Radhaballabh Debnath

Site-dependent luminescence of Cu+ ions in silica glass

Absorption and luminescence spectra for interconfigurational 3d10 ↔3d94s transitions of Cu+ ions in a glass of near-pure silica have been studied. The observed dual luminescence characteristics (≈ 433 and ≈ 490 nm) and their excitation dependence have been ascribed to two different Cu+ ion sites in the glass. Correlation of the two excitation spectra with the absorption spectrum allowed us to identify the oxygen coordination of the individual sites as tetragonal (≈ 490 nm) and cubic (≈ 433 nm) in behavior. The long lifetime and bi-exponential behaviour of each emitting state indicated that the emitting states are triplets (3Eg, 3T2g) each of which is split by spin-orbit interactions.

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+ .

Inhibiting effect of AlPO4 and SiO2 on the anatase → rutile transformation reaction : an x-ray and laser Raman study

The anatase —> rutile transformation reaction of TiO2 prepared in contact with amorphous aluminum phosphate and with fume silica has been investigated as a function of temperature by x-ray diffraction methods. It is observed that the anatase —> rutile transformation in such systems becomes greatly inhibited and the Raman spectra of titania on such surfaces exhibit features that indicate substrate-TiO2 interfacial reactions.

On the excitation of the 3E luminescent state of Cu+ ions in glass

Abstract Experimental decay times of the emitting state of Cu+ ions have been compared with the value calculated from the UV absorption in the case of two Cu+-containing glasses. In view of the observed discrepancies, it is argued that, as in crystals, in copper (I) doped glasses the absorption and emission occur respectively through singlet→singlet and triplet→singlet transitions.

Dual luminescence of Cu+ in glass

Absorption, emission and excitation spectra of Cu+ ((2.528 ± 0.25) × 1018 ions/cm3) in a glass of near pure silica (96%) composition have been measured. It is suggested that the spectral data are consistent with Cu+ ions in octahedral coordination of oxygen in the silica network. The two observed emitting states (∼ 450 nm and ∼ 490 nm), each of which has been characterized as triplet (3Eg), are ascribed to the existence of two relatively predominant slightly different copper(I) sites with the same coordination. The bi-exponential nature of each emitting state on the other hand is assumed to be the result of splitting of the triplets due to spin-orbit interaction.

Surface-bound titania-induced selective growth and stabilization of tridymite aluminum phosphate

The crystallization behavior of amorphous aluminum phosphate (AlPO4) has been studied by X-ray diffraction as a function of the treatment temperature (500–1150°C) in the presence of small amounts of titania (TiO2) anchored onto its surface. The compound, which like silica can exist in several possible crystalline modifications depending on the treatment temperature, is found to grow in the presence of TiO2 exclusively in a single phase in the entire range of temperatures studied. The phase has been shown to have the tridymite structure belonging to the orthorhombic system with cell parameters a = 9.638 ± 0.0019, b = 8.664 ± 0.0017, and c = 18.280 ± 0036A.

Thermally reversible γ-ray-induced redox reaction between substitutional iron and aluminum impurity centers in a silica glass

The magnetic properties of the substitutional iron and aluminum impurity centers in a sintered Vycor silica glass were studied before and after 1.1-1.3 MeV gamma irradiation. Observation of two overlapping spin resonances (g similar to 4.20-4.28) in the spectra of both the irradiated and preirradiated glasses indicated the existence of two types of tetra coordinated substitutional iron centers of the [FeO(4)(-)/Na(+)](0) type. The intensity of these electron-paramagnetic resonance (EPR) signals decreased upon gamma irradiation of the glass with concomitant generation of aluminum hole center [AlO(4)](0), which was manifested by the occurrence of a new six-line EPR signal with g = 2.009, while thermal annealing of these aluminum oxygen hole centers restores the intensity of the iron centers almost to their preirradiation level. This result suggests that if not the whole, a major fraction of the electrons released in the process of gamma -ray-induced hole trapping at the Al site are captured by the substitutional iron centers. The electron traps, thus formed, are quite stable and can be deactivated by thermal stimulation.

Control of phase formation of AlPO4 with titania

Abstract Polymorphous AlPO 4 has been found to grow exclusively in a single phase (tridymite) over a temperature range 500–1150°C in the presence of titania (TiO 2 ) anchored onto its surface. A solid state epitaxial type of effect has been proposed to be active in the process.

Site-dependent thermoluminescence of copper(I) ions in silica glass

Similar to our earlier observations of site-dependent photoluminescence of Cu+ ions in a silica glass a phenomenon of site-sensitive copper(I)-related thermoluminescence has been observed in the glass after gamma -irradiation. The results have been interpreted in terms of a process of site-to-site direct recombination of different copper-associated aluminium hole centres with their respective electrons.

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.