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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