Study On Defect Regulation And Luminescence Properties In Apatite-type Compounds

The luminescent properties of inorganic solid luminescent materials（commonly known as phosphors）are largely determined by point defects in solids,including intrinsic defect and impurity.Defects are inevitable in solid sand seriously affect the optical,magnetic and electrical properties of solid materials.In recent years,there have been endless reports on controllable syntheses of high efficiency luminescent materials based on defect engineering.Anion vacancies are particularly important for the regulation of self-activated luminescence,photoluminescent thermal stability,persistent luminescence and so on.However,the research on the cognition of defect microstructure and the mechanism of defect formation is not mature enough.In general,the defect concentrations related to optoelectronic properties in solids is very low（dilute limit）so that the experimental characterization technology is not enough to provide accurate defect structural information.First-principles calculation has become an important means to supplement experiments and characterize defects.Apatite-type compound A₁₀（BO₄）₆X₂（A=Ca/Sr/Ba/La;B=P/Si;X=F/Cl/Br/O）is a well-known host material for inorganic phosphors because of its excellent chemical stability,rich chemical compositions and high defect tolerance.Our previous work found that apatite materials synthesized in reducing atmosphere formed a certain amount of anion vacancy defects（V_Oand V_X）and generated defect-related self-activated luminescence or persistent luminescence.In this work,we apply density functional theory（DFT）based computational calculations.Taking the representative Ba₁₀（PO₄）₆Cl₂as an example,we propose corrections on the calculation formula of atomic potentials and anion defect formation energy in polyanion-contained oxalate compounds.Then,these corrections are used to investigate the formation rules of anion vacancy defects and defect-related luminescence mechanism in A₁₀（BO₄）₆X₂compounds.In addition,Hubbard model（DFT+U method）are applied to calculate the impurity defect formation energy,electronic structure and possible persistent luminescence mechanism of Ba₁₀（PO₄）₆Cl₂:Eu²⁺crystal.The research results provide a theoretical basis for the defect design and regulation of polyanion oxalate luminescent materials.1.Corrections on atomic chemical potentials and formation free energy calculations of anion vacancy defects in Ba₁₀（PO₄）₆Cl₂in reducing atmosphere.Anionic vacancy defects exist widely in solid materials,which depend heavily on the synthesis atmosphere and restrict the optical properties of the materials.Formation energy calculations of anion vacancy defects has been developed for years inmonoanionic compounds.However,in oxalate salts and polyanion-contained compounds,the boundary conditions of anion atomic potentials are complex,which makes it more difficult to calculate the defect formation energy.In addition,the redox reaction occurs on the surface of solids under the condition of oxidizing orreducing atmosphere.Thus,the thermodynamic process of defect formation should be considered in the calculation of defect formation energy.In this paper,taking Ba₁₀（PO₄）₆Cl₂as an example,the calculation method of polyatomic potential is modified,and the effects of synthetic atmosphere and temperature on defect formations are taken into account.The simultaneous modifications of the calculation of two kinds of anion vacancy defects(oxygen vacancy,V_O;chlorine vacancy,V_Cl)formation free energy in chlorophosphate under reducing atmosphere.Moreover,the modified calculation method proposed is used to calculate the free energy of V_Oformation of Zn O in H₂reduction atmosphere.The results show that it is in good agreement with the experimental and theoretical values in the literature,which fully verifies the rationality of the modified method.2.The formation law of anion vacancy defects in apatites in reducing atmosphere and the self-activated luminescence mechanism.Apatite-type compounds synthesized under weak reduction atmosphere will form different concentrations of anion vacancy defects,in which A₁₀（BO₄）₆X₂（A=Ca/Sr/Ba/La;B=P/Si;X=Cl/Br/O）exhibits defect-related self-activated luminescence properties.The DFT calculation is carried out by the standard Perdew-Burke-Ernzerh（PBE）using the generalized gradient approximation（GGA）density functional and implemented in Vienna Ab initio Simulation Package（VASP）.Using the corrected calculation method proposed,the change trends of V_Oand V_Xformation energies were compared.The results show that in reducing atmosphere,the anion defect formation energy of M₁₀（PO₄）₆X₂（M=Ca/Sr/Ba）solid solution decreases with the increase of temperature,and the defect formation energy of V_Ois always lower than that of V_X.Therefore,V_Odefects are dominant in alkaline earth metal halide phosphates synthesized under weak reducing conditions.The differential charge density and electronic structure analyses of two anionic vacancy defect structures in Ba₁₀（PO₄）₆Cl₂have revealed that V_Ohas a greater effect on the crystal structure and the V_Oenergy level is more localized than V_Cl,suggesting that the V_Odefect energy level is more likely to form a local luminescence center.The calculated results of charge thermal transition energy levels show that V_O^··/·acts as the emission recombination center,which is the main factor of apatite self-activated luminescence.However,in La₈M₂（Si O₄）₆F₂and La₈M₂（Si O₄）₆O₂,the actual synthesis temperature is more favorable for the formation of V_X,which is related to the rigid channel structure of X site anions and the mobility of X^-ions.Low concentration of V_Xcan promote the formation of self-activated luminescence of silicate-based apatite.3.The impurity defect formation energy and persistent luminescence mechanism of Ba₁₀（PO₄）₆Cl₂:Eu²⁺.Eu²⁺-doped Ba₁₀（PO₄）₆Cl₂phosphor has long blue persistent property of Eu²⁺.The previous work of our research group shows that the anion vacancy defects formed in reducing atmosphere act as electron traps.Apatite has two types of cation sites（4f and 6h）,and Eu²⁺-doped defects accounting for 5%of defects are constructed in 1×1×2 Ba₁₀（PO₄）₆Cl₂supercell.Comparing the defect formation energies of Eu²⁺in different sites,it is considered that Eu²⁺occupies Ba1 site most preferentially.By comparing and analyzing the charge transfer energy levels of Eu _Ba,V_Oand V_Cl,it is found that the 5d electrons of Eu²⁺thermally ionize into the conduction band,and Eu²⁺ionizes into Eu³⁺,while the free electrons in the conduction band are captured by V_Odefect energy level,and the V_O^··/·defect energy level acts as an electron trap,and the captured electrons are released and return to Eu 5d energy level,thus realizing Eu²⁺long persistent emission.