosphors.To exploit the temperature sensing behaviors in Er³⁺/Yb³⁺and Tm³⁺/Yb³⁺co-,and Er³⁺/Tm³⁺/Yb³⁺tri-doped GZT UC phosphors,the detail temperature sensing performances in diverse doped systems were studied systematically.The main works and consequenses are listed below:（1）A series of Er³⁺single-,Er³⁺/Yb³⁺and Tm³⁺/Yb³⁺co-,and Er³⁺/Tm³⁺/Yb³⁺tri-doped GZT UC phosphors were prepared by a solid state method.The results of XRD structure and Rietveld refinement by GSAS software indicate that GZT belongs to the double perovskite structure and the structure of main phase GZT cannot be changed by introducing Er³⁺single-,Er³⁺/Yb³⁺and Tm³⁺/Yb³⁺co-,and Er³⁺/Tm³⁺/Yb³⁺tri-doped way.The vibration peak of Gd-O band locates at 300⁶00 cm^-1,indicating the GZT host has low phonon energy.SEM and EDX energy spectrum reveal that although the morphology of prepared phosphors are inrregular and in a wide range,all chemical elements of compounds are distributed through the particles uniformly.Under 980 nm excitaion,all of Er³⁺single-,Er³⁺/Yb³⁺and Tm³⁺/Yb³⁺co-,and Er³⁺/Tm³⁺/Yb³⁺tri-doped GZT UC phosphors exhibit superior UC emissions and bright emissions of green,blue,red,cyan and white can be obtained of them.Power pump dependency,fluorescence lifetime,and the variation of fluorescence intensity ratios are used to demonstrate the UC mechanisms in the Er³⁺single-,Er³⁺/Yb³⁺and Tm³⁺/Yb³⁺co-,and Er³⁺/Tm³⁺/Yb³⁺tri-doped GZT systems.Bright and multicolor emissions reveal the Er³⁺/Tm³⁺/Yb³⁺doped GZT UC phosphors have great application potential.（2）The temperature sensing behaviors of several doping phosphors were investigated in detail owing to the obtained great UC properties and stability of Er³⁺/Yb³⁺and Tm³⁺/Yb³⁺co-,and Er³⁺/Tm³⁺/Yb³⁺tri-doped GZT UC phosphors.Based on the Fluorescence intensity ratio（FIR）technique,the maximum absolute sensitivity（S_A）of 47.5×10^-4 K^-1（473 K）is achieved on the strengh of the thermally coupled levels（TCLs）in the Er³⁺/Yb³⁺co-doped GZT phosphors.Besides,Another higher maximum S_A of 1.7351 K^-1（473 K）is received on account of the superiority of non-thermally coupled levels（N-TCLs）of Tm³⁺in the Tm³⁺/Yb³⁺co-doped GZT phosphors.Incorporating characters of Er³⁺and Tm³⁺into one part,the maximum S_A of TCLs and N-TCLs,with the values of 84.6×10^-4 K^-1（473 K）and 5.02 K^-1（473 K）respectively,are gained in the Er³⁺/Tm³⁺/Yb³⁺tri-doped GZT phosphors.Comparing the variations of S_A of TCLs(Er³⁺)and N-TCLs(Tm³⁺)and the discrepancy of UC emission intensities in the three systems mentioned above,The primary cause for large diffrences of the maximum S_A is ascribed to the energy gap and emission intensity ratios of two emission levels.High snesitivities in Er³⁺/Yb³⁺,Tm³⁺/Yb³⁺and Er³⁺/Tm³⁺/Yb³⁺doped GZT phosphors indicate the three kinds of phosphors have great potential in temperature sensing application.（3）Because the effects of energy gap and FIR ratios on the S_A are mainly restricted to the TCLs of Er³⁺,The relative magnitudes of emission intensities of emission levels are fixed with the temperature variation as the excitation and and doping concentration are comfiremed.To overcome this issue,a strategy applied to the TCLs of Er³⁺was proposed to enhance the sensitivity.The basic idea of this strategy is that the sequence of FIR is changed according to the magnitude of emission intensity of emission levels.For instance,the FIR is calculated with I₁/I₂ as the emission intensity of upper emission level（I₁）is larger than that in the lower emission level（I₂）,vice versa.By utilizing the proposed strategy,a maximum S_A of 218.8×10^-4 K^-1（313 K）of TCLs of Er³⁺is gained in the Er³⁺/Yb³⁺co-doped GZT phosphors,which outclasses the maximum S_A obtained by conventional FIR technique.Most importantly,the universality of this proposed strategy is demonstrated by detail argument in the Er³⁺/Yb³⁺co-doped NaYF₄phosphors.