Electronic Structure And Persistent Luminescence Of Ce³⁺/Eu²⁺-doped(Oxy)nitride Compounds

To fulfill the demands of pratical applications in security monitoring,food safety,solar-like LED health lighting and persistent lighting,the development of new types of efficient deep-red to near-infrared luminescence materials,violet/blue-excitable efficient rare-earth luminescence materials,and effeicent persistent luminescence(PersL)materials is currently under the spotlight.Ce3+/Eu2+-doped nitrides and oxynitrides have the great potential due to their impressive structure diversity,excellent chemical stability,and unique luminescence property.Herein,we systemically study the effect of composition and structure of host material on electronic structure of the host material and doped rare-earth ion to construct the composition-structure-property correlations in Ce3+/Eu2+-doped nitrides and oxynitrides,providing powderful design tool toward new efficient luminescence material for specific applications.Meanwhile,based on electronic structure,we successfully develop new persistent phosphor ?-Sialon:Eu2+ and improve the performance of red-emitting persistent phosphor Sr2Si5N8:Eu2+,providing guideline to motivate future research on developing high performance persistent phosphor.The specific contents and main conclusions are as follows:(1)The quantitive correlation between the composition and structure of host material and the crystal field splitting of 5d level of Ce3+and Eu2+in Ce3+/Eu2+-doped nitride phosphors is revealed by anaylzing the structural and spectral data.By virtue of the statistical method,it is demonstrated that crystal field splitting is related to the shape and size of the polyhedron,while being irrelevant of the anion types.Larger coordination number leads to smaller crystal field splitting,and crystal field splitting is proportional to inverse square of the average bond length.It is verified that crystal field splitting of Ce3+ and Eu2+ is correlated by a multiplication factor 0.76.(2)The quantitive correlation between the composition and structure of host material and the centroid shift of 5d level of Ce3+ in Ce3+-doped nitride phosphors is revised,and a method to determine the coordination number of the cation in nitrides is proposed.The nitrogen which contributes more than 4%to the bond valence of the central cation is generally considered as coordination atom,while the threshold value can be slightly modified to fulfill the requirement of coordination polyhedron stability.By analyzing the structural and spectral data,it is demonstrated that the spectroscopic polarizability is linearly related to the inverse square of the average electronegativity of the cations in Ce3+-doped nitride phosphors.Combined with crystal field splitting theory,the excitation property of Ce3+-doped nitride phosphors can be directly predicted based on the composition and structure of host material,which is of guiding importance for development of Ce3+-doped nitride phosphors.(3)The qualitative correlation between the composition and structure of host material and the Stokes shift in Ce3+/Eu2+-doped inorganic phosphors is revealed through statistical analysis of the structural and spectral data.Stokes shift has a positive correlation to the effective average bond length.It decreases in going through the halogenide series from fluorides to bromides,and the chalcogenides from oxides to selenides,respectively;while it is almost irrelevant of the anion types in the same period(nitrides,oxides,and fluorides).It is verified that Stokes shift has no obvious correlation with coordination number of the cation.(4)The PersL property and electronic structure of ?-Sialon:Eu2+are first reported.Utilizing DFT calculations,it is revealed that the trap levels responsible for PersL are impurity levels induced by Si-O introducing via random substitution of Al-O for Si-N in ?-Si3N4.Through spectra analysis,it is clarified that electrons captured by the traps are directly excited from the 4f7 ground state to the CB.From this,a novel strategy to construct the HRBE schemes is proposed and applied in constructing the electronic structure of ?-Sialon to give insights into the PersL mechanism.(5)The improved PersL property of red-emitting persistent phosphor Sr2Si5N8:Eu2+is achieved through co-doping H3BO3.By analyzing PersL decay curves,it is confirmed that both B3+and O2-contribute to the optimized performance.Thermoluminescence spectrum reveals that Sr2Si5N8:Eu2+,B3+,O2-phosphor possesses larger trap density compared with the other phosphors,giving rise to their improved PersL property.