THE MECHANISM OF SUPERCONDUCTIVITY SUPPRESSION IN THE YBASRCU3-XSNXOYSYSTEM

Carrier concentration, superconductivity, and crystal structure in a s eries of YBaSrCu3-xSnxO3 samples (0 less than or equal to x less than or equal to 0.25) have been investigated. The X-ray diffraction analys is and structural computer simulations reveal that at low doping level s (x < 0.15) Sn preferentially occupies the Cu(1) site, but gradually goes into both the Cu(1) and Cu(2) site at higher doping levels. The r ate of T-c suppression caused by Sn doping is significantly slower for low dopant concentrations than for high dopant concentrations, indica ting that different mechanisms operate at the Cu(1) and Cu(2) sites. T he hole concentration, determined by both Hall effect and oxygen conte nt, shows a systematic decrease with the dopant concentration. Our res ults suggest that one of the mechanisms of the superconductivity suppr ession is the reduction of the carrier concentration, which dominates the T-c-x relationship at low dopant concentrations. Another mechanism may be the impurity-scattering effect caused by the Sn doping at CuO2 planes which is responsible for the dramatic suppression of T-c at hi gh dopant concentrations. The doping behaviour in the present system a nd in the Sr free system are also compared.