UV-VUV Photoluminescence And Host Electronic Structure Of Several Typical Rare-earth Silicates

Silicates are excellent hosts for photoluminescence (PL) materials for their various crystal structures and high physical-chemical stability. Rare-earth (RE) ions are proper activators due to their abundant energy levels in ultraviolet (UV) and vacuum ultraviolet (VUV) region and various excitation paths, e.g.f-f,f-d, charge transfer (CT) transitions. In the present work, the UV-VUV PL properties of RE ions in several typical silicate hosts were systematically investigated combined with the electronic and linear optical properties calculation.The UV-VUV spectroscopy study were applied to LiYSiO₄:Eu³⁺, Ba₂Mg_1-xZn_xSi₂O₇:Eu²⁺(0≤x≤0.4),SrB₂Si₂O₈:RE(RE=Eu³⁺,Eu²⁺,Tb³⁺,Ce³⁺), Sr₂Mg_1-xZn_xSi₂O₇:Eu³⁺(0≤x≤1), CaMgSi_2xO_6+2x:Eu²⁺(1.00≤x≤1.20), Na₃(Y, Gd)Si₃O₉:RE(RE=Tb³⁺,Eu³⁺), M³⁺₂Si₂O₇:Eu³⁺(M=La,Y), SrZrSi₂O₇:Eu³⁺ and Ca₃ZrSi₃O₁₂:Eu³⁺. The excitations in UV region of these phosphors were mainly correlated directly with the activators and included O^2--Eu³⁺CT,Eu²⁺ f-d,Tb³⁺f-d,Ce³⁺f-d and etc.. In VUV region, besides the host absorption in the range of 140-190 nm, the excitations such as the high energy states of Eu³⁺f-d,Tb³⁺f-d and Ce³⁺f-d, and the O^2--Ce³⁺ CT and O^2--Tb³⁺ CT transitions were usually involved In some of the phosphors, the "abnormal" strong absorption appeared around 200 nm in the VUV excitation spectra in contrast to the UV excitation spectra. The comparing and contrasting of the UV-VUV excitation spectra of different phosphors related the "abnormal" strong absorption to the bound exciton formed near the metal cations such as Sr²⁺,Ba²⁺,Y³⁺ and La³⁺ in the hosts. The energy of the CT transition between the coordination anion (O^2-) and the central RE cations, such as the O^2--Eu³⁺ CT transition, shifted when the Eu³⁺ were located in different hosts: 233 nm in Na₃(Y,Gd)Si₃O₉:Eu³⁺, 250 nm in SrB₂Si₂O₈:Eu³⁺ and Sr₂ZnSi₂O₇:Eu³⁺, and 258 nm in Sr₂MgSi₂O₇:Eu³⁺. In some specific host such as SrB₂Si₂O₈:Eu³⁺, trivalent RE ions (e.g. Eu³⁺) can be reduced spontaneously to the divalent RE ions (e.g. Eu²⁺) even in air and the emission could be switched by different excitation wavelengths. It was also found that ion doping and non-stoichiometry affected the PL properties obviously. the substitution of Zn²⁺ for Mg²⁺ in Sr₂(Mg,Zn)Si₂O₇:Eu³⁺ enhanced the UV PL intensity and shifted the O^2--Eu³⁺ CT energy by up to 8 nm; the UV PL intensity increased with the increasing Zn²⁺ content in Ba₂Mg_1-xZn_xSi₂O₇:Eu²⁺(0≤x≤0.4); the incorporation of Gd³⁺ strengthened both the Tb³⁺ and the Eu³⁺ emissions in Na₃(Y,Gd)Si₃O₉:RE(RE = Tb³⁺, Eu³⁺); the excess SiO₂ improved the PL performance of CaMgSi₂O₆:Eu²⁺ prepared in a reducing atmosphere. In some of the phophors such as LiYSiO₄:Eu³⁺ and Ca₃ZrSi₃O₁₂:Eu³⁺, selective excitations were observed.The possibility of Na₃(Y,Gd)Si₃O₉:RE(RE=Tb³⁺,Eu³⁺), M³⁺₂Si₂O₇:Eu³⁺(M=La,Y), SrZrSi₂O₇:Eu³⁺ and Ca₃ZrSi₃O₁₂:Eu³⁺ as novel promising VUV phosphors were exploited. Among them, Na₃Y_0.45Gd_0.15Si₃O₉:0.4Tb³⁺ showed a brightness of 88% and shorter decay time compared with the commercial green phosphor Zn₂SiO₄:Mn²⁺ when excited by 147 nm. The result indicated that Na₃Y_0.45Gd_0.15Si₃O₉:0.4Tb³⁺ would be a potential VUV green phosphor. The comparison of the Eu³⁺ and Tb³⁺ PL behaviours in Na₃(Y,Gd)Si₃O₉ indicated that Tb³⁺ f-d transition in VUV region contributed significantly to the excellent PL performance of Na₃Y_0.45Gd_0.15Si₃O₉:0.4Tb³⁺. Y₂Si₂O₇:Eu³⁺ and SrZrSi₂O₇:Eu³⁺ exhibited the emission intensity of 40% and 45% and better color purity when compared with the commercial red phosphor (Y,Gd)BO₃:Eu³⁺.To further understand the PL properties of the above phosphors, the electronic and linear optical properties calculation were performed on LiYSiO₄, Ba₂MgSi₂O₇, Ba₂ZnSi₂O₇,SrB₂Si₂O₈, Sr₂MgSi₂O₇, Sr₂ZnSi₂O₇, CaMgSi₂O₆, La₂Si₂O₇ and Y₂Si₂O₇. The calculated results demonstrated that: (1) the valence band top of these silicate crystals was mainly composed of the O 2p orbibals, also formed from the Si 3p and 3s orbitals; (2) the low conduction band contained the Si 3p and 3s orbitals, also included the metallic-element orbitals such as the Sr 4d, Ba 5d, Y 4d and La Sd orbitals; (3) the transition-metal-element orbitals such as Zn 3d orbitals contributed significantly to the valence band top and contracted the band gap. The results suggest that the optical absorption of silicate hosts was mainly derived from the Si-O group related absorption, also from the electron transitions between the metallic cations such as Sr²⁺, Ba²⁺, Y³⁺, La³⁺ and Zn²⁺ and their coordination anion (O^2-).