Neutron Powder Diffraction on the Ill High Flux Reactor and High TC Superconductors

Abstract

Neutron powder diffraction, with both reactor and pulsed neutron sources, has been important for our understanding of the structures of the oxide superconductors. These structures appeared very different when seen with neutrons, which emphasized the oxygen lattice, compared with X-rays, which emphasized the heavy metal atoms. Neutron diffraction first demonstrated the two dimensional nature of the early materials, and this lead to the successful search for new 2D superconductors based on the well known ‘Aurivillius’ structure. Neutron powder diffraction also played an important role in the developement of ‘charge transfer’ ideas of the electronic doping mechanism in oxide superconductors. Although it is not known if these ideas are relevant to the superconducting mechanism itself, they have guided chemists in the discovery of new materials. They have also helped us understand why oxidation sometimes increases Tc, and sometimes kills superconductivity, and how the distribution of electron holes between the different layers of a superconductor can be changed by effects such as pressure, which often produce unexpectedly large changes in Tc.