Positron trap studies in Zn1-xCuxCr2Se4 spinels

The results of the positron annihilation measurements and the calculations of the vacancy parameter carried out in single and polycrystals of Zn1+xCuxCr2Se4 spinel series are discussed within the framework of a two state trapping model. The single crystal values of the vacancy concentration remain almost constant with increasing copper concentration x. They are higher than the polycrystal ones (which linearly increase) suggesting, that the single crystals are more defected than polycrystals. The heat treatment influences the values of the vacancy concentration showing, that oxidation and adsorption behaviour of the spinels seems to be different on the zinc rich side (where the antiferromagnetic component dominates) as compared with the copper rich side (where the ferromagnetic component dominates). In particular, the results are in good agreement with the values of the vacancy parameter which is related to the vacancy concentration.