BaO-LuPreparation Of Phase Diagram Thermodynamics And Mesophase Upconversion Of 2 O 3 -SiO 2 System

The BaO–Lu₂O₃–SiO₂ ternary system are important matrix systems for the rare earth luminescent materials,which are of great interest because of the potential application of the intermediate compounds in the single-phase phosphor,upconversion luminescence materials and inorganic scintillator.However,the information of phase relationship and thermodynamics are insufficient for further design and optimization of luminescent materials.With the help of the CALPHAD technology based on experimental phase diagram and relevant thermodynamic data,the consistent thermodynamic description of the Lu₂O₃–SiO₂system and BaO–Lu₂O₃–SiO₂ ternary system have been obtained from limited experimental information.Base on the phase information of the intermediate compound Ba3Lu4O9 from the phase diagram of the BaO–Lu₂O₃ system,the color-tunable Er³⁺/Yb³⁺co-doped Ba3Lu4O9 upconversion luminescence materials were synthesized.The results can be listed as follows:The Lu₂O₃–SiO₂ system was optimized and calculated by CALPHAD technology based on available phase diagram and relevant thermodynamic data of RE₂O₃–SiO₂（RE=Lu,Yb,Y）binary systems as well as our experimental data of Lu₂O₃–SiO₂ system.The Gibbs free energy of high temperature solution was described by an ionic two-sublattice model as(Lu³⁺)P（O₂-,SiO₂0）Q.The calculated phase diagram below 1873 K is in good agreement with experimental data at 1573,1773 and 1873 K.The calculated Gibbs energies of two intermediate phases Lu2SiO5 and Lu2Si2O7,the activity of Lu₂O₃ and SiO₂ and specific heat capacities of intermediate phases are similar with experimental results of similar Y₂O₃–SiO₂system.Base on the phase relations and thermodynamics of the Ba O-Lu₂O₃,BaO-SiO₂ and Lu₂O₃-SiO₂ binary systems,the phase diagram of the BaO–Lu₂O₃–SiO₂ ternary system have been obtained by extrapolation.The isothermal section of the Ba O–Lu₂O₃–SiO₂ system at1473 K,1573 K,1673 K and vertical section of the BaO–Lu₂O₃–SiO₂ system when W（Lu₂O₃）= 0.5 are in good agreement with experimental information.According to the the phase diagram of the BaO-Lu₂O₃ binary system,the intermediate phase were obtain,meanwhile,a series of novel Er³⁺ doped and Er³⁺/Yb³⁺ co-doped Ba3Lu4O9 phosphors were synthesized by a simple high-temperature solid-state reaction method.The crystal structure and morphology of the samples were characterized by XRD and SEM analysis.Under 980 nm laser diode excitation,the green（at 537 nm and 560 nm）and red（at660 nm）UC emission were observed,which could be attributed to the（2H11/2,4S3/2）→4I15/2and4F9/2→4I15/2 transitions,respectively.The sintering temperature and doping concentration of Ba3Lu4-x-y O9: x Er³⁺,y Yb³⁺ were optimized to be x=0.1 and y=0.6 at 1550℃.The emission intensity ratio of Green/Red keep declining monotonically with increasing Er³⁺ or Yb³⁺concentration,which is due to the cross-relaxation effect and cooperative energy transfer between the two neighboring Er³⁺ ions-as well as back energy transfer from Er³⁺ to Yb³⁺ ions.Based on the pump-power dependence and UC luminescence decay curves,the energy level diagram,the possible energy transfer mechanism of Er³⁺doped and Er³⁺/Yb³⁺co-doped system were investigated in detail.In addition,according to the energy gap law,the possibilities of non-radiative transition between energy levels of Er³⁺ ions were calculated.