STRUCTURAL-CHANGES IN YBA2CU3O7 AND YBA2CU4O8 BY ZN DOPING

Atomistic simulation techniques were used to determine the Zn substitu tion sites in YBa2Cu3O7 (123) and YBa2Cu4O8 (124). It was found that Z n dopant is energetically favourable to substitute at the Cu(2) site. Zn substitution at both Cu(1) and Cu(2) sites is also possible, but Zn substitution at Cu(1) site seems unlikely. Our result implies that mo st Zn dopants go to the Cu(2) sites and a small percentage of Zn dopan ts goes to the Cu(1) sites, which is in agreement with the experimenta l results. We also found that the structural changes in Zn doped 124 a nd Zn doped 123 are opposite to that in 124 under pressure and in YBa2 Cu3O6.6 under pressure, respectively (note that T-c is enhanced in the latter two cases). It is found that Zn substitution for Cu(2) causes similar atomic displacements in YBa2Cu3O7 and YBa2Cu4O8. We also found that Zn doping on the Cu(2) site causes a shortening of the Cu(1)-O(1 ) bond length and an elongation of the Cu(2)-O(1) bond length in these two materials. These structural changes may induce the charge carrier s to transfer from CuO2 plane to CuO chain resulting in T-c decrease.