Elsevier

Journal of Luminescence

Volume 129, Issue 8, August 2009, Pages 829-835
Journal of Luminescence

Investigation of defect centres responsible for TL/OSL in MgAl2O4:Tb3+

https://doi.org/10.1016/j.jlumin.2009.03.001Get rights and content

Abstract

Magnesium aluminate doped with Tb3+ (MgAl2O4:Tb3+) was prepared by combustion synthesis. Three thermoluminsence (TL) peaks at 120, 220 and 340 °C were observed. PL and TL emission spectrum shows that Tb3+ acts as the luminescent centre. Optically stimulated luminescence (OSL) was observed when stimulated by 470 nm blue light.

Electron spin resonance (ESR) studies were carried out to identify the defect centres responsible for the TL and OSL processes in MgAl2O4:Tb3+. Two defect centres were identified in irradiated MgAl2O4:Tb3+ phosphor by ESR measurements which was carried out at room temperature and these were assigned to V and F+ centres. V centre (hole centre) is correlated to 120 and 220 °C TL peaks and F+ centre (electron centre), which acts as a recombination centre is correlated to 120, 220 and 340 °C.

Introduction

Magnesium aluminium oxide spinel (MgAl2O4) is used in various technological applications such as light emitting devices including laser, optical, electrical applications and also in radiation environment [1]. Rare-earth-doped MgAl2O4 are reported to be a long persistent afterglow emission phosphors used in light emitting device and color display [1], [2]. Many of its properties viz. density, melting point are intermediate between those of its constituent oxides (MgO and Al2O3) [3]. The crystal structure of MgAl2O4 is a face-centered cubic lattice of oxygen ions, with a lattice parameter of 8.08 Å. It has the cubic space group Fd3m. Unit cell is formed with eight molecules, in which there are 64 tetrahedral symmetry sites and 32 octahedral sites. MgAl2O4 of natural origin have 8 magnesium ions occupying tetrahedral sites and 16 aluminium ions occupying octahedral sites. However, synthetic MgAl2O4 crystal can have up to 30% of cation antisite disorder [4], [5], [6], [7], [8]. Antisite formation due to interchange of the ions on tetrahedral and octahedral lattice positions by divalent and trivalent ions causes numerous trapping sites for the electron and hole on irradiation. Further, irradiation causes damage to the defect centres and impurities by changing their charge states [9]. These defect centres play a vital role in many of the luminescent and optical properties of the crystal. Optically stimulated luminescence (OSL) emission in pure MgAl2O4 crystal is reported by Yoshimura and Yukihara [10].

A detailed study on V centre (hole trapped in a cationic vacancy) and F centre (two electrons trapped in anion vacancy) in MgAl2O4 were carried out extensively by optical absorption and electron spin resonance (ESR) on irradiation with neutron, UV and γ-rays in correlation with MgO and Al2O3 [4], [11], [12], [13], [14], [15].

Combustion synthesis or self-propagating high temperature synthesis (SHS) provides an attractive practical alternative to the conventional synthesis of producing advanced materials, such as ceramics, composites etc. The underlying basis of SHS is highly efficient energetic exothermic reaction with the evolution of various gases along with the high intense flame. It volatilizes low boiling point impurities and results in purer products than those produced by the other conventional synthesis [16].

This paper reports photoluminescence (PL), thermoluminescence (TL) and optically stimulated luminescence properties of MgAl2O4 doped with terbium, prepared by combustion synthesis. ESR studies were carried out to identify the defect centres playing role in TL and OSL.

Section snippets

Experimental details

Magnesium nitrate, aluminium nitrate, urea and terbium nitrate were used as starting materials. MgAl2O4 was prepared in stoichiometric ratio (oxidation to reducing valency) O/F=1. Metal nitrates, urea and desired amount of dopant are dissolved in de-ionized water in a glass beaker. The beaker was kept inside a furnace which was set at 550 °C. Once the water boiled off, the metal nitrate and urea react and ignite. The energy released from the reaction can produce temperatures in excess of 1500 °C

XRD

Fig. 1 shows the XRD pattern of as-prepared MgAl2O4 and standard XRD pattern (JCPDS file 75-0905). XRD of MgAl2O4 shows various planes of diffraction with miller indices (1 1 1), (2 1 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0). This result confirms the formation of the crystal phases of MgAl2O4 which includes the Al3+ ions in octahedral symmetry and Mg2+ occupying the tetrahedral symmetry site.

Photoluminescence

Fig. 2(a) displays the PL excitation and emission spectra of (0.5 mol%) Tb3+-doped MgAl2O4. The emission

Conclusions

MgAl2O4:Tb3+ phosphor is prepared by combustion synthesis. Three TL peaks are observed at 120, 220 and 340 °C. It exhibits OSL when stimulated with 470 nm light. The sample thermally treated at 900 °C gives maximum PL and OSL emission intensity. Two defect centres, centre I and II are identified in irradiated MgAl2O4:Tb3+ phosphor and these centres are assigned to V and F+ centres, respectively. V centre (hole centre) appears to correlate with the 120 and 220 °C TL peaks while F+ centre which acts

Acknowledgements

The authors are thankful to Dr B.C. Bhatt, Emeritus Scientist, CSIR for his critical comments and suggestions. The authors are also thankful to Mrs P. Ratna, RSSD, BARC for her help to record TL glow curves, Dr M.S. Kulkarni and Dr D.R. Mishra for providing OSL reading facility and to Dr. V. Sudarshan, Ch. D, BARC for recording the XRD spectra.

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