Lattice properties of YBa2Cu3?xCoxO7: Pilot calculations using effective medium theory

We report preliminary total energy calculations for YBa₂Cu_3–xCo_xO₇ of the lattice parameters and of the Raman-active phonon modes, particularly of the apex oxygen, within the so-called effective-medium theory. Experimentally it is found that substitution of Cu(1) by Co (or Al) introduces a correlation between the apex oxygen mode and the critical temperature, and that this correlation reflects the variation in distance between the O(4) and Cu(2), that decreases with doping, and between the Ba and Cu(2), that on the contrary increases with doping. The picture that emerges from this study is the following: as the Co (or Al) atoms partially substitute the Cu(1), there is a reduction of the hole carriers in the plane. This picture is supported by the present pilot calculation.