Role of flux and co-dopants on the luminescent properties of BaSO4:Eu2+ phosphor synthesized by co-precipitation route

Abstract

ABSTRACTThe thermoluminescent (TL) and photoluminescent (PL) sensitivities of BaSO4:Eu2+ synthesized by co-precipitation technique in aqueous medium were found to enhance drastically with Na2SO4/BaCl2 molar ratio. The above result reported for the first time is explained on the basis of stabilization of defects and Eu impurity by the synergistic effect of combined flux incorporated during co-precipitation and subsequent sintering at high temperatures in reduced atmosphere. The PL sensitivity of BaSO4:Eu2+ made with Na2SO4/BaCl2 ratio of 3 was 40% of the commercial phosphor CaMgSi2O6:Eu2+. In the absence of flux, Eu could not be incorporated into BaSO4 host lattice. The increased conversion efficiency of EuCl3 to Eu2(SO4)3 with increased Na2SO4 content could also be a part of the above stabilization process. Such a synergistic action giving rise to a white colored phosphor powder with high luminescence efficiency could not be achieved by solid-state diffusion reactions involving physical mixtures of BaSO4 and Eu2O3 along with different flux materials. XRD data revealed that the precipitate phase obtained is always BaSO4 irrespective of the Na2SO4/BaCl2 molar ratio. This shows that point defects created during co-precipitation of impurities provided by the flux materials rather than the crystal structure itself influence the luminescence properties of BaSO4:Eu2+. Incorporation of Na+ at Ba2+ sites or the incorporation of interstitial ions would create anion vacancies which could act as electron traps. Oxygen or chlorine interstitial ions could act as hole traps. Increased TL with Na2SO4/BaCl2 molar ratio could occur due to increased number of such electron and hole traps. Though conductivity measurements indicate that precipitation reaction is complete at a molar ratio of Na2SO4/BaCl2 = 1, weight measurements showed for the first time that the quantity of BaSO4 precipitated is less than the expected value at molar ratios <1 and more than the expected value at molar ratios >1. This could be attributed to the instability of xBaSO4·yBaCl2 mixed crystal formed in aqueous medium at low molar ratios and incorporation Na2SO4 into the BaSO4 host forming a stable 0.84BaSO4·0.16Na2SO4 mixed crystal at the molar ratio of 3. BaSO4:Eu2+ exhibits a major TL peak at 235°C with shoulder peaks at 195°C and 290°C. The hole traps corresponding to these peaks were tentatively assigned to 2-hole interstitial traps (either or ) associated, respectively, with or Ba2+ ions and to Cl− associated with Na+ ions. The TL response of 195°C peak increased linearly with gamma dose in the high-dose region while other two peaks increased sublinearly. As a result, the integrated glow curve area increased sublinearly before saturating around 3 kGy.