Dilute Metals
  Superconductivity (Tc), like any other property of a condensate, depends critically on the concentration of atoms. "Physical" dilution of metals exists in nonstoichiometric compounds. In such stoichiometric compounds as oxides, oxygen initiates "chemical" dilution of the metal, but its efficiency can be estimated only if the real radius of the oxygen ion, r0 ~ 0.5 A, is used in the calculation. The ground-state radii of metal atoms rm ~ (1.3--2.0 A) >> r0, so that atoms of metals occupy in the lattice ~90% of the total volume. Therefore, the lattice parameter and the electronic properties are determined by the metal-atom ground states. (For TiO2, the parameter c = 2.95 A ~ 2rTi = 2.94 A; for TiO, a*sqrt(2) = 5.99 A ~ 4rTi = 5.90 A; for BaTiO3, c = 4.05 A ~ 2rBa = 4.12 A; for Y-Ba-Cu-O, c = 11.68 A ~ (4rBa + 2rY) = 11.63 A, and so on.) Each atomic quantum state can be identified with a specific physical property of the condensate. As a result of superposition of the ground and ionic states of each metal atom, the fraction of the ground states decreases at the expense of the oxygen-excited ionic ones   (thus reducing the effective concentration of atoms). The bands narrow down (in the limit, to the electron localization length), with the metal becoming an insulator, as is the case, for instance, with YBa2Cu3O8-x as x decreases from xM to 0. Conceivably, reducing the lattice deficiency in oxygen by saturating part of its valence bonds with H, Li, or B atoms could increase the Tc (YBa2Cu3O7-x ~ YBa2Cu3O7-x(1 + x)H2O). "Self-dilution" of lattices of excited atoms by ground-state atoms was observed to occur in Pd and inert gases. 
V.Bogomolov