O/N Modulation Or Ce³⁺ Or Eu²⁺ Activates Y-Si-Al-ON And Ba(Ca)-Si-ON System Nitrogen Oxide Structure And Luminous Performance

Recently,oxy-nitride phosphors have been widely attracted researchers’attentions due to their abundant structures,chemical stabilities,and excellent luminescence et.al.As a key part to pc-LED,its property have directly influence on the performance of white LED.In this thesis,Y-Si-Al-O-N and Ba（Ca）-Si-O-N series of oxy-nitrides were synthesized by solid state reactions,w-hich were designed through the chemical unit co-substitution and combination strategies based on phase diagram,such as[Si⁴⁺-N³⁺]→[Al³⁺-O²⁺]and 2N³⁺→3O²⁺.Structures of these oxy-nitrides were investigated by X-ray Powder diffraction,Neutron diffraction and Electron diffraction techniques.Anions occupancies and orders of O and N in these structures were carefully analysised.Development of this research work would have a promoting effect on understanding the dependence of structural and luminescence among these phases.Luminescence properties of Ce³⁺ or Eu²⁺ doped samples of the studied phases were executed.Both activators show broad band emissions in the UV to visible range,which are observed and attributed to 5d → 4f transitions.The excitation states of 5d orbitals are strongly depend on the coordination environment in the host lattices.Thus,emission and excitation spectra of phosphors could be achieved by tuning the chemical composition among matrixes compositions.Due to the larger electronegativity of N（3.07）,comparing with O（3.61）,there should be stronger Ce³⁺/Eu²⁺-N covalent bond and large nephelauxetic effect by comparison with Ce³⁺/Eu²⁺-O.Moreover,N³⁺has a higher formal charge（-3）than that of O²⁺（-2）,which could lead to a larger crystal-field splitting of 5d orbital for Ce³⁺ and Eu²⁺.Both two factors are attributed to the lowest 5d excitation band of Ce³⁺ or Eu²⁺ shift to lower energy and make oxy-nitride phosphors suitable for UV-or Blue-LED.The main results were concluded as follows:（1）According to Y₄Al₂O₉-Y₄Si₂O₇N₂ binary phase diagram,Y₄Al_2-2xSi-xO_9-2xN_2x（0.00 ≤ x ≤ 1.00）oxy-nitrides were designed by module substitution method with([Si⁴⁺-N³⁺]→[Al³⁺-O²⁺])and synthesized by high temperature solid state reactions.The phase relations with N contents in the binary system was studied.The unit cell parameters were acquired by Rietveld refinement against XRD data.A jump for unit cellβ was observed when x changed from 0.40 to 0.50,which also caused abrupt of the cell volume.Thus,we divided the binary system into two parts with one for x = 0.00-0.40and the other for x = 0.50-1.00.（2）The structure of middle phase Y₄AlSiO₈N（x = 0.50）was analysized by powder X-ray and neutron diffraction data as well as electron diffraction data.The occupancies and orderings of O and N atoms in the structure were specially determined by Rietveld refinement of its neutron data.The structural framework of Y₄AlSiO₈N could be considered as[（Si,Al）₂（O,N）₇]bitetrahedron units were alternatively packed along c direction.N atom was not preferential to occupy the bridge site.Only half of N located at the bridging site,and the other N located at the terminal site of the Si（O,N）₄ tetrahedron.（3）The luminescent properties of the Y₄Al（2-2x）Si_2xO_9-2xN_2x:0.01Ce³⁺（0.00 ≤ x ≤1.00）solid state solutions were investigated.In the solid solution I（0.00 ≤ x ≤ 0.04）,the excitation spectra of samples in these areas are in the range of 255nm to 330nm whose maximum peak located at 290nm.Correspondingly,the emission spectra of Ce³⁺ is in the UV to the blue range from 310～450nm with maximum at 350 nm.In the solid solution Ⅱ（0.50 ≤ x ≤ 1.00）,both the emission and excitation spectra of Ce³⁺ had red-shifts.It turned out that the maximum peak site of excitation spectra extended to 450nm and the emission spectra shown blue-green emission from 400nm to 600nm,which resulted from more N atoms were involved in the coordination environment.（4）Two oxy-nitride phases Ba₃Si₆O₉N₄ and Ba₃Si₆O12N₂ were prepared by solid state reactions,which based on "equal charge" substitution method.The crystal structure refinements were carried out by the Rietveld method based on both the X-ray and neutron powder diffraction data.Structure of these two oxy-nitrides are constructed by the layered framework of[Si₆O_12-3xN_2+2x]c^6-,which are linked by corner-sharing and packed along c direction.The Ba²⁺ cations are located between the layers.The results indicated that O atoms placed at bridging sites and terminal sites of tetrahedral,N atoms sited at the triple-link site.Ba₃Si₆O12N₂ could be transfereed to Ba₃Si₆O₉N₄ when 302-were replaced by 2N³⁺,which meant the N（SiO3）3 groups in Ba₃Si₆O12N₂ were broken and new tetrahedrons were rearranged to form the N（SiO2N）3 groups leading to the formation of Ba₃Si₆O₉N₄.The luminescent properties of Ce³⁺/Li+ or Eu²⁺ doped Ba₃Si₆O₉N₄ and Ba₃Si₆Oi2N₂ were investigated,respectively.Especially,we studied the energy transfer processes occurring among different Eu²⁺ sites in Ba₃Si₆O₉N₄:Eu²⁺phosphors and assigned the luminescent centers based on their coordination environments.（5）We found Ca₃Si₄O₂N₆ oxy-nitride by "chemical combination" approach,which meant a large number of samples were prepared by traditional solid state method.All of these samples were identified,compared（ICSD card as reference）and selected.Finally,Ca₃Si₄O₂N₆ were acquired and its structure was determined by the Rietveld structure based on the X-ray powder diffraction and electron diffraction.Ca₃Si₄O₂N₆ belong to cubic systems,space group is P a-2（No.205）and lattice constant a = 15.4636（3）A.The luminescence properties of Ce³⁺ and Eu²⁺ doped Ca₃Si₄O₂N₆ were investigated.The results are as follows:the excitation spectra of Ca2-2xO14O2N6:xLi+,xCe³⁺ samples is a wide band,which changes from 200nm to 480nm.Its corresponding emission spectra varied from blue light regions to green light regions by tuning the concentration of Ce³⁺/Li+ when they were excited by 350nm.For Eu²⁺ doped Ca₃Si₄O₂N₆ samples,the excitation spectra of them are in the range of 200nm to 500nm.And their emission spectra are asymmetric band from 450nm to 700nm when they are excited by 360nm.The emission spectra varied from green light areas to yellow light areas at the different Eu²⁺ concentration.Both of these phosphors matched well with near-fare LED chip,which could be applied in white LED.