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

Abstract

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  tude re  ve Á le l'importance des recuits thermiques (500±1150 C) sous air du CaSO 4 : Eu sur les e  missions des ions Eu 2+ et Eu 3+ , pour des e  chantillons synthe  tise  s dans des conditions oxydantes et pour diffe  rents dopages compris entre 10 et 2000 ppm. Les spectres d'excitation optique, d'absorption optique, de fluorescence et de cathodoluminescence sont donne  s avant et apre Á s recuit. Le ro à le des excitations UV, X ou e  lectronique est souligne Â. On confirme une conversion irre  versible Eu 3+ 3 Eu 2+ en fonction de la tempe  rature du recuit (sous air), principalement dans l'intervalle 600±900 C, et cela me à me pour de tre Á s faibles dopages (< 50 ppm). Il existe une forte variation du rapport des e  missions Eu 2+ /Eu 3+ autour de 25±100 ppm. De plus ce travail indique de nouveau qu'il faut e à tre prudent dans l'interpre  tation de la pre  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  recuit thermique et/ou irradiation ionisante (X, rayonnement cathodique).

Synthesis and properties of submicron range CaSO4:Eu particles

“…Later, Moharil et al [11,12] gave recipe for preparing CaSO 4 :Eu phosphor with Eu in predominantly Eu 2+ or Eu 3+ form and used these phosphors for establishing the mechanism of thermoluminescence in CaSO 4 :RE systems [13,14]. Recently, Lapraz et al [15] have also presented the work on these aspects. They also reported cathodoluminescence in CaSO 4 :Eu.…”

TL and PL in BaSr(SO4)2:Eu mixed sulphate

“…A small amount of desulphatation observed (after the departure of oxygen) is an indication of the occurrence of such a reaction. We strongly feel that at lower concentrations (as low as 0.1 mol%), formation of Eu-aggregates is unlikely as these ions are substitutes for Ba 2+ and Sr 2+ sites; therefore, in our system, the explanation given by Lapraz et al [30] seems to be more appropriate. Moreover, in the present mixed-sulphate system it could also be more complicated and related to the incorporation of the Eu ions of two different sizes corresponding to two different ionic states at various Ba 2+ and Sr 2+ sites as described earlier.…”

“…Therefore while the impurity ions are becoming incorporated in two different states they may enter into the lattice substitutionally as a good fit for their sizes; if they do not do so during preparation, they may do so by annealing, thus creating a favourable environment for producing sufficient number of traps and giving rise to more TL intensity. At higher temperatures, of course, the reasons may be those stated earlier [30]. However, more experiments, such TL emission spectra and electron spin resonance measurements, are needed for a more in-depth study.…”

“…The annealing temperature effect has also been found to occur in some sulphate systems. Nair et al [20] have attributed such an annealing effect to the dissociation of "Eu-aggregates" to give Eu 3+ and Eu 2+ on annealing, while Lapraz et al [30] have concluded from their luminescence studies that at least two configurations of Eu 3+ occupy Ca 2+ positions in CaSO 4 :Eu, some with local charge compensation and others with neighbouring charge compensation. In the latter case interstitial O i ions, due to the method of preparation in strongly oxidizing atmosphere, play an important role and the RE 3+ ion substitutes for Ca 2+ ions without any local charge compensation, and would not be modified by thermal annealing treatments.…”

Ce3+ emission in hexagonal and orthorhombic phases of CaSO4

“…Of the several sulfates, rare earth-doped CaSO 4 is studied extensively for the possible application as phosphor for photoluminescence crystal liquid display (PLLCD), optical storage material and cathodoluminescent material [3][4][5][6][7]. Several studies have been made with regard to control of particle size and morphology of CaSO 4 [8,9].…”