Synthesis And Luminescence Properties Of Rare Earths Doped Ca₉Sr（PO₄）₆Cl₂ And Na₂Ca₃Si₂O₈ Phosphors

White light emitting diodes（WLEDs） are considered as an ideal candidate for the replacement of conventional lighting resource because of their remarkable advantages such as long operation lifetime, high luminous efficiency,energy saving and eco-friendliness. Typically, the commercial WLEDs were assembled by a combination of a blue emitting InGaN chip and a yellow emitting Y₃Al₅O₁2:Ce³⁺(YAG:Ce³⁺) phosphor. However, this type of device suffers from inevitable drawbacks including a low color rendering index（CRI <75） and high correlated color temperature（CCT 3 4500K） due to the lack of red component in the visible region. Nowadays, in view of these problem, WLEDs fabricated by ultraviolet（UV） or near-ultraviolet（n-UV） LED chips and tricolor（red, green and blue） phosphors are expected to dominate the solid-state lighting（SSL） market in the future because the white light outputting from this type of WLEDs has controlled color temperature and exceptional color rendering index.Obviously, the performances of n-UV LEDs to a great extent, strongly depend on the luminescence performance of phosphors used. There are also somedisadvantages in the developed tricolor phosphors for n-UV LEDs, for example,the poor fit with the main emission of the n-UV LED chips, poor thermal stability, leading to the WLED has lower efficiency and luminous intensity.Therefore, the immediate work is to develop suitable tricolor phosphors with high luminescent efficiency and high stability for n-UV LEDs.Ce³⁺-doped, Ce³⁺-Tb3+ co-doped and Eu²⁺-doped Ca₉Sr（PO₄）₆Cl₂,Eu³⁺-doped Na₂Ca₃Si₂O₈ phosphors have been synthesized via a high temperature solid-state reaction method. The crystal characteristics,luminescence properties as well as the energy transfer mechanism from different ions of the phosphors are investigated in detail.（1） Ca₉Sr（PO₄）₆Cl₂:Ce³⁺ phosphors were synthesized by the high temperature solid-state reaction method. All of the profiles match well with the Standard file The Inorganic Crystal Structure Datebase ICSD 24237 for Ca₉Sr（PO₄）₆Cl₂. No detectable phase from impurity can be observed in all the as-prepared samples even though Sr²⁺, Ce³⁺, and Tb³⁺ ions were introduced into the host lattice, which indicating that pure phase of Ca₉Sr（PO₄）₆Cl₂:Ce³⁺ can be obtained at 1400 oC for 5 h. The asprepared Ca₉Sr（PO₄）₆Cl₂:Ce³⁺ phosphors exhibit a broad excitation band ranging from 220 to 385 nm and blue lightemitting band centered at 431 nm, which corresponds to the 4f-5d transitions of Ce³⁺ ion. It is observed that the PL intensity is affected by Ce³⁺concentration. With the increase of Ce³⁺ ions concentration, the emission intensity increases and reaches a maximum at 5 mol%. The energy transferbetween Ce³⁺ions occurred via the electric dipole-dipole interaction（d-d）mechanism in the Ca₉Sr（PO₄）₆Cl₂:Ce³⁺.（2） The asprepared Ca₉Sr（PO₄）₆Cl₂:Ce³⁺ phosphors exhibit a broad excitation band ranging from 220 to 385 nm and a broad emission band ranging from 360 to 700 nm, so it can be regarded as sensitizer.Ca_9-0.05-y Sr（PO₄）₆Cl₂:0.05Ce³⁺, yTb³⁺ phosphors were synthesized by the high temperature solid-state reaction method. Under 324 nm excitation, the emission spectra showed a broad blue emission at about 431 nm which corresponds to the5d-4f transition of Ce³⁺ and several green emission with a maximum at about541 nm of Tb³⁺ corresponding to the 5D4-7F5 transition. The luminescent intensities of Tb³⁺ ions were dramatically enhanced by the introduction of Ce³⁺in the Ca₉Sr（PO₄）₆Cl₂:Tb³⁺phosphors because of the efficient energy transfer from Ce³⁺toTb³⁺ions, generating tunable blue-green emission colors. The mechanism of energy transfer between Ce³⁺ and Tb³⁺ ions was demonstrated to be an electric dipole-quadrupole interaction. Moreover, the energy transfer efficiency was evaluated up to 75% based on the analysis of the emission spectra.When the temperature reach to 150℃, the emission intensity of Ca_9-0.1Sr（PO₄）₆Cl₂:0.05Ce³⁺, 0.05Tb³⁺ phosphors decreased to 57.6% compared with the room temperature, indicating that the phosphors have excellent thermal stability.（3） Ca_9-x Sr（PO₄）₆Cl₂:xEu²⁺ phosphors were synthesized by the high temperature solid-state reaction method at 1500℃ for 10 h under CO reducingatmosphere. Under 383 nm excitation, Ca₉Sr（PO₄）₆Cl₂: Eu²⁺phosphor showed a broad emission band centered at 489 nm, ranging from 420 nm to 700 nm. The concentration quenching of Eu²⁺ ions occurs when the Eu²⁺ concentration is beyond 1 mol% and the relevant quenching mechanism is dipole-dipole interaction. All of which indicate that the Ca_9-xSr（PO₄）₆Cl₂:xEu²⁺ phosphors has been evaluated as a color converter for white LEDs.（4） Eu³⁺ doped new red-emitting phosphors Na₂Ca₃Si₂O₈ were synthesized by the high temperature solid-state reaction method. The phosphors can be efficiently excited by near UV light 394 nm to realize an intense red luminescence. The calculated intensity ratio of I（5D0â†'7F2）/I（5D0â†'7F1） is 1.43,indicating that Eu³⁺ ion occupied the lattice site of noncentrosymmetric environment in Na₂Ca₃Si₂O₈. Na₂Ca_3-xSi₂O₈: xEu³⁺have excellent thermal stability. The emission intensity of the selected Na₂Ca_3-0.2Si₂O₈:0.2Eu³⁺phosphor decreased to 90.34% when temperatue reached 225℃ compared with in the room temperature.