From oxides to oxyhalides: modelling the properties of high TCsuperconductors

Abstract

Atomistic computer simulation techniques are used to investigate the defect properties of high temperature superconductors, in an attempt to illustrate the type of information that can be obtained from the modelling of these complex materials. Attention is focused on two systems: the mercury cuprate HgBa₂CuO_4+δ, which is the first member of the homologous series HgBa₂Ca_n-1Cu_nO_2n+2+δ, and the copper oxychloride Ca₂CuO₂Cl₂, which is part of a new family of oxyhalide superconductors. Effective interatomic potentials are used to reproduce accurately their observed crystal structures. A range of redox and dopant substitution reactions relevant to high T_C behaviour have been examined. The results are consistent with the experimental observation that HgBa₂CuO_4+δ becomes superconducting by oxygen incorporation at interstitial sites, while Ca₂CuO₂Cl₂ shows superconductivity only on doping with alkali metal ions at the calcium site. We also consider the energetics of dopant substitution on the copper sublattice of the oxychloride. Our study indicates the value of computer simulation methods in examining the nature of defect reactions at the atomic level.