Materials Science and Engineering B 136 (2007) 140–147 Effect of doping of divalent and trivalent metal ions on the structural and electrical properties of magnesium aluminate Muhammad Javed Iqbal ∗ , Saima Farooq Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan Received 21 July 2006; received in revised form 26 August 2006; accepted 8 September 2006 Abstract Nanosized magnesium aluminate materials doped by divalent cations (Ca 2+ , Ba 2+ , Sr 2+ ) and trivalent cations (Cr 3+ , Mn 3+ , Fe 3+ ) having nominal compositions Mg 1-x M x Al 2 O 4 and MgAl 2-x M x O 4 (x =0.02–0.1), respectively, were synthesized by the sol–gel method. The samples were char- acterized by X-ray diffraction (XRD) and dc electrical resistivity measurements. The XRD data showed that all the samples were spinel single phase cubic closed packed crystalline materials having crystallite sizes between 6 and 35 nm. The lattice constant and X-ray density were found to be affected by the ionic radii of doped metal cations investigated here. Both the bulk density (d b ) and X-ray density (d x ) of doped materials increased whereas the porosity percentage (P) decreased with the increase in the contents of the substituents. The dc-electrical resistivity of all the samples was measured in temperature range 160–400 ◦ C by a two-point probe method and was found to decrease with temperature as expected for semiconductors. It was observed that alkaline earth metal dopants increased the resistivity of MgAl 2 O 4 more than that by transition metal dopants. Arhenius activation energy of hopping of electron for all the samples was also calculated. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Magnesium aluminate; dc-Electrical resistivity; Two-point probe method; X-ray density 1. Introduction Nanophase materials (<100 nm) exhibit greatly altered physicochemical properties compared to their normal bulk coun- terparts [1–3]. Because of their theoretical and technological relevance, spinel magnesium aluminates and their related struc- tures have been studied thoroughly. It is a mixed oxide whose physical properties range between those of magnesium oxide and aluminum oxide. Magnesium aluminate (MgAl 2 O 4 ) spinel has received a great deal of attention as a technologically impor- tant material on account of its attractive properties such as high melting point (2135 ◦ C), high mechanical strength at elevated temperatures, high chemical inertness and good thermal shock resistance [4–6]. Therefore, it has been extensively used for var- ious purposes such as refractory material [7], catalyst or catalyst support in the field of environmental catalysis [8], active element in humidity sensors [9] and structural material in fusion reac- tors [10]. It has also been employed as an excellent transparent ∗ Corresponding author. Tel.: +92 51 9219811/0642143; fax: +92 51 2873869/271689. E-mail address: mjiqauchem@yahoo.com (M.J. Iqbal). ceramic material for high-temperature arc-enclosing envelops and alkali-metal vapor discharge devices [11]. The physical properties of ceramic materials are influenced by the nature of grains, such as shape, size, orientation, grain boundaries, voids, inhomogeneities, etc. The electronic properties and electrical conductivity of mesoscopically small crystals change compared to their bulk counterparts owing to their small grain sizes. The change also depends on the large percentage of their atoms in grain boundary environments and the interactions between grains [12]. Studies concerning substitution of Ce 3+ , Cr 3+ , Mn 3+ and Ti 2+ at Al 3+ site of MgAl 2 O 4 spinel have been carried out by several workers [13–16]. The synthesis of Ca 2+ , Ba 2+ , Sr 2+ , Cr 3+ , Mn 3+ and Fe 3+ substituted MgAl 2 O 4 nanoparticles in order to investigate the substitution effect on structural and electrical properties is the focus of the present study. 2. Experimental The compounds used in the synthesis of samples were of analytical grade i.e. Al(NO 3 ) 3 ·9H 2 O (Merck, 95.0%), Mg(NO 3 ) 2 ·6H 2 O (Merck, 99.9%), ethylene glycol (Merck, 99.0%) and aqueous NH 3 (Reidal, 33.0%) and were therefore used as such. Samples were prepared by the sol–gel method [17]. 0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.09.009