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329Average positron lifetime and the bulk positron lifetime were further calculated using the formula τ Ave τ 1 I1 τ 2 I2 I1 I2 and τ B τ 1 I1 τ 2 I2 I1 I2 respectively and plotted against the stoichiometry as Figure 5c The nature of both the graphs is similar and as typical the value of τ B is more than τ Ave indicating the presence of a vacancy defect in the sample which increases with increasing the non stoichiometry in AgCuS that has a significant impact on the vanishing p n p conduction switching in Ag1 xCuS and AgCu1 xS samples Doppler broadening of positron annihilation radiation DBPAR line shape parameter S parameter defined in experimental section provides us a quantitative idea about the number of positrons being annihilated with the lower momentum valence electrons An increase in the value of S parameter implies that the positrons are being more annihilated by the lower momentum electrons Figure 5d shows the variation of S parameter of the different samples plotted against their stoichiometry With the corresponding increase of vacancies the S parameter value decreases As vacancy concentration increases in AgCuS the low momentum electrons which are comprised mainly of the valence shell electrons of the cations decreases This decrease leads to a downfall in the count of positrons annihilated by the lower momentum electron Since the total number of positrons incident on the samples remains same the decrease in the numerator suggests us that the S parameter value will decrease which is indeed the case Table 1 represents the variation of carrier concentration with the stoichiometry of Ag and Cu i e Ag Cu in AgCuS It is interesting to note that the nature of the carrier concentrations of these samples is in agreement with the positron annihilation lifetime results Figure 5c but the nature of Figure 5d is reverse as compared to Figure 5c Thus positrons are more annihilating with the lower momentum electrons when the Ag Cu is around 1 coupled with a higher change in thermopower during the p n conduction switching as measured by temperature dependent Seebeck coefficient As S parameter decreases with changing Ag Cu ratio the change in thermopower also decreases
