Influence of Rare Earth Doping on Microstructure and Luminescence Behaviour of Sodium Sulphate

Na2SO4, Na2SO4: Li, and Na2SO4: Li, Eu, Dy phosphors were prepared by using slow evaporation technique followed by subsequent calcination at 400°C for 4？h. Doping with Li+ ion stabilized the thenardite phase of host matrix, while codoping with RE3+ stabilized the phase transformation from stable thenardite to metastable mirabilite crystal structure. The microstructure and morphology were studied by using scanning electron microscopy and transmission electron microscopy. The thermoluminescence studies revealed that isovalent doping of Li+ served as a quencher and addition of codopant introduces the additional trap sites in the host matrix. The room temperature emission spectra of Li-doped, RE3+-codoped, and undoped Na2SO4 were studied under ultraviolet radiation. For pure Na2SO4 the two peaks which appeared are at 364 and 702？nm, respectively. The emission intensities of RE3+-codoped samples increase with increase in dopant concentration. 1. Introduction Alkali sulphates have been known for a long time as versatile and excellent phosphor materials. These sulphates have attracted the attention of many workers in view of their potential applications in radiation dosimetry, TV screens, cathode ray tubes, and so forth. A variety of defect centres are likely to be formed in sulphate based phosphors [1–6]. Sulphate based radiation dosimeter materials doped with rare earth (RE) ions have been extensively investigated due to their high luminescence sensitivity [7]. Significant advancements have been made in thermoluminescence (TL) and photoluminescence (PL) experiments during the last couple of decades [8–10]. Up to date, sodium sulphate is extensively investigated from the prospect of phosphor material attributed to its simple chemical composition and defect rich crystal. Na2SO4 exhibits a variety of phase transitions between its five anhydrous polymorphs (labelled I–V). The phase transformation sequence among the Na2SO4 polymorphs can be described as Na2SO4 forms two naturally occurring minerals: mirabilite (Na2SO4·10H2O) and thenardite (Na2SO4). Both are in thermodynamic equilibrium at 32°C which may be lowered to 18°C in the presence of foreign ions [11]. At room temperature, phase V (thenardite) is reported to be stable while phase III is metastable. Phases I and II are high-temperature polymorphs; however, phase II is reported to have a narrow stability zone. Phase IV is considered to be metastable and its phase relation and structure have yet to be well established [12–15]. Correcher et al. [16] observed the spectra of infrared-stimulated luminescence