On the role of europium in CaSO4 : Eu phosphor

This study addresses the important influence of air thermal preannealings (500±1150 C) on the emissions of Eu 2+ and Eu 3+ ions in CaSO 4 samples doped with various Eu concentrations (10±2000 ppm) and synthesized under oxidizing conditions. The characteristics of optical excitation, optical absorption, fluorescence and cathodoluminescence spectra are given before and after the annealings. The role of UV, X-ray or electron irradiation is discussed. An irreversible conversion Eu 3+ 3 Eu 2+ is obtained beyond 600±900 C, even for low Eu concentrations (<50 ppm). There exists an intense change in the ratio of Eu 2+ /Eu 3+ emissions at about 25±100 ppm. Moreover, the present paper shows again how careful one must be with the interpretation of the presence of Eu 2+ in CaSO 4 : Eu when considering cathodoluminescence results only. Two types of Eu 2+ centres seem to be obtained in CaSO 4 : Eu 3+ by thermal preannealings and/or under ionizing (X-ray, cathode-ray) irradiations.Cette e Â tude re Â ve Á le l'importance des recuits thermiques (500±1150 C) sous air du CaSO 4 : Eu sur les e Â missions des ions Eu 2+ et Eu 3+ , pour des e Â chantillons synthe Â tise Â s dans des conditions oxydantes et pour diffe Â rents dopages compris entre 10 et 2000 ppm. Les spectres d'excitation optique, d'absorption optique, de fluorescence et de cathodoluminescence sont donne Â s avant et apre Á s recuit. Le ro Ã le des excitations UV, X ou e Â lectronique est souligne Â. On confirme une conversion irre Â versible Eu 3+ 3 Eu 2+ en fonction de la tempe Â rature du recuit (sous air), principalement dans l'intervalle 600±900 C, et cela me Ã me pour de tre Á s faibles dopages (< 50 ppm). Il existe une forte variation du rapport des e Â missions Eu 2+ /Eu 3+ autour de 25±100 ppm. De plus ce travail indique de nouveau qu'il faut e Ã tre prudent dans l'interpre Â tation de la pre Â sence d'ions Eu 2+ dans CaSO 4 : Eu au vu des seuls spectres de cathodoluminescence. Deux types de centres Eu 2+ semblent se former dans CaSO 4 : Eu 3+ apre Á s pre Â recuit thermique et/ou irradiation ionisante (X, rayonnement cathodique).

“…9). In the excitation and absorption spectra, we do find the charge transfer band at about 250±260 nm, as mentioned by Danby [13] and Dhopte et al [8], but not the f±f transitions.…”

Section: Discussionsupporting

confidence: 57%

Effect of Thermal Preannealing on Fluorescence, Thermostimulated Luminescence and Cathodoluminescence of CaSO4 Doped with Europium

Lapraz

Prévost

Baumer

et al. 2000

“…Mere presence of Eu 3+ emission (in peaks III and V) does not support model I automatically. For example, an earlier study [6] luminescence when the irradiation temperature was increased from 350 to 450 K. But the above redox model predicts an increase in Eu 2+ luminescence during the readout of peak IV at 400 K. The energy transfer model [13] is, however, more consistent with these results. Table 1 shows that the X-ray induced TSL emission spectra of Da-II also shows more or less a similar behaviour to Y-700, the only difference being that the Eu 2+ emission could not be seen at all in peaks III and V of Da-II.…”

Section: X-ray Induced Tslsupporting

confidence: 66%

“…But no TSEE emission is seen in UV irradiated samples (Fig. 7); and finally (vi) a TSEE mechanism based on the Auger-like decay of electrons rather than`Maxwell-tail' escape electrons would be consistent with the widely accepted energy transfer' [2,3] and charge transfer models [5,6] for the TSL emission in RE doped CaSO 4 phosphors after X-ray and UV irradiations, respectively.…”

Section: X-and Uv Radiation Induced Tseementioning

confidence: 59%

“…Dy and Tm) are those which emit intense TSL (model II also known as energy transfer model). Both Eu 2+ and Eu 3+ ions in CaSO 4 can be excited by the above lattice emission though not to their optimal efficiencies [5,6]. Our recent studies [16] emissions.…”

Section: X-ray Induced Tslmentioning

confidence: 88%

“…For example, Danby [2] on the basis of photoluminescence (PL) studies states that in the CaSO 4 3+ /RE 2+ redox model for the 473 K peak, one possible explanation suggested by them was the Eu 2+ /Eu + redox process for the 383 K peak. In contrast, Upadeo et al [5] and Dhopte et al [6] is negligibly small and claim that this explains the absence of ESR signal due to Eu 2+ in g-irradiated CaSO 4 : Eu. In general, no correlation between TSL and Eu 3+ 3 Eu 2+ conversion was seen by them.…”

Section: Introductionmentioning

confidence: 88%

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Thermostimulated Luminescence, Exoelectron Emission and X-Ray Induced Luminescence Studies in CaSO4 : Eu and CaSO4: Eu, Na

Lakshmanan

Tomita

1999

X‐ and UV radiation induced thermostimulated luminescence (TSL), thermostimulated exoelectron emission (TSEE) and X‐ray induced luminescence (XIL) studies in CaSO4 : Eu and CaSO4 : Eu, Na reveal that the X‐ray induced TSL peaks exhibit Eu2+ (blue) and Eu3+ (red) emissions — but their relative ratio varies with the peak. In contrast, UV induced TSL peaks exhibit only Eu3+ emission. Even in the CaSO4 : Eu sample, which contains both Eu2+ as well as Eu3+, no Eu2+ emission is seen in all the TSL peaks produced by UV irradiation. These results confirm that the X‐ray induced TSL in these phosphors is best explained by the energy transfer model while UV induced TSL is best explained by the charge transfer model. In CaSO4 : Eu, the X‐ray induced TSEE peak coincides exactly with the 400 K TSL peak whereas the UV induced TSEE is negligible. This indicates an Auger‐like decay of electrons following the self‐absorption of 400 nm Eu2+ emission due to the 400 K TSL peak. Intense XIL in CaSO4 : Eu occurs mainly due to Eu2+ emission which decreases with increasing irradiation temperature from 100 K perhaps due to thermal quenching. But the significant variation in XIL intensity in the temperature interval 350 to 450 K is attributed to Eu2+ → Eu3+ oxidation. XIL in CaSO4: Eu increases with X‐ray intensity but is nearly independent of X‐ray exposure time. These results suggest that XIL emission is probably caused by metastable Eu2+ centers formed during X‐irradiation.

Thermostimulated Luminescence, Exoelectron Emission and X-Ray Induced Luminescence Studies in CaSO4 : Eu and CaSO4: Eu, Na

Lakshmanan¹,

Tomita²

1999

X-and UV radiation induced thermostimulated luminescence (TSL), thermostimulated exoelectron emission (TSEE) and X-ray induced luminescence (XIL) studies in CaSO 4 : Eu and CaSO 4 : Eu, Na reveal that the X-ray induced TSL peaks exhibit Eu 2+ (blue) and Eu 3+ (red) emissions ± ± but their relative ratio varies with the peak. In contrast, UV induced TSL peaks exhibit only Eu 3+ emission. Even in the CaSO 4 : Eu sample, which contains both Eu 2+ as well as Eu 3+ , no Eu 2+ emission is seen in all the TSL peaks produced by UV irradiation. These results confirm that the X-ray induced TSL in these phosphors is best explained by the energy transfer model while UV induced TSL is best explained by the charge transfer model. In CaSO 4 : Eu, the X-ray induced TSEE peak coincides exactly with the 400 K TSL peak whereas the UV induced TSEE is negligible. This indicates an Auger-like decay of electrons following the self-absorption of 400 nm Eu 2+ emission due to the 400 K TSL peak. Intense XIL in CaSO 4 : Eu occurs mainly due to Eu 2+ emission which decreases with increasing irradiation temperature from 100 K perhaps due to thermal quenching. But the significant variation in XIL intensity in the temperature interval 350 to 450 K is attributed to Eu 2+ 3 Eu 3+ oxidation. XIL in CaSO 4 : Eu increases with X-ray intensity but is nearly independent of X-ray exposure time. These results suggest that XIL emission is probably caused by metastable Eu 2+ centers formed during X-irradiation.A.R. Lakshmanan and A. Tomita: TSL, Exoelectron Emission and Luminescence