COMPUTER SIMULATION OF HIGH TEMPERATURE SUPERCONDUCTORS

Abstract

Computer simulation techniques have been used to study the structural changes caused by Zn, Ce, Pr, Th, Ca, Sr, Fe, Co and Ni doping in YBa2Cu4O8 . Our results suggest that Zn energetically prefers to substitute at the Cu(2) site whereas Pr and Th prefer to substitute at the Y site. These results are in agreement with the experimental results. Our results also suggest that Ce preferentially substitutes for the Cu(2) site. The detailed analysis of the structural changes caused by Zn, Ce, Th and Pr doping were carried out, and the results were compared with the results of Ca, Sr, Fe, Co and Ni doping in YBa2Cu4O8. We found that beneficial dopants (Ca and Sr) and deleterious dopants (Pr, Th, Zn, Ce, Fe, Co and Ni) caused very different structural changes in Cu(2)-O(1), Cu(1)-O( 4'), z(Ba)-Z(O(1)), Cu(2)-O(2) and Cu(2)-O(3) It appears that Ca-doped YBa2Cu4O8 is under "positive chemical pressure" while deleterious dopants doped YBa2Cu4O8, behave as if it is under "negative chemical pressure" Our calculated changes in lattice parameters of YBa2Cu4O8 due to Th and Ce doping are in qualitative agreement with the experimental results. Atomistic simulation techniques were used to determine the Zn substitution sites in YBa2Cu4O8 (123) and YBa2Cu4O8 (124). It was found that Zn dopant is energetically favourable to substitute at the Cu(2) site. Zn substitution at both Cu(1) only and Cu(2) sites is also possible, but Zn substitution at Cu(1) site only seems unlikely. Our result implies that most Zn dopant go to the Cu(2) sites and a small percentage of Zn dopant goes to the Cu(1) sites, which is in agreement with the experimental results. We also found that the structural changes in Zn-doped 124 and Zn-doped 123 are opposite to that in 124 under pressure and in YBa2Cu4O8 under pressure respectively (note that Tc 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 carriers transfer from CuO2 plane to CuO chain resulting in Tc depression. A correlation of the distance Cu(2)-Cu(2) with dTc/dP is established for YBa2Cu4O8 and YBa2Cu3O6.6 . A model, proposed by Zhang and Catlow, is used and it yields dTc/dP = 5.07 KGPa-1 for YBa2Cu4O8 and dTc/dP = 4.65 KGPa-1 for YBa2Cu3O6.6 both of which are in excellent agreement with the experimental data. It is also found that YBa2Cu3O6.6 has similar pressure induced structural changes as YBa2Cu4O8, but different from YBa2Cu3O6.93. The structure of HoBa2Cu4O8 was filled using the 'relaxed" fitting approach. The set of potential parameters obtained reproduced the structure with deviations from experimental data smaller than 0.3% in the a- and b- directions and less than 1.9% in the c-direction. Computer simulation techniques have been used to study the structural changes caused by Zn, Ce, Pr, Th, Ca, Sr, Fe, Co and Ni doping in HoBa2Cu4O8. We found that Sr-doped HoBa2Cu4O8 has similar trend as Sr-doped YBa2Cu4O8 and we deduce that Sr-doped HoBa2Cu4O8 enhances Tc. Similarly Pr, Ce, Fe, Co and Ni dopants cause similar structural changes in HoBa2Cu4O8 and YBa2Cu4O8 and we deduce that these do pants depress Tc.