R Sahoo

A new type of charge compensating mechanism in Ca5(PO4)3F:Eu3+ phosphor

Abstract The X-ray powder diffraction, reflectance, photoluminescence, photoluminescence excitation and ESR spectra of Ca 5 (PO 4 ) 3 F:Eu 3+ phosphor have been studied. Three distinct variants of calcium substitutional Eu 3+ -sites have been observed in this host and the charge compensating species related to each of these sites has been identified. It is noted that the host related trace impurities those have prospects of acting as charge compensator, and the reaction environment that exists during the preparation of the material, greatly influence the preferential substitution of different Ca 2+ -sites by the Eu 3+ ions. It is also noted that the charge compensating species in a suitable case, takes part in the photophysical process of luminescence of the Eu 3+ .

Strong green and red emission of a newly developed calcium fluoroaluminate:Eu3+ phosphor

A phase of new bichromatic (green, red) phosphor Ca 2 Ak 3 O 6 F:Eu(III), was found to grow in a small fraction along with Ca 5 (PO 4 ) 3 F:Eu(III) phase if aluminum is previously added to the reaction mixture of the latter andflourish to a sizable concentration on subsequent heat treatment. The luminescence spectrum of the as-prepared sample, where Ca 5 (PO 4 ) 3 F: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 5 D 0 → 7 F J (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 5 D 2 → 7 F 3 transition of Eu(III) occurs concomitant with a splitting of almost all the 5 D 0 → 7 F J 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 (F 2 n ) - type in the matrix. It is argued that the (F 2 n ) - holes are localized in the interstitial of the Ca 2 Al 3 O 6 F 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.

Long lived photoinduced electron-transfer in (lead, zinc)phosphate glass–C60 composites

Abstract A phenomenon of long lived photoinduced electron transfer from the Pb2+ ions of the glass structure to the incorporated C60-fullerene has been observed in a group of newly developed (lead, zinc)phosphate glass–(C60) fullerene composites upon 230–400 nm UV irradiation. Measurements of post irradiation absorption (PIA) and ESR (PIESR) spectra, of two such composites revealed the photogeneration of Pb3+ type of holes and C60 related anions in the matrix. An analysis of the carrier dynamics suggested that the photogenerated electrons in such systems migrate from the lead hole-sites to the fullerene sites in a tunneling mode.