Luminescence of Ce3+ and Li+ co-doped MgO synthesized using solid-state reaction method

https://doi.org/10.1016/j.nimb.2021.07.012Get rights and content

Highlights

  • The synthesis of MgO phosphors co-doped with Ce3+ and Li1+ using the solid-state reaction.

  • The compositional analysis and crystal structure analyzes of MgO:Ce,Li.

  • RL, PL and TL studies of MgO:Ce,Li phosphors.

  • Determination of kinetic parameters associated with trap levels.

Abstract

MgO:Ce3+,Li+ was synthesized by the solid-state reaction (SSR) method and its luminescence properties were investigated. The X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images were used for the structure analysis. The luminescence properties were studied using thermoluminescence (TL), photoluminescence (PL), and radioluminescence (RL) techniques. The step annealing experiment was conducted to find the preheating temperature. The dose–response features as a function of β-ray exposure extended from 0.2 Gy to 2 kGy were investigated. The kinetic parameters such as the activation energy (Ea), frequency factors, and order of kinetics of MgO:Ce3+,Li+ were found using the computerized glow curve deconvolution (CGCD), variable heating rate (VHR) methods, and Tm-Tstop method in combination with the Initial Rise (IR) method. The experimental results of the TL, PL, and RL analyses can serve for a better understanding of the luminescent mechanism in Ce3+ and Li+ activated MgO for the potential use of designing phosphor as a dosimeter and optical ceramic material.

Introduction

Magnesium oxide (MgO) is one of the most remarkable materials among oxides. It was first introduced as a thermoluminescent phosphor by Thomas et al. in 1964 [1] and has been continuously researched and developed by many researchers due to its near tissue-equivalence (Zeff = 10.8) [2], wide bandgap (Eg = 7.8 eV) [3], high chemical stability (melting point = 2800 °C), cubic lattice structure, and easy production using many chemical methods such as sol–gel [4], precipitation [5], solution combustion [6], [7]. The presence of various types of defects such as oxygen vacancies, F-centers/F+-centers, and surface defects causes localized energy levels in the wide bandgap with improved luminescent properties [8].

MgO has been an interesting material for luminescence dosimetry for many years. The first interest was UV and neutron dosimetry. TL properties of pure and lanthanide doped MgO possess important dosimetric features and have been extensively reported since the 1970 s. Having no fixed pattern to TL properties for reasons such as sample sources, pre-exposure annealing, exposure followed by annealing, and thermal behavior led to the rejection of this material for dosimetry applications at the beginning. Later, Bos et al. described the dosimetric properties of Tb3+ doped MgO phosphors and ignited an increasing interest in MgO [9]. A thermally stimulated emission spectrum of Tb3+ doped MgO exhibited the characteristic lines of Tb3+ in a broadband emission. The TL sensitivity of the main TL glow peak at 315 °C was found as 1.7 times higher than that of the TL of Al2O3:C. However, the OSL sensitivity of the phosphor did not possess the same performance as that of Al2O3:C. McKeever et al. have reviewed some of the earlier results on MgO dosimetry combining a good background knowledge [10].

In the last decade, lanthanides with well-known luminescence properties and lifetimes have been used as dopants for MgO phosphor [2], [11], [12], [13], [14], [15]. Designing a new TL (or optically stimulated luminescence-OSL) material with optional features for distinguishing utilization demands, a clear understanding of the role of defects on TL or OSL mechanisms is necessary. It is known that the most widely used commercial TL and OSL dosimeters were developed after trying several different methods until the method works well or by luck rather than deliberate construction according to a theoretical model. However, the emergence of an experimental model for predicting the energy level position of lanthanides and understanding of the charge trapping and release mechanisms changed this picture. Lanthanide-doped MgO is of current interest due to the unique properties of the various 4f ions and their potential applications as new optical materials [13], [15]. The influence of activator type and the amount on luminescence and structural properties of MgO samples were investigated by different researchers [16], [17], [21], [22]. In addition to the lanthanide doping, some other transition metal dopants (notably Fe, Cr, Mn, Al, Ti, Ni, V) intentionally incorporated into the crystal structure have also been proposed for obtaining more intense luminescence [16], [17], [18].

The solid-state reaction (SSR) method has been rarely applied to MgO in the literature. In this work, the SSR procedure, for MgO phosphor doped with a versatile rare earth Ce3+ and alkali metal Li+ ions was conducted. The structural properties of the phosphor were determined using XRD and SEM. The samples were characterized using TL, photoluminescence (PL) and RL techniques. Some main dosimetric characteristics (sensitivity, dose–response and energy storage) were measured, and the kinetic parameters of MgO:Ce,Li were determined.

Section snippets

Materials and method

A solid-state reaction process was used to prepare Ce3+ and Li+ co-doped MgO phosphors. Analytical grade reagents MgO (Sigma Aldrich 98%), Li2CO3 (Sigma Aldrich 99%), and CeO2 (Sigma Aldrich 98%) were used as the starting materials. Stoichiometric amounts of the starting materials were well-grounded together using an agate mortar and pestle to form a homogeneous solid solution. Temporary defects and charges, which may cause triboluminescence, can occur during grinding. To minimize any

Compositional analysis and crystal structure

The XRD spectra of the as-prepared sample are given in Fig. 1. The sharp peaks related to the MgO cubic structure of the periclase phase with the Fm-3 m (2 2 5) space group are visible and match well with the JCPDS card number 45–0946. Some Ce and Li related impurity peaks in the spectra show that a significant amount of the dopants is not incorporated into the MgO lattice. These peaks matched with the peaks observed in JCPDS #15–0401 for Li and 31–0325 for Ce (metallic). The sharpness of the

Conclusions

In the present study, Ce3+ and Li+ co-doped MgO phosphors were synthesized using the solid-state reaction technique, and their luminescence properties were investigated. The MgO:Ce,Li phosphor was determined as polycrystalline with a cubic structure of periclase phase with Fm-3 m space group. It was observed that MgO:Ce3+ (1 mol%), Li+ (1 mol%) phosphor possess considerably high PL, RL and TL intensities when compared to the other studied concentrations. The samples were characterized with the

CRediT authorship contribution statement

V. Guckan: . : Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Visualization. S.W. Bokhari: Validation, Resources, Writing – original draft. V. Altunal: Conceptualization, Methodology, Investigation. A. Ozdemir: Investigation, Methodology. W. Gao: Resources, Writing - review & editing. Z. Yegingil: Conceptualization, Writing - review & editing, Visualization, Project administration, Funding acquisition, Supervision.

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.

Acknowledgments

The authors would like to acknowledge the NATO SPS MYP for financially supporting this research under project contract number G5647. This research has also been supported by the Cukurova University Research Projects Unit through the contract numbers FDK-2018-10599, FAY-2020-13089, and FBA-2020-13126. One of the authors has been granted by TÜBİTAK (The Scientific and Technological Research Council of Turkey) through a Doctoral Researcher Program with project number 2211-C. The author is grateful

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