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Positron annihilation spectroscopy PAS has been a Cornerstone

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 spectroscopy and the Ag1 xCuS and AgCu1 xS samples spectroscopy 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 τ 1 of about 156 ps is considered to the free annihilation of the positron The intermediate lifetime component τ 2 is due to the positron annihilation at defect sites In the present studies the intermediate lifetime τ 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 τ 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 τ 2 is about 272 ps and had been identified as Ag vacancy 13 It is interesting that τ 2 is minimum around Ag Cu 1 Upon decreasing the Ag Cu ratio to values less than unity τ 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 τ 2 is relatively faster in case of Cu vacant samples 320 ps for AgCu0 99S to 339 ps for AgCu0 96S The increase of τ 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

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