Positron annihilation spectroscopic studies of Mn substitution-induced cubic to tetragonal transformation in ZnFe_2–xMn_xO₄ (x = 0.0–2.0) spinel nanocrystallites

Abstract

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, Mössbauer 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 ~10 nm.

Keywords:

• defects
• ferrites
• Mössbauer spectroscopy
• spinels
• substitutional effects
• positron annihilation
• vacancies
• X-ray diffraction

Acknowledgements

The authors wish to thank Dr Dipankar Das, UGC-DAE Consortium for Scientific Research, Kolkata, for useful discussion on the subject. They also like to express their gratefulness to Dr Mahuya Chakrabarti for helping in the Mössbauer spectroscopic data analysis.

Disclosure statement

No potential conflict of interest was reported by the authors.