Oxygen “disorder” and the structures of high-Tc superconductors by neutron powder diffraction

All of the high-T
c
perovskite superconductors appear to show disorder of certain oxygen atoms. In (La,Sr)
2
CuO
4
and perhaps also in YBa
2
Cu
3
O
7
this is associated with a structural transition. The Bi and Tl superconductors, for which we now have neutron structural data on four different phases, also show oxygen “disorder” which may be associated with valence fluctuations. In Tl
2
Ba
2
CuO
6
, electron holes are created by the absence of 1/8 of the atoms in the TlO plane, producing a marked superstructure. However, this material is not superconducting if the superstructure is well ordered, with an orthorhombic (strictly monoclinic) structure. The T
c
appears to depend on the disorder of the superstructure to produce a pseudotetragonal metric in which the oxygen atoms within the TlO plane are distributed over four equivalent sites about the center of the Tl square.