Synthesis And First-Principles Study Of Eu²⁺ Activated Phosphors

Rare earth luminescent materials are widely used in the fields of novel monitors and lighting because of their high color purity and stability.In order to pursue higher brightness and better color quality of monitors,narrow-band emitting phosphors have become a research hotspot in this field.However,the current methods for obtaining narrowband emitting rare earth phosphors lack corresponding theoretical guidance,which greatly slows down the research progress of the phosphors with narrowband emission of green and red light.In this study,four corresponding luminescent materials were synthesized by doping Eu²⁺with four kinds of Ca₂B₅O₉Cl,Ca₂BO₃Cl,Cs₂MP₂O₇（M=Ca,Sr）compounds by high-temperature solid-state method.The phase and crystal structure data and the steady-state excitation/emission spectra of the four phosphors were obtained by XRD,SEM,Rietveld structure refinement and steady-state fluorescence measurement.Based on ABINIT and VASP software,the relevant parameters of the four phosphors were calculated,and the influencing factors of the three parameters including the emission full width at half maximum（FWHM）,thermal blue shift and thermal quenching performance were further analyzed.The specific research contents are as follows:（1）Eu²⁺-doped Ca₂B₅O₉Cl and Ca₂BO₃Cl phosphors were synthesized experimentally by high-temperature solid-state method,and their phase composition,microscopic morphology and element distribution were determined by XRD and SEM tests.The steady-state fluorescence emission peaks of the two phosphors were measured at 455 nm and 560nm,respectively,and the emission full width at half maximum are 1789.6 cm^-1 and 2825.7cm^-1,respectively.In view of this difference,this study uses ABINIT software to calculate the relevant parameters of the two phosphors:the values of the effective vibrational masses（M）are 36.10 and 22.55 respectively;the values of the excitation relaxation（ΔQ）are 14.55and 9.83 respectively;the values of the average phonon frequency（Ω）are 19.95 me V and16.23 me V respectively.Combined with the configuration coordinate model（1D-CCM）,theΔQ,M andΩof Eu²⁺-doped phosphors are positively correlated with their emission half-peak width.（2）Cs₂MP₂O₇:Eu²⁺（M=Ca,Sr）phosphors were synthesized by a high-temperature solid-state method,and the phase composition,microscopic morphology and element distribution of the two phosphors were determined by XRD and SEM tests.The steady-state fluorescence emission peaks of the two phosphors are measured at 570 nm and 535 nm,respectively,and the emission full width at half maximum are 2859.7 cm^-1 and 3000.7 cm^-1,respectively.At room temperature,the Cs₂Ca P₂O₇:Eu²⁺phosphor showed concentration quenching when the Eu²⁺doping concentration was 5%,while the Cs₂SrP₂O₇:Eu²⁺phosphor did not show obvious quenching concentration.In this study,the relevant parameters of the fluorescence properties of the two were calculated by using ABINIT software:the values of the degree of distortion of the coordination environment（W）are 0.0286 and 0.0417respectively;the values of the values of the relaxation effective mass（M）are 30.44 and20.47 respectively;the values of the average effective phonon frequency（Ω）are 9.62 me V and 8.83 me V respectively.The highly symmetric Eu²⁺-doped environment,the smaller relaxation mass and phonon frequency in the 4f-5d transition the three together constitute this important reason for the narrow-band emission of Cs₂MP₂O₇:Eu²⁺phosphor.（3）The values of the emission peak blue shift of Cs₂Ca P₂O₇:Eu²⁺and Cs₂SrP₂O₇:Eu²⁺phosphors were 44 nm and 29 nm in the process of heating from 77 K to 500 K,respectively,and the luminous intensity increased to 147.90%and 131.34%,respectively.In this study,the linear expansion coefficients of the two phosphors were calculated by using VASP software to be 2.67×10^-5 K^-1 and 3.19×10^-5 K^-1,respectively,which proved that thermal expansion of the crystal is an important factor affecting the heat induced blue shift.The trap level inside the crystals of the two phosphors is the reason for their negative thermal quenching,furthermore,the structural rigidity of the Eu²⁺-doped phosphors is positively correlated with its thermal stability.