Positron lifetime and microstructures in Chevrel phase compounds

Abstract

Positron lifetime measurements and SEM analysis of REMo₆S₈ series (RE = La, Gd, Yb, and Ho) and Cu_xMo₆Se_7.6 (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 λ_f with increase of Cu concentration indicates that there is a charge transfer from Cu atoms to Mo₆ clusters. The enhanced values of λ_f for sulphides reveals the formal charge reduction for selenides by 0.2 electron units in Chevrel phase compounds.