Elsevier

Solid State Sciences

Volume 123, January 2022, 106777
Solid State Sciences

Thermoluminescence and electron paramagnetic resonance correlation studies in lithium silicate phosphor

https://doi.org/10.1016/j.solidstatesciences.2021.106777Get rights and content

Highlights

  • By solid-state reaction synthesis, Li2SiO3 powder phosphors obtained.

  • Prepared phosphor was well characterized using XRD, TL and EPR spectroscopy.

  • The TL glow curve of lithium silicate exhibits three peaks.

  • Defect centers were identified in γ irradiated phosphor due to O ion, and Ti3+ and F+ centers.

  • The EPR signal due to Ti3+ center is associated to the 155 °C TL peak.

Abstract

Lithium silicate phosphor, synthesized by the solid-state reaction method, displays three thermoluminescence (TL) peaks at 155 °C, 240 °C and 430 °C. Activation energy, frequency factor and kinetic order associated with the TL peaks have been determined using E-Tstop and glow curve deconvolution (GCD) methods. Electron Paramagnetic Resonance (EPR) studies have been carried out to identify the defect centers induced in the phosphor by gamma irradiation and also to find the centers responsible for the TL process in the system. The observed EPR spectrum arises from a superposition of three defect centers. One of the centers (center I) with a g-value 2.0097 is identified as the O ion. Center II with an isotropic g-value 2.0002 is assigned to a F+-type center (singly ionized oxygen vacancy). The F+ center is likely to be associated with the TL peak at 156 °C. Center III characterized by a rhombic g-tensor with g1 = 1.9831, g2 = 1.9676, and g3 = 1.9208 is identified as a Ti3+ center. The Ti3+ center relates to the low temperature TL peak at 155 °C.

Introduction

Thermoluminescence (TL) properties of a material depend on the genesis of the system or on the synthesis process and on the impurity contents which require a details phenomenological study of TL for each material. Therefore, a study of the TL properties and elucidation of defect centers responsible for TL emission of each material is essential to determine the TL parameters and to understand the mechanism of charge capture and recombination leading to light emission process during TL readout. This understanding allows us to establish the suitability of a material for a given application like for example, radiation dosimetry.

Silicates are a class of materials that are being widely investigated as dosimeters using the TL technique [[1], [2], [3], [4], [5], [6]]. Pure and doped lithium silicates have been synthesized using different techniques like sol-gel [7], combustion synthesis [8] and solid-state reaction [9]. These materials have physicochemical stability at high temperatures, compatibility with other types of structural materials, radiation stability and adequate heat transfer. The main feature is their high sensitivity to ionizing radiation for a wide energy range [[10], [11], [12], [13], [14], [15]]. Also, Li2SiO3 (Zeff = 10.5) and Li4SiO4 (Zeff = 9.04), being low Z materials, may find application in the field of radiation dosimetry.

Cruz et al. [13] reported a detailed study on the thermal stability and crystal structure of lithium silicate. Li2SiO3 crystallizes in orthorhombic structure having space group Cmc21 with lattice parameters a = 9.392 Å (2), b = 5.397 Å (2) and c = 4.660 Å (1). The lithium atom is tetrahedrally coordinated with a mean Li–O distance of 2 Å [16]. On the other hand, Li4SiO4 at room temperature is monoclinic; with a space group P2/m and lattice parameters a = 11.546 Å, b = 6.090 Å and c = 16.645 Å [17,18].

Nur et al. [14] reported that lithium silicate synthesized from natural amethyst quartz is a potential phosphor for applications using TL. Barve et al. [15] also indicate that Cu-doped lithium silicate exhibits interesting TL and OSL (optically stimulated luminescence) properties. However, there is still no consensus on understanding of the detailed mechanism of TL in these materials. It is known that the emission is due to energy storage in traps, derived from defects in the crystal lattice or even in ions with high electronic affinity in this class of materials.

Electron paramagnetic resonance (EPR) is one of the most informative methods for the identification of defects in ionic crystals [19]. EPR can detect unpaired charges and in the TL phenomenon, recombination of some of these charges with the luminescent centers leads to the observed thermoluminescence. The results obtained by EPR can be correlated with those obtained by TL. As a result, the defect centers that are responsible for the TL emission can be identified.

In the present study, EPR technique has been used to identify the defect centers induced by gamma irradiation in pure lithium silicate (LSO) that has been synthesized by the solid-state reaction method. Further, an attempt has been made to understand the TL process in LSO in terms of the observed defect centers.

Section snippets

Experimental

Lithium silicate (LSO) phosphor was synthesized by the solid-state reaction method. Lithium carbonate (Li2CO3, Aldrich Chemicals, 99.0%) and silicon dioxide (SiO2, Aldrich Chemicals, 99.0%) were mixed in their stoichiometric ratio according to the Li2SiO3 chemical formula. The stoichiometric mixture of these powders was thoroughly homogenized in a mill using alumina spheres for 4 h and then transferred to alumina crucibles. The homogenized mixture was heated in air at 900 °C for 10 h in a

Results and discussion

The diffractogram of the LSO synthetic material is shown in Fig. 1. The sample diffractogram was analyzed using the X'Pert HighScore Plus program version 3.0 for phase identification. The results of XRD characterization show predominant presence of lithium metasilicate, Li2SiO3, (ICDD reference code, 00-029-0829) and at the same time, a little amount of lithium orthosilicate, Li4SiO4, (ICDD reference code, 00-020-0637) was also detected. Performing the Rietveld refinement, using the program

Conclusions

Lithium silicate phosphor sample was prepared via solid-state reaction process. XRD studies confirmed the predominant formation of lithium metasilicate (Li2SiO3) and a little amount of lithium orthosilicate, Li4SiO4.

Lithium silicate phosphor sample exhibits TL glow peaks at about 155 °C, 240 °C and 430 °C. The TL intensity as function of dose in log-log representation of the first two peaks (155 °C and 240 °C) has shown that their TL intensity grows linearly in the dose range of 1 – 100 Gy.

Credit author statement

S.C. Aynaya-Cahui: Data curation, Formal analysis, Investigation, Methodology. N.F. Cano: Conceptualization, Formal analysis, Funding acquisition, Methodology, Supervision, Validation, Visualization, Writing – original draft. A.H. Lopez-Gonzales: Conceptualization, Formal analysis, Investigation, Methodology, Visualization. T.K. Gundu Rao: Formal analysis, Investigation, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing. M.B. Gomes: Formal analysis,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to express thanks to Ms. E. Somessari from the Institute for Energy and Nuclear Researches (IPEN), Brazil, for kindly carrying out the γ irradiation of the samples. This work was supported by CONCYTEC-FONDECYT, Peru, in the framework of the call E038-01 (Process number 037–2019).

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