Matrix Modification And Luminescence Properties Of Apatite-Type Structure Phosphors Based On Density Functional Theory

Natural apatite is a kind of mineral with abundant reserves in the earth.Recently,the apatite compounds have attracted wide attention and are considered as"star materials"that can be used as matrices of luminescent materials owing to their good chemical stability.The optical material host with apatite structure has tunable crystallographic lattice sites,which can provide different crystal field environment for rare earth ions,and provide conditions for studying the relationship between composition,structure and luminescence properties of materials.The apatite host doped with rare earth ion Dy³⁺can act as a single-phase white light emitting phosphor with excellent luminescence performance,and has the potential to be applied in the field of white light emitting diodes?w-LEDs?.Moreover,the luminescence electric dipole and magnetic dipole transition from Dy³⁺are closely related to the surrounding crystal field splitting effect,which can be used as a structural probe to reflect the crystal field changes.In this thesis,a series of apatite structure phosphors A₉Ln?PO₄?₅?MO₄?X₂:Dy³⁺?A=Ca,Sr,Ba;Ln=La,Y,Sc;M=Si,Ge,Sn;X=F,Cl,Br?were successfully synthesized by high temperature solid phase method.The effects of divalent cations,trivalent cations in the polyhedra,tetravalent cations in tetrahedron and channel anions on the structure and luminescence properties of these apatite structure phosphors were studied in detail through systematic characterization combined with density functional theory?DFT?calculation.The main research contents and results are as follows:1.A series of A₉La?PO₄?₅?SiO₄?F₂:Dy³⁺?A=Ca,Sr,Ba?phosphors were synthesized to study the effect of divalent cations substitution.The compositional changes,structure,luminescence properties of the apatite phosphors and the relationship between them were studied.It is found that with the substitution of Ca,Sr,Ba,the unit cell parameters of the apatite structure gradually increase,and the optical band gap gradually decreases.Since the highest occupied molecular orbital?HOMO?is mainly composed of the p-orbital electrons of the[PO₄]and[SiO₄]tetrahedra,which have no significant change,and the decrease of the optical band gap is result from the decrease of the lowest unoccupied molecular orbital?LUMO?.With the substitution of Ca,Sr and Ba,the La 5d energy level in the matrix is further decreased,indicate the crystal field around Dy³⁺,which replace for La,is further splited.As a result,the electric dipole transition of Dy³⁺ions is stronger than the magnetic dipole transition to generate the warm white light.2.A series of Ca₉Ln?PO₄?₅?SiO₄?F₂:Dy³⁺?Ln=La,Y,Sc?phosphors were synthesized by studying the substitution of trivalent cations.It can be seen that with the substitution of La,Y,Sc,the unit cell parameters of the apatite structure gradually decrease,and the optical band gap remains basically unchanged.The composition of HOMO is still mainly composed of p-orbital electrons of[PO₄]and[SiO₄]tetrahedrons.The composition of LUMO is mainly composed of d-orbitals of rare earths and 3s orbital electrons of P.The substitution of La,Y,Sc,the d level of the rare earth in the matrix has a downward trend,and the crystal field around the rare earth ion is splited obviously,and the phosphor color is warm white light.3.After[GeO₄],[SnO₄]substituted for[SiO₄]tetrahedron,the unit cell parameters of Ca₉La?PO₄?₅?MO₄?F₂:Dy³⁺?M=Si,Ge,Sn?phosphors gradually increase,and the optical band gap gradually reduces.It is found that with the substitution of Sn,Ge and Si,HOMO is closer to the Fermi surface,and the 5d energy level of La has obvious centroid offset,which makes the crystal field around the rare earth ions split,and further broadens the Dy³⁺ion excitation spectrum.The illuminating color is also warm white light.Moreover,the[SiO₄]was replaced by[GeO₄],the thermal stability of the as-prepared phosphor was significantly improved.4.After channel anion Br^-/Cl^-is substituted for F^-,the unit cell parameters of Ca₉La?PO₄?₅?SiO₄?X₂:Dy³⁺?X=F,Cl,Br?phosphors increase gradually,and the optical band gap decreases.It is found that with the substitution of Br,Cl and F,the p-electron energy levels of Cl^-and Br^-are closer to the Fermi surface than the 2p energy level of F^-,which improves HOMO,and the 5d energy level of La has obvious splited to decrease the LUMO,which further makes the earth ions surrounding crystal field split strong,and the fluorescent color of the phosphor is warm white light.