Positron annihilation lifetime changes across the structural phase transition in nanocrystalline Fe 2O 3

S. Chakrabarti, S. Chaudhuri, P. M.G. Nambissan

Research output: Contribution to journalArticle

28 Citations (Scopus)

Abstract

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.

LanguageEnglish
Article number064105
JournalPhysical Review B - Condensed Matter and Materials Physics
Volume71
Issue number6
DOIs
Publication statusPublished - 1 Feb 2005

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Positron annihilation
Positrons
positron annihilation
positrons
Phase transitions
life (durability)
Atoms
atoms
Doppler effect
Free volume
Gamma rays
Vacancies
line spectra
line shape
x ray diffraction
Diffraction
grain size
trapping
Heat treatment
routes

Cite this

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abstract = "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.",
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Positron annihilation lifetime changes across the structural phase transition in nanocrystalline Fe 2O 3. / Chakrabarti, S.; Chaudhuri, S.; Nambissan, P. M.G.

In: Physical Review B - Condensed Matter and Materials Physics, Vol. 71, No. 6, 064105, 01.02.2005.

Research output: Contribution to journalArticle

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