Effect of doping of divalent and trivalent metal ions on the structural and electrical properties of magnesium aluminate

Nanosized magnesium aluminate materials doped by divalent cations (Ca2+, Ba2+, Sr2+) and trivalent cations (Cr3+, Mn3+, Fe3+) having nominal compositions Mg1−xMxAl2O4 and MgAl2−xMxO4 (x = 0.02–0.1), respectively, were synthesized by the sol–gel method. The samples were characterized 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 (db) and X-ray density (dx) 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 MgAl2O4 more than that by transition metal dopants. Arhenius activation energy of hopping of electron for all the samples was also calculated.