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

ABSTRACT

The 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.

KEYWORDS:

• Co-precipitation
• BaSO₄:Eu²⁺
• molar ratio
• flux
• co-dopants
• interstitials
• point defects
• anion vacancies
• luminescence
• 2-hole traps
• high dose–response

Acknowledgements

Authors are thankful to Dr M. T. Jose and O. Annalakshmi from RSD, IGCAR, Kalpakkam for providing ⁶⁰Co gamma irradiation facility. We also thank Dr Ruby Thomas and B. Yamini of Saveetha Engineering College for providing the conductometry data. We are grateful to Dr S. Kalavathi from Materials Science Group, IGCAR, Kalpakkam for recording the XRD data of different BaSO₄:Eu powder samples synthesized in this study.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

R. Sangeetha Rani http://orcid.org/0000-0002-4764-6242

J. Nandha Gopal http://orcid.org/0000-0003-1050-3210

Bhaskar Sanyal http://orcid.org/0000-0002-2698-703X

K. N. Marimuthu http://orcid.org/0000-0003-1400-3248

Additional information

Funding

Authors are thankful to the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy, Govt. of India, for providing financial assistance to carry out this work under the research project on High Level Radiation Dosimetry sanctioned via letter No. 2013/36/47-BRNS/2422.