Positron intermediate lifetime (?2) component is in

Positron
annihilation spectroscopy (PAS) has been a cornerstone in recent history for
characterizing and identifying the chemical nature of the defects in different
solids.12,13,27-30 The positron annihilation lifetime (PAL) and Doppler
broadening (DB) measurements of the Ag1-xCuS
and AgCu1-xS samples are the two main techniques; one probe the electron density distribution whilst
the other probes the electron momentum distribution in the studied material. Structural
phase transitions in different sulfide
samples using these two positron annihilation
techniques have been studied successfully.13,25

We have characterized all the as synthesized samples (Ag0.85CuS,
Ag0.90CuS, Ag0.99CuS, AgCu0.99S, AgCu0.98S
and AgCu0.96S) by positron
lifetime spectroscopy and Doppler broadening of positron annihilation radiation
spectroscopy. The entire lifetime spectrum has been analyzed
by PATFIT 88 program with proper source correction. The best fit of the
spectrum (variance of fit < 1 per channel) is with three lifetime components fitting, having a long lifetime of  1.3 ns with  4 % intensity. This lifetime component is due to the formation of positronium at the surfaces or at the void spaces inside the sample. Figure 5a represents the positron annihilation lifetime spectrum for Ag0.99CuS and AgCu0.96S samples. The shortest lifetime component (?1) of about 156 ps, is considered to the free annihilation of the positron. The intermediate lifetime component (?2) is due to the positron annihilation at defect sites. In the present studies, the intermediate lifetime (?2) component is in the range of 321 ± 5 to 347 ± 5 ps with the relative intensity of 43 to 53 %. Figure 5b represents the variation of ?2 with the stoichiometric ratio of Ag and Cu (i.e. Ag/Cu) in AgCuS sample. It has already been observed earlier that for a high-quality crystalline ingot of AgCuS, the value of ?2 is about 272 ps, and had been identified as Ag vacancy.13 It is interesting that ?2 is minimum around Ag/Cu ~ 1. Upon decreasing the Ag/Cu ratio to values less than unity, ?2, there is a gradual increase from 343 ps for Ag0.99CuS to 346 ps for Ag0.90CuS and then slightly decreases to 339 ps for Ag0.85CuS, while the increment of ?2 is relatively faster in case of Cu vacant samples (320 ps for AgCu0.99S to 339 ps for AgCu0.96S). The increase of ?2 suggests the agglomeration of cation defects at a particular defect site and hence increases of positron lifetime value, which indicates towards an increase in cation vacancy in Ag1-xCuS and AgCu1-xS samples. Average positron lifetime and the bulk positron lifetime were further calculated using the formula ?Ave = (?1× I1 + ?2 × I2)/(I1+I2) and ?B = (?1/ I1 + ?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 ?B is more than ? 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.

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