VUV-excited Luminescence Properties Of MAl₂B₂O₇: Re(M=Sr, Ba, Ca; Re=Eu～(3+), Tb～(3+), Eu～(2+))

In recent years, more attention has been paid to PDP (Plasma display panel )because of its potential market in the illumination. Special attention has been paid to the development of new and efficient VUV-excited phosphors. On the other hand, borates are of interest as host lattices for luminescent ions, because of their large band gap. Based on this consideration, it is valuable to investigate the alkaline borate family. In this paper, The VUV excitation and luminescence characteristics of MAl₂B₂O₇:Re(M = Sr, Ba, Ca; Re= Eu³⁺,Tb³⁺, Eu²⁺) have been studied.In this work, a series of powder samples of MAl₂B₂O₇:Re(M=Ca, Sr, Ba; Re= Eu³⁺,Tb³⁺, Eu²⁺)was prepared under a reducing atmosphere by solid-state reaction.. The X-ray powder diffraction data of MAl₂B₂O₇:Re are in agreement of JCPDS standard card which indicates that single-phased phosphor can be obtained in such synthesis process.The excitation spectrum of MAl₂B₂O₇: Eu³⁺ (M=Sr, Ba, Ca) are composed of two broad bands. One band in the range from 130 nm to 170 nm is attributed to the borate groups, and the other band at 230 nm is due to the charge-transfer state (CTS) of the Eu³⁺ ion. The excitation peak shifts from 167nm to 172nm when the Sr²⁺ ions are completely replaced by the Ba²⁺ and Ca²⁺ ions. It is obvious that the wavelength positions of the excitation bands depend much more on hosts. The lattice parameters of the phosphors increase and the crystal-field strength decrease with increasing of the ionic radius of Ca²⁺, Sr²⁺ and Ba²⁺.Because Eu³⁺ ions doped into CaAl₂B₂O₇ occupy centrosymmetric site, the emission of magnetic dipole transition 5D₀→7F₁ is stronger than electric dipole transition. As Eu³⁺ ions doped into MAl₂B₂O₇ (M=Sr, Ba) occupy Ba²⁺ or Sr²⁺ sites which is non-centrosymmetric site, so that the emission of electric dipole transition ⁵D₀→⁷F₂ is stronger than magnetic dipole transition in those series sample. The strong emission line at 613 nm is due to the hypersensitive electric dipole transition of ⁵D₀→⁷F₂. The weaker emission around 592 nm is due to the magnetic dipole transition ⁵D₀→⁷F₁.As for MAl₂B₂O₇:Tb³⁺ (M=Sr, Ba, Ca) the VUV excited spectra consists of one broad band in the range from 120 nm to 170 nm and the band peaking around 226 nm, and 270 nm respectively. The band (130～170nm) is also assigned to absorption of the borate groups. The bands which cover the spectral range from 160 to 270nm are due to the transitions of Tb³⁺(4f⁸→4f⁷5d ).When Ce³⁺ is doped into the samples, the absorption band of samples obviously decreased in VUV. In PL spectra of MAl₂B₂O₇:Tb³⁺ (M=Sr, Ba, Ca), there are four peaks at 487 nm, 546 nm, 589 nm and 619 nm corresponding to ⁵D₄ →⁷F_J (J=6,5,4,3) transition of Tb³⁺ respectively. Among these transitions, the green emission around 545 nm (⁵D₄→⁷F₅) is the strongest because it is a magnetic dipole allowed transition with â–³J=±1The excitation spectrum of BaAl₂B₂O₇: Eu²⁺show a high absorption band which obviously decreased in VUV. The emission spectrum of Ba_1-xSr_xA₁₂B₂O₇:Eu²⁺move to longer wavelength with increasing of Sr²⁺ mole concentration.