ELECTRONIC-STRUCTURE AND CRYSTAL-CHEMISTRY OF TLBA2CUO5 AND RELATED CUPRATE SUPERCONDUCTORS

In contrast to the fixed Cu valence (2+) in the Tl-bilayer cuprate superconductors, the average formal valence of Cu in the Tl-monolayer compounds TlBa2Can-1CunO2n+3 varies as (2+n-1)+. This characteristic is reflected in linear augmented-plane-wave band-structure results for the simplest n=1 member of this Tl-monolayer homologous series, TlBa2CuO5, where the filling (0.16) of the planar Cu(3d)-O(2p) * band is reduced well below one-half. It is shown that the 50-50 Ba-La alloy is an appropriate parent compound for this n=1 phase since the half-filled-band condition is restored. For any member of this Tl-monolayer series, the optimal doping for high-temperature superconductivity should involve a combination of structural and chemical contributions. © 1990 The American Physical Society.