Positron Lifetime and Microstructures in Chevrel Phase Compounds

Positron lifetime measurements and SEM analysis of REMo6S8 series (RE = La, Gd, Yb, and Ho) and CuxMo6Se7.6 (x = 0.0, 0.7, 1.0, 1.8, 2.0, and 2.3) are performed. The average grain size R of the polycrystalline samples are obtained from SEM pictures and ranges from 0.7 to 2.78 μm. So surface annihilation is expected. But the trend of I2 with R is incompatible with Brandt model. So it is concluded that Chevrel phase compounds are full of intrinsic defects such as channels. The second lifetime component τ2 is attributed to channel trapping in Chevrel phase compounds. The estimated D +app lies within Brandt's detectability limit 2 × 10−3 to 0.2 cm2/s. The progressive increase of λf with increase of Cu concentration indicates that there is a charge transfer from Cu atoms to Mo6 clusters. The enhanced values of λf for sulphides reveals the formal charge reduction for selenides by 0.2 electron units in Chevrel phase compounds. Copyright © 1987 WILEY‐VCH Verlag GmbH & Co. KGaA