Strong green and red emission of a newly developed calcium fluoroaluminate: Eu 3+ phosphor
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A phase of new bichromatic (green, red) phosphor Ca2Al3O6F:Eu(III), was found to grow in a small fraction along with Ca5(PO4)3F:Eu(III) phase if aluminum is previously added to the reaction mixture of the latter and flourish to a sizable concentration on subsequent heat treatment. The luminescence spectrum of the as-prepared sample, where Ca5(PO4)3F:Eu(III) is the dominant phase, shows a strong band at 612 nm along with a series of less intense bands at 573, 584, 644, and 692 nm due to different 5D0 → 7FJ (J ⳱ 0, 1, 2, 3, 4) transitions and indicates that the presence of aluminum in the system forces the Eu(III) ions to occupy exclusively one type of site rather than multiple types of sites. As the aluminate phase grows, a strong green emission band around 520 nm due to the 5D2 → 7F3 transition of Eu(III) occurs concomitant with a splitting of almost all the 5D0 → 7FJ bands. The excitation spectrum of the green emission (520 nm) shows a strong absorption band at 393 nm, and the electron spin resonance spectrum of this material shows existence of a fluorine-related hole center of (F2n)− type in the matrix. It is argued that the (F2n)− holes are localized in the interstitial of the Ca2Al3O6F phase near the calcium-substituted Eu(III) ions to maintain the charge balance and form a complex with the latter, which plays a vital role in the process of green emission.
I. INTRODUCTION
Blue, green, and red phosphors are used in color display, fluorescent lamps, and different optoelectronics. Because of higher thermal stability and better color rendering index, rare-earth-based phosphors are preferable to the conventional transition-metal ion-based phosphors for these applications. The efficiency and the nature of the emission of a rare-earth ion in a phosphor, however, varies with the variation of host compositions, method of preparation, nature of host guest interaction, phonon structure, etc.1–4,5,6 It has already been noted that the excited lanthanide ions in condensed phase do not normally cascade like gaseous atoms but undergo nonradiative de-excitation.2,7 For example, a Eu(III) ion which has several possible emitting levels in its excited state, gives radiative emission only from its lowest excited “J” state (5D0) in solids of high energy phonons like oxides, sulfides, phosphates, borates, etc. All the excited states normally decay nonradiatively to the 5D0 level, and the most common fluorescence in this case is due to 5D0 → 7F1 magnetic dipolar transition (which is almost independent of surroundings) along with weak 5 D0 → 7F2 and 5D0 → 7F4 emissions. However, the same ion in hosts of low phonon energies, such as LaCl3 and LaBr3 crystals1,2 or germanate, telluride, and fluoride glasses,7–11 shows luminescence from upper 5D3, 5D2, J. Mater. Res., Vol. 18, No. 3, Mar 2003
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and 5D1 levels in addition to the emission from 5D0 level. Thus, in a host of low phonon energy, lanthanides may exhibit emissions from different upper J levels. Formation of a charge transfer
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