Chemical Bond Type, Atom Packing, and Superconductivity of YBa2Cu3O7 and   NaxWO3
  The YBa2Cu3O8-x and NaxWO3 superconductors are treated not as an ionic compounds but rather as an arrays of atoms featuring both covalent and metallic bonding. We accept as particle radii the positions of the principal maxima in the radial distribution functions of the charge density, which are very close to the known covalent and metallic radii of atoms while differing strongly from the traditional ionic radii. The graphic pattern of atomic packing in the lattices reveals a number of features inherent in nonstoichiometric compounds. The network of atoms Y and Ba in medium Cu-O and of atoms Na in medium WO3 may be considered not as a dopant but rather as the second component of the nanocomposite. The scheme of the onset of the superconducting state and possible methods of its verification are discussed. 
V.Bogomolov