Indian Journal of Pure & Applied Physics Vol. 47, June 2009, pp. 420-425 Role of defect centres in thermoluminescence mechanism of Tb 3+ doped MgAl 2 O 4 E Alagu Raja, Bhushan Dhabekar, Sanjeev Menon, S P More, T K Gundu Rao* & R K Kher Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085 *Regional Sophisticated Instrumentation Centre, IIT, Powai, Mumbai 400 076 E-mail : e_alaguraja@yahoo.com Received 24 February 2009; accepted 20 April 2009 Magnesium aluminate (MgAl 2 O 4 ) doped with terbium was prepared by combustion synthesis. Defect centres of MgAl 2 O 4 :Tb are studied by correlating various techniques such as Electron Spin Resonance (ESR), Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL). TL studies on MgAl 2 O 4 :Tb 3+ show two TL peaks at 120 and 340°C. MgAl 2 O 4 :Tb 3+ exhibits OSL when stimulated with 470 nm blue light. ESR study shows two defect centres with g = 2.011 and g = 2.0052 which are assigned to V- centre and F + centre, respectively. Step annealing ESR technique study suggests that V-centre is correlated to 120°C TL peak whereas F + centre is correlated with both the 120 and 340°C TL peaks. A TL mechanism is proposed. ESR–OSL correlation studies are carried out. The results of ESR-OSL correlation study along with TL bleaching study support the proposed TL mechanism of 120°C TL peak. Keywords: Thermoluminescence, MgAl 2 O 4 , Defect centres, Electron spin resonance 1 Introduction Magnesium aluminium oxide spinel (MgAl 2 O 4 ) is an important material in technological applications as host materials for various phosphors and light emitting devices including laser, and for various optical and electrical applications and some being in radiation environment 1 . Many of its properties are intermediate between those of its constituent oxides (MgO and Al 2 O 3 ). Consequently, although much is now known about anion vacancy centers and cation vacancy (V-type) centers in both MgO and -Al 2 O 3 , relatively little is known about these centers in MgAl 2 O 4 . The spinel crystalline structure is a face centered cubic lattice of oxygen ions, with a lattice parameter 2 of 8.08 Å for MgAl 2 O 4 . It has the cubic space group Fd3m. Eight molecules form its unit cell, in which there are 64 tetrahedral symmetry sites and 32 octahedral ones. Only 8 magnesium ions occupy tetrahedral sites and 16 aluminum ions occupy octahedral sites in MgAl 2 O 4 of natural origin. However, upto 30% of cation antisite disorder 3-5 occurs in synthetic MgAl 2 O 4 crystal. 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 in the synthetic as prepared samples of MgAl 2 O 4 . Further, irradiation causes damage to the defects centre and impurities by changing their charge states 6 . This plays a vital role in many of the luminescent and optical properties of the crystal, especially thermoluminescence (TL) and optically stimulated luminescence (OSL). OSL was reported in pure MgAl 2 O 4 crystal 7 . Rare earth doped MgAl 2 O 4 are reported to be a long persistent afterglow emission phosphors used in light emitting device and colour display 1,8 . A detailed study on V- type centres (hole trapped in a cationic vacancy) and F-centres in MgAl 2 O 4 were carried out extensively by optical absorption and Electron spin resonance (ESR) on irradiation with neutron, UV and -rays in correlation 9-14 with MgO and Al 2 O 3 . Various TL studies have also been carried out to find the defects responsible for the TL peaks 15-17 . The defects responsible for the observed first TL glow peak (80°C) were ascribed to electron traps 16 (Al 3+ ion at Mg 2+ site). 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, 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. Preparation of MgAl 2 O 4 sample by combustion synthesis has been reported 18 .