Thermoluminescence Properties of UV-irradiated CaLa₂ZnO₅ Phosphor Doped with Sm³⁺

A.S. Gour^1,*, B. Netam², P. Verma³, A. Shakya⁴

^1-4 SoS in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur, Chhattisgarh, India 

Abstract

In this study Sm³⁺ activated CaLa₂ZnO₅ phosphor was synthesized using a solid-state reaction method. The structural study of this phosphor was conducted using X-ray powder diffraction (XRD) technique. Thermoluminescence (TL) properties were investigated after UV irradiation(254nm).The thermoluminescence glow curve of the CaLa₂ZnO₅:3mol%Sm³⁺  showed a prominent peak at 387.15K. Computerized glow curve deconvolution revealed two trapping centers. Furthermore, the essential TL parameters, viz., activation energy (E) and frequency factor (s) were evaluated using Chen’s peak shape method. The results indicate the presence of moderately deeper traps. These findings provide useful information about the trapping behavior of CaLa₂ZnO₅:Sm³⁺  and suggest its potential for further investigation in luminescent and dosimetric applications.

Keywords: CaLa₂ZnO₅:Sm³⁺ phosphor, XRD, Thermoluminescence, UV irradiation.

Introduction

Recently, significant attention has been given by researchers worldwide to the development of novel materials for radiation detection and measurement applications. Materials that exhibit high chemical stability, heat resistance and strong resistance to moisture are especially preferred, as these properties make the material suitable for radiation dosimetry applications. Among the available methods, Thermoluminescence (TL) has emerged as an effective technique widely employed in dosimetric studies[1]. Thermoluminescence refers to the light emitted by a material when they are heated after exposure to ionizing radiation[2]. During irradiation, defects in the materials trap electrons and holes. When the materials are heated, these trapped charge carriers gain enough thermal energy to escape from the traps and recombine, releasing energy in the form of light[3]. Therefore, it is important to analyze about the trap levels and defect structures in materials and study the key parameters, such as  trap activation energy (E) and kinetic frequency factor(s) which possibly evaluated from the TL intensity curve. The dosimetric behavior of these materials are mostly influenced by the kinetic characteristics of the corresponding peaks[1].

Currently, phosphor-based thermoluminescent materials are receiving considerable interest in radiation dosimetry applications because of their compact size, simple preparation methods, and cost effectiveness[4]. Furthermore, oxides are considered excellent host materials for various luminescent activator ions due to their remarkable optical, mechanical, chemical, and electronic characteristics along with promising dosimetric performance. Phosphors developed from these materials have numerous technological applications such as WLEDs, cathode ray tubes(CRT), medical imaging instruments, highway traffic guidance systems, catalysts, optical storage, anti-counterfeiting and improving the performance of solar cells[4,5]. In this context, ternary metal oxides with general formula MR₂ZnO₅ (where M=Ca, Ba and R= Gd, Y, La) fulfill these diverse functional requirements making them highly attractive for next generation multifunctional applications. In particular, CaLa₂ZnO₅ has been established as an effective host for both up conversion and downcoversion photoluminescence[6,7]. However, to the best of our knowledge, its thermoluminescence properties have not been reported so far. In this study, we used the solid-state reaction method for the preparation of CaLa₂ZnO₅:Sm³⁺ phosphor and studied its structural properties and thermoluminescence properties.

Materials and methods

Synthesis method-

Pure and Sm³⁺ activated CaLa₂ZnO₅ phosphors were synthesized via the solid-state reaction method. The stoichiometric proportions of CaCO₃(Sigma Aldrich, 99.0%), La₂O₃(Loba Chemie, 99.9%), ZnO (Loba Chemie, 99%) and Sm₂O₃(Otto, 99.9%) were weighted and gathered them in to an agate mortar pestle for grinding. A small amount of (CH₃)₂CO (Loba Chemie, 99.5%) was added to obtain homogeneity during the grinding process. The materials were ground for an hour. These materials were placed in an alumina crucible and heated in a muffle furnace at 900°C for about 7 h. Finally, the prepared product was brought to room temperature and then converted into a fine powder through grinding and used for further characterization.

Characterization techniques-

The crystal structure of the obtained CaLa₂ZnO₅:Sm³⁺ powder was characterized by X-ray diffraction (XRD). The XRD profile was measured with a D2 Phaser Model:08 discover, Bruker (Table Top Model) and the data were obtained within the 2θ range of 10-80° at ambient temperature. The thermoluminescence intensity curves were measured using a Nucleonix TL reader (Model:1008) and the corresponding temperature interval for these measurements was 300 to 600K.

Result and discussion

XRD analysis-

The XRD pattern of CaLa₂ZnO₅:Sm³⁺ (fig.1) shows  sharp and well-defined peaks, indicating good crystallinity. The major peaks match well with the previously documented patterns by Bandi et al. in[9]. A few minor additional peaks were also observed, which may be attributed to secondary phases or slight variations in synthesis conditions. The average crystallite size of the prepared sample was estimated from XRD data using the well-known Debye’s Scherrer equation (1)

                                   ……. Eq. (1)           

Where D denotes crystallite size, k is the Scherrer constant, λ is wavelength of X-ray source,  ꞵ is FWHM and θ is peak position. The calculated crystallite size of the prepared sample was found 48.26 nm, indicating the formation of nanocrystalline particles[10].

Thermoluminescence Analysis-

To investigate the thermoluminescence behavior of Sm³⁺ doped CaLa₂ZnO₅ phosphor, the temperature vs intensity spectrum was analyzed. Fig.2 shows the thermoluminescence intensity curve of CaLa₂ZnO₅:3mol%Sm³⁺ phosphor. During this study, the glow curves of Sm³⁺ doped samples were exposed to UV radiation at a wavelength of 254 nm for 5 to 15 min, followed by thermal stimulation from room temperature at a linear heating rate of 5K/s[11]. The effect of UV irradiation time on TL intensity is shown in fig.3(a). The TL intensity increased with increasing irradiation time from 5 to 15 min, indicating progressive filling of traps. Further, to determine the number of trapping centers and their kinetic parameters, the TL glow curve of the 3 mol% Sm³⁺ doped sample irradiated for 15 min was deconvoluted using the computerized glow curve deconvolution (CGCD) technique implemented through the tgcd package in R software, as shown in Fig. 3(b)[12]. Chen’s peak shape method was utilized to calculate the kinetic parameters, and the corresponding results are provided in Table (1). The symmetry factor was evaluated using Eq. (2) defined as

                              …… Eq. (2)

Where T_m represents the maximum temperature of the intensity peak, while T₁ and T₂ correspond to the initial and final temperatures of the glow peak, respectively. The parameters τ = (T_m – T₁), δ = (T₂ – T_m) and ω = (T₂ – T₁) define the width at half of the intensity peak. Typically, µ values of nearly 0.42 and 0.52 indicate first-order and second-order kinetics respectively. Since the experimental glow curve did not follow first or second order, a general order kinetics was used to calculate trapping parameters. The activation energy was calculated using Eq. (3)  considering general order kinetics in terms of ɛ (ɛ = τ, δ, ω).

                                                                                 ………Eq. (3)

Where c_ɛ and b_ɛ are constants that depend on the symmetry factor µ. These constants were obtained using standard empirical relations given below

The kinetic frequency factor (s) was subsequently evaluated by Winer and Chen relation given in Eq. (4)

         ……. Eq. (4) 

Where ꞵ denotes the heating rate and k is the Boltzmann constant. The FOM (Figure of Merit) given by Eq. (5)

                  ……. Eq. (5)          

Where Y_exp is experimental glow curve, and Y_fit is fitted curve. The calculated FOM value is 1.69% confirms the accuracy of the deconvolution, as it falls well below the accepted 5% threshold for reliability[13]. The activation energy values obtained from  Chen’s peak shape analysis was found to lie in the range of 0.91-1.14 eV, indicating the presence of moderately deep trapping centers in the phosphor material. Furthermore, the frequency factor values were observed in the order of 2.93×10¹¹ to 1.35×10¹³ , which are typical for thermoluminescent materials [11].

Conclusion

In summary, CaLa₂ZnO₅ phosphor was successfully synthesized using a solid-state reaction method. The phase purity and crystalline nature of the prepared material were confirmed by X-ray diffraction analysis, which indicated the formation of a well-defined crystalline structure. Under UV irradiation, the optimized 3mol% Sm³⁺ doped sample exhibited strong thermoluminescence with a main peak near 387.15 K. Glow curve deconvolution revealed two trapping centers with activation energies of 1.14eV and 0.91 eV. The present results demonstrate promising thermoluminescent behavior in CaLa₂ZnO₅:Sm³⁺. Further detailed investigations including different doping concentrations, gamma/X-ray irradiation, dose-response linearity, thermal fading and reusability studies are required to evaluate its full potential for radiation dosimetry applications.

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Fig.1 X-ray diffraction pattern of CaLa₂ZnO₅ and CaLa₂ZnO₅ doped with Sm³⁺.

Fig.2 TL Glow Curve of Sm³⁺ doped CaLa₂ZnO₅ for 15min UV ir-radiation.

            Fig.3(a) TL Glow Curve of Sm³⁺ (3mol%) doped CaLa₂ZnO₅ for 5-15min UV-irradiation.

Fig.3(b) Deconvoluted Graph of Sm³⁺ (3mol%) doped CaLa₂ZnO₅ for 15 min UV-irradiation.

Table.1 TL parameters of deconvoluted graph of Sm³⁺ (3mol%) doped CaLa₂ZnO₅ for 15 min UV-irradiation.