Positron lifetime spectroscopic studies of nanocrystalline $ZnFe2O4$

By carrying out positron lifetime measurements in zinc ferrite $(ZnFe2O4)$ samples of various grain sizes down to 5 nm, the defect microstructures have been identified. In the bulk samples composed of grains of large sizes, positrons were trapped by monovacancies in the crystalline structure. Upon reduction of the grain sizes to nanometer dimensions, positrons get trapped selectively at either the diffused vacancies on the grain surfaces and the intergranular regions. Below about 9 nm, the grains undergo the transformation from the normal spinel structure to the inverse phase. A concomitant lattice contraction results in substantial reduction of the octahedral site volume, and hence, a fraction of the $Zn2+$ ions with larger ionic radius fails to occupy these sites. This leaves vacancies at the octahedral sites which then turn out to be the major trapping sites for positrons. $ZnFe2O4$ samples prepared through different routes were investigated, which showed similar qualitative features, although those synthesized through the hydrothermal precipitation method showed remarkably larger lifetimes for trapped positrons upon nanocrystallization in comparison to the samples prepared through the citrate route.