SAIP2026

Quantization of radiation-induced defects in SnO2 using positron lifetime components.

8 Jul 2026, 16:20

1h 20m

Great Hall ( University of the Western Cape)

Poster Presentation Track A - Physics of Condensed Matter and Materials Poster Session 2

Speaker

Ms Dineo Motjope (First author)

Description

In this work a two-component density functional theory is employed in the modelling of defects in ion and neutron-irradiated SnO2 using positrons as probes. Since defects are localized, the local density approximation (LDA) is used, which is part of DFT. Although LDA gives a good approximation of positron lifetimes and electron-positron annihilation momentum density, it does not consider the variational nature of the electron density. This has an unintended consequence of having over estimated annihilation rates or lower positron lifetimes compared to experimental values. This deficiency in LDA is corrected by using the generalized gradient approximation (GGA) which considers the variational nature of electron density. The accumulation of annihilation spectrum using coincidence setup, is utilized to allow for the determination of annihilation parameters, S and W. The spectrum consists of positron annihilations at defect sites as well as annihilations in the bulk (defect-free region). The annihilation curve of the spectrum also consists of annihilations of positrons with core electrons (high momentum electrons) and this specifically allows the calculation of the W-parameters. The low and high momentum distribution of electrons will be used to characterize the Doppler broadening which will tell us about the quantity of radiation-induced defects in SnO2 in terms of calculating S-parameter, which is the ratio of the annihilation centroid area to the total area of the annihilation curve. Calculated S parameters are then compared with the experimentally obtained S parameters. The nature of the defects is theoretically obtained from the annihilation rates or equivalently from the calculated positron lifetimes in SnO2. The concentration of defects per cubic centimeter in both the Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA) frameworks was obtained. The concentration of defects per cubic cm in the framework of both the LDA and GGA were obtained. These concentrations were influenced by the positron lifetime components, the bulk lifetime, the defect lifetime as well as the average positron lifetime as 241.05 ps, 263 ps and 252.01 ps respectively. The concentration of tin vacancies (VSn) was found to be 1.09 x 1017 cm^-3 in the framework of GGA and 1.18 x 1017 cm^-3 in the framework of LDA. The discrepancy between these values is mainly due to the fact that LDA overestimate the annihilation rates whereas the GGA considers the charge density variation in atoms including the at the defects.

Keywords: positron annihilation, Doppler broadening, Local density approximation, positron lifetime, S-parameter