Positron annihilation studies in single and polycrystals of Zn1-xCuxCr2Se4 spinel series

Positron annihilation studies were carried out on single and polycrystals of Zn1-xCuxCr2Se4 spinels. The largest average positron lifetime is observed for the single crystals, suggesting that for the spinel structure single crystals contain more vacancies than polycrystals. The positron lifetime connected with vacancies increases with x (copper concentration) as long as the crystals are semiconducting and decreases with x in the metallic region. These effects are probably caused by the different electron densities in the semiconducting and metallic regions of the Zn1-xCuxCr2Se4 spinel series together with different vacancy concentrations. In particular, the results are in good agreement with a vacancy model which allows to calculate the vacancy concentration using a single vacancy parameter.