Positron annihilation spectroscopic studies of Mn substitution-induced cubic to tetragonal transformation in ZnFe2-xMnxO4 (x=0.0-2.0) spinel nanocrystallites

The replacement of cations at the B-sites in the spinel ferrite ZnFe2O4 by Mn3+ ions brings in several interesting changes, the most striking among them being a transformation from the spinel cubic structure to a body-centered tetragonal one. Concomitantly, there are variations in the nanocrystallite sizes and also in the lattice parameters. These are examined through high-precision X-ray diffraction measurements and transmission electron microscopic analysis. A more interesting aspect is the success of positron annihilation spectroscopy comprising of the measurements of positron lifetime and coincidence Doppler broadening measurements in understanding the effects of cation replacement and the resultant generation of vacancy-type defects. There are definite changes in the positron lifetimes and intensities which show positron trapping in trivacancy-type defect clusters and the nanocrystallite surfaces. The presence of ortho-positronium atoms within the extended intercrystallite region is also identified, although in small concentrations. The cubic to tetragonal transformation is indicated through definite decrease in the values of the positron lifetimes. We also performed a model analysis to predict the expected effect of substitution on the positron lifetime in the bulk of the sample and the experimentally obtained positron lifetimes significantly differed, indirectly hinting at the possibility of a structural transformation. Finally, Mossbauer spectroscopic studies have indicated a ferromagnetic nature for one of the samples, i.e. the one with Mn3+ doping concentration x=0.4, which incidentally had the lowest crystallite size similar to 10nm.