Nanocrystalline samples of γ-Fe 2O 3 were prepared through chemical routes and the particles were allowed to grow by thermal treatment at different temperatures. γ-Fe 2O 3 transformed to the a phase at a temperature relatively lower than that reported for coarse-grained samples. Positron lifetimes were measured in both the phases of the samples and a significant reduction was observed across the transformation. This is attributed to the expected reduction in the unit cell volume and the nature of orientation of the nearest-neighboring atoms around the positron trapping sites. Positrons are basically annihilating from the octahedral vacancy sites in γ-Fe 2O 3 and the lifetime decreases as the nearest-neighboring atoms increasingly influence the annihilation probability of the positrons when the sample changes to the a phase. The reduction in the positron lifetime has helped to envisage the onset of the transformation earlier than that indicated by x-ray diffraction. The positron lifetimes in the intergranular region increased due to the increase in free volume associated with the atoms on the grain surfaces when the grain size reduced. Doppler broadening measurements of the annihilation γ-ray spectral line shapes were also carried out and the results supported these findings.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published (in print/issue) - 1 Feb 2005|