Combustion synthesized MgAl 2 O 4 :Cr phosphors—An EPR and optical study Vijay Singh a,Ã , R.P.S. Chakradhar b,c , J.L. Rao d , Dong-Kuk Kim a a Department of Chemistry, Kyungpook National University, Daegu 702701, Republic of Korea b Glass Technology Laboratory, Central Glass and Ceramic Research Institute, Kolkata 700032, India c Council of Scientific & Industrial Research, New Delhi 110001, India d Department of Physics, Sri Venkateswara University, Tirupati 517 502, India article info Article history: Received 28 December 2007 Received in revised form 22 July 2008 Accepted 21 August 2008 Available online 13 September 2008 Keywords: Combustion XRD BET EPR Cr 3+ ions Photoluminescence Phosphors abstract Magnesium aluminate (MgAl 2 O 4 ) doped with trivalent chromium (Cr 3+ ) was synthesized by the combustion method. The prepared sample was characterized by X-ray powder diffraction, Brunauer– Emmet–Teller (BET) adsorption isotherms and diffuse-reflectance UV–vis spectroscopy techniques. Electron paramagnetic resonance (EPR) and photoluminescence (PL) studies have been performed at room temperature and at 110K. The EPR spectrum exhibit resonance signals at g ¼ 5.37, 4.53, 3.82, 2.26 and 1.96 characteristic of Cr 3+ ions. The luminescence of Cr 3+ -activated MgAl 2 O 4 exhibits a red emission peak around 686 nm from the synthesized phosphor particles upon 551 nm excitation. The luminescence is assigned to a transition from the upper 2 E g - 4 A 2g ground state of Cr 3+ ions. By correlating EPR and optical data the crystal field splitting parameter (D q ), Racah inter-electronic repulsion parameter (B) and the bonding parameters have been evaluated and discussed. The bonding parameters suggests that the ionic nature of Cr 3+ ions with the ligands and the Cr 3+ ions are in distorted octrahedral environment. & 2008 Elsevier B.V. All rights reserved. 1. Introduction Spinel-type compounds doped with several kinds of transition metal ions continue to draw attention in view of their wide range of applications such as solid-state lasers and non-linear optic devices. It is well known that impurity elements play a major role because they can influence the properties of materials. When new laser materials based on oxides and glasses are developed, chromium ions are often chosen as impurities [1,2]. This dopant has its specific use as active media in solid-state laser. Cr 3+ ions more effectively interact with lattice and have high luminescent quantum efficiency as dopants in several host materials [3–5] and, therefore are widely investigated for numerous optical spectro- scopic and luminescence properties. Optical and electron para- magnetic resonance (EPR) spectrum is an ideal method for studying the role of Cr in host lattices. It is the intention of this paper to investigate the influence of chromium impurities on the optical properties of magnesium aluminate (MgAl 2 O 4 ) host. MgAl 2 O 4 , a high-melting ceramic with a wide band-gap and spinel structure, is one of the candidates to host rare earth and transition metal ions. Spinel has been extensively studied for its possible use in several applications such as metallurgical, chemical, catalysis and electronic industries [6–8]. In the distant past as well as in recent years spinel crystals (MgAl 2 O 4 ) doped with rare earth and transition metal ions, such as Ce 3+ , Fe 3+ , Mn 2+ and Ti 3+ have been synthesized and evaluated for luminescence properties by different groups [9–11]. In this paper, Cr 3+ ions are introduced into the MgAl 2 O 4 host using combustion process which is simple, fast, safe and has been prepared at relatively low temperatures. The prepared sample was characterized by X-ray diffraction (XRD) and Brunauer–Emmet– Teller (BET) surface area measurements. In addition, from EPR and optical studies, the spin-Hamiltonian parameters, crystal field splitting parameter (D q ) and Racah parameter (B) have been evaluated and discussed. 2. Experimental 2.1. Powder preparation MgAl 1.99 Cr 0.01 O 4 was prepared by the combustion method. Analytical reagent grade 3.43 g magnesium nitrate, 10.00 g aluminium nitrate, 0.05 g chromium nitrate and 5.33 g urea were used as the starting materials. The starting materials were crushed and ground in an agate mortar and due to large crystallization water in aluminium nitrate, a thick paste was ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence 0022-2313/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.08.011 Ã Corresponding author. Present address: LPQM UMR CNRS 8537, ENS de Cachan, 61, Avenue du President Wilson, 94235 Cachan, France. Tel.: +33147405561; fax: +33147405567. E-mail address: vijayjiin2006@yahoo.com (V. Singh). Journal of Luminescence 129 (2009) 130–134