Synthesis and luminescence properties of double-perovskite SrLa(1-x)LnxMgTaO6 (Ln=Eu, Dy) phosphors for white LED

Abstract

The work described in this thesis explores the structures and luminescence properties of perovskite-type SrLa(1-x)LnxMgTaO6 (Ln = Eu, Dy) phosphors. The phosphors were synthesized by two steps modified solid state method. Phase purity and formation of phosphor were confirmed by XRD technique. The XRD patterns of perovskite-type SrLa(1-x)LnxMgTaO6 (Ln = Eu, Dy) phosphors that were not found in the JCPDS files, but we could obtain from the Atom Work Database at the National Institute for Materials Science (NIMS). Photoluminescence spectra of Eu3+ and Dy3+ doped SrLaMgTaO6 phosphors showed intense emission peaks in purity red and white region, respectively. According to two different activator ions (Ln = Eu, Dy), Eu3+-doped SrLaMgTaO6 could be excited by the near UV (350-400 nm) and blue lights, and this phosphor exhibited intense and narrow-band red emission that can improve the color qualities of warm LEDs. Due to the Eu3+ ions being an intrinsically asymmetric cation environment, the dominant emission band emitted from the transition 5D0→7F2 is extraordinarily stronger than that of 5D0→7F1, the ratio I0-2 / I0-1 was about 8.67. And owing to the most Eu3+ ions locating at same crystal field environment, a full-width at half-maximum (FWHM) of this band was very narrow (6.49 nm) at room temperature. Energy transfer between Eu3+ ions were demonstrated from dipole-dipole interaction, and the critical distance was estimated to be 9.31 Å. When Eu3+ ions’ concentration reaches 30%, the emission intensity is observed about three times higher than that of a commercial red phosphor Y2O3: Eu3+, and the Commission International de L’Eclairage chromaticity coordinates was calculated at (0.66, 0.33). Dy3+-doped SrLaMgTaO6 also could be excited by the near UV and exhibited blue (484 nm), yellow (577 nm) and red (648 nm) emission corresponding to 4F9/2–6H15/2, 4F9/2–6H13/2 and 4F9/2–6H11/2 transitions, respectively. Additionally, the spectral overlap happened between excitation spectra of SrLa0.93MgTaO6: 0.07Dy3+ and the emission spectra of SrLaMgTaO6 host, this fact implied that there existed energy transfer from host lattices (group TaO6) to doped Dy3+ ions in the compound. So the group TaO6 can be a sensitizer and it can effectively transfer energy to activator ion. Due to Dy3+ ions being an intrinsically asymmetric cation environment, a similar phenomenon can be found in Dy3+, the transition of 4F9/2-6H13/2 is much strong than 4F9/2-6H15/2, the ratio of I(4F9/2→6H13/2) to I(4F9/2→6H15/2) was about 8.6. Energy transfer between Dy3+ ions were also demonstrated from dipole-dipole interaction, and the critical distance was estimated to be 15.04 Å. The intensities increased with increasing of Dy3+ concentration in the range of 0.01 < x < 0.07, and then decreased in the x > 0.07 due to the conventional concentration quenching. And both cool and warm white light could be obtained from the double perovskite phosphor with excited at 354 nm and 390 nm, which matched well with the output wavelength of near-UV chips used phosphor-converted white LEDs.