Stabilization of On and High Oxidation States

We might well have asked why 3d-4s hybridization on Cu1 did not lead to an egg shaped core instead of a disk shaped core. To a first approximation, these two situations would indeed to be degenerate. With hybridization to an egg shaped core, the occurrence of Cu1 in three-fold of four-fold planar coordination should be no surprise. This, of course, actually happens in the n-type copper oxides such as Nd2 xCexCu04. 

High polarizibility of atoms or ions requires easy hybridization and/or large size. We see that for both copper and bismuth we have ample opportunity for hybridization that could lead to high polarizibility. For copper, it is 3d-4s hybridization for bismuth, it is 6s-6p hybridization. We expect this polarizibility to be more pronounced for the lower oxidation states of bismuth and copper. Oxygen polarizibility (2s-2p) may also be significant. 

Another type of polarizibility results from the near degeneracy of the metal levels and the oxygen 2p levels. This is directly related to the high covalency in these systems thus, this type of polarizibility will be greater for the higher oxidation states of copper and bismuth. Both of these polarizibility contributions are likely very important for theories of superconductivity based on charge fluctuations. 

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