Partial inverse spinel structure of manganese-doped gahnite: XRD and EPR spectroscopy studies

Abstract

Manganese-doped ZnAl 2O 4 (gahnite) samples, with doping levels 0, 4.3, 7.6 and 11.7 at.% Mn (substituting for Zn), were prepared using a sol–gel technique and characterized by X-ray powder diffraction. Diffraction lines were broadened indicating on nanocrystalline samples, the line broadening depended on Mn doping level. The samples were cubic with a spinel-type structure, space group F d 3 ¯ m . Manganese doping of gahnite caused an increase of unit-cell parameter. The structure of the gahnite samples was refined by the Rietveld method, simultaneously with the analysis of diffraction line broadening. Valence state and the location of manganese ions in the structure were determined by EPR spectroscopy. Pure gahnite possessed the normal spinel structure, while Mn-doping induced the appearance of partial inverse spinel structure with Mn 2+ ions residing on both tetrahedral and octahedral cation sites. Metal-oxide distances in the (Zn,Mn,Al)O 4 tetrahedra and (Al,Mn)O 6 octahedra increased as the Mn-doping level increased, resulting in the unit-cell parameter enlargement. Crystallite size decreased and lattice strain increased on the increase of Mn loading.