LIMITS TO TC IN OXIDE SUPERCONDUCTORS STUDIED BY NEUTRON POWDER DIFFRACTION

The T(c) of electron-hole doped superconductors appears to be controlled by charge transfer to the CuO-planes from intermediate "charge reservoir" layers, such as CuO-chains, BiO-planes, etc. We must understand the mechanisms of and limits to the charge transfer in order to understand the effect of different intermediate layers, and the doping of these layers with other cations. In this paper we will review neutron diffraction measurements of changes in Cu-0 distances indicating charge transfer in YBa2Cu3O7 on oxidation-reduction, and in YBa2Cu4O8 by the application of external pressure. New measurements on YBa2Cu3O7 indicate a possible explanation of why T(c) is limited to 90 K, while in similar materials it can go as high as 125 K.