Microstructure of sol-gel derived Mn-doped gahnite: Correlation of TEM and XRD investigations

Powder ZnAl2O4 (gahnite) samples doped with 0, 4.3, 7.6 and 11.7 at% Mn (in relation to Zn) were prepared by a sol-gel technique followed by thermal treatment. The effect of manganese doping on microstructure of the samples has been investigated by transmission electron microscopy and X-ray diffraction. Diffraction patterns showed that all samples were cubic with a spinel structure, space group Fd3¯m. Diffraction lines were broadened indicating nanocrystalline samples. Transmission electron microscopy investigations confirmed that the samples were nanocrystalline. Particles (grains) in the samples, as observed by transmission electron microscopy, were evenly shaped and grain size distribution could be fitted to a lognormal function. Grain sizes determined by TEM analysis and crystallite sizes obtained from diffraction line-broadening analysis were similar. Both decreased with increasing manganese doping level. Increase in doping level from 0 to 11.7 at% Mn caused the change in grain size from 25.0(7) nm to 14.2(3) nm and the change in crystallite size from 24.1(1) nm to 18.0(1) nm. Doping Mn2+ cations acted as defects in the gahnite structure that increased lattice strain from 0.07(1)% for undoped sample to 0.21% for sample doped with 11.7 at% Mn, and disturbed the crystallites to grow.