Luminescence of Ce doped MgAl 2 O 4 prepared by the combustion method W.A.I. Tabaza, H.C. Swart, R.E. Kroon n Department of Physics, University of the Free State, IB51, Box 339, Bloemfontein 9300, South Africa article info Keywords: Luminescence Cerium MgAl 2 O 4 Combustion synthesis abstract Magnesium aluminate (MgAl 2 O 4 ) has received special attention as a technologically important material because of its attractive properties, such as mechanical strength, chemical inertness, a wideband gap, relatively low density, high melting point, high thermal shock resistance, low thermal expansion coefficient, resistance to neutron irradiation and low dielectric loss. It has also been used as a phosphor host activated by a variety of transition metal and lanthanide ions. A simple combustion method was employed for the preparation of Ce doped MgAl 2 O 4 nanocrystals using metal nitrates as precursors and urea as a fuel in a preheated furnace at 520 1C. The as-prepared samples were annealed in a hydrogen atmosphere to improve their optical properties. The samples thus obtained were characterized by X-ray diffraction (XRD), UV–vis spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy and photoluminescence spectroscopy (PL). The XRD data showed that all the samples had the spinel structure and the average particle size of the as-prepared samples was about 25 nm. PL spectra of Ce doped MgAl 2 O 4 using an excitation wavelength of 350 nm produced broad green emission bands centred at 500 nm. Maximum green emission was obtained for the sample doped with 0.75 mol% Ce. UV–vis diffuse reflectance spectra and XPS were used to obtain more information on the conversion of Ce ions from the non-luminescent Ce 4 þ to the luminescent Ce 3 þ charge state. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Spinel oxides find many applications as magnetic, electric, ceramic, catalysis and optical materials. They have the formula AB 2 O 4 , where A and B represent divalent and trivalent cations respectively [1] and belong to the cubic space group Fd 3m [2]. The arrangement of Mg, Al and O atoms in the unit cell of magnesium aluminate (MgAl 2 O 4 ) is shown in Fig. 1, as drawn with the Diamond crystal software [3]. The cations occupy twice the number of octahedral sites as tetrahedral sites, and in the normal spinel arrangement these are occupied by Al 3 þ and Mg 2 þ ions, respectively. MgAl 2 O 4 has received a great deal of attention as a technologically important material because of its mechanical strength, chemical inertness, wide bandgap (6.8 eV), relatively low density, high melting point (2105 1C), high thermal shock resistance, low thermal expan- sion coefficient, resistance to neutron irradiation and low dielectric loss [4,5]. It has been employed in several applications, such as humidity sensors [6] and tuneable solid-state lasers [7]. MgAl 2 O 4 has also been assessed as a phosphor host doped with Tb [8,9], Eu [10,11], Ti [12],V [13], Cr [14] and Mn [15] ions. Long afterglow emission from defects in MgAl 2 O 4 was reported to be enhanced by doping with Ce [16]. Ce is an important activator for phosphors e.g. YAG:Ce [17], SiO 2 :Ce [18], ZnAl 2 O 4 :Ce,Tb [19] and Dorenbos [20] has collected the emission properties in many hosts since knowledge about the Ce luminescence can be extrapolated to other lanthanide ions. There are many methods to prepare spinel powders, such as solid-state reaction [21], precipitation [22], hydrothermal synthesis [23], sol–gel methods [24], co-precipitation [25], and the combustion or microwave combustion methods [26]. Combustion synthesis occurs via a highly exothermic redox reaction between metal nitrates and an organic fuel. This technique can produce a homogenous product in a short amount of time without using expensive high temperature furnaces and has been used to prepare new nanocrys- talline phosphor powders MgAl 2 O 4 :Ce 3 þ with different concentra- tions of Ce from 0.25 to 2 mol%. 2. Materials and methods MgAl 2 O 4 was prepared from analytical grade magnesium nitrate Mg(NO 3 ) 2  6H 2 O, aluminium nitrate Al(NO 3 ) 3  9H 2 O and urea (CH 4 N 2 O). Approximately 10 mmol of aluminium nitrate was added to stoichiometric amounts of the other reactants in about 6 ml of distilled water, which was stirred vigorously for 30 min to obtain a homogeneous transparent solution. The transparent solution of reagents was transferred into a porcelain crucible Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B 0921-4526/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2013.10.060 n Corresponding author. Tel.: þ27 51 4012884; fax þ27 51 4013507. E-mail address: KroonRE@ufs.ac.za (R.E. Kroon). Please cite this article as: W.A.I. Tabaza, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.10.060i Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎