  Superconductivity of some compounds may be explained as resulting from
Bose-Einstein condensation (BEC) of atomic electron pairs of divalent atoms or
electron pairs of diatomic molecules made up of univalent atoms. "Artificial"
superconductors of such types can be tailored using nonstoichiometric ompounds.
Synthesis of "natural" stoichiometric superconductors is a much more
complicated problem. In these cases, we have two methods of obtaining dilute
metals in a state intermediate between the metal and the insulator.
