Physics – Condensed Matter – Superconductivity
Scientific paper
2005-05-30
Physics
Condensed Matter
Superconductivity
3 pages, 1 figure
Scientific paper
Traditionally, when one describes the crystallographic structure of oxides, the oxygen ion radius r02 is assumed to be approximately equal to 1.4A. The oxygen ions occupy in this case 80-90% of the crystal volume. Metal atoms are considered then as ions playing a role of donors with rather small radius of (0.5 - 0.8) A. However, the atomic packing picture and, therefore, physical properties such as electric conductivity and superconductivity of oxides will be essentially different, if we assume r02- ~ 0.56 A. Such magnitude of the radius is known from the quantum mechanics calculations [2]. According to this picture, 80-90 % of the crystal volume is occupied by the metal atom orbitals with radius (1.3-1.9) A, while the oxygen ions play a role of acceptors, which reduce occupancy of these orbitals ("indirect" dilution of the metal). A "direct" dilution of metals takes place in stoichiometric matrices. When r02-~0.56 A, channels with diameter 3.6 A present in the hexagonal matrix Ag3Pb2H2O6 directed along the "c" axis. The channels are filled by chains of the Ag2 molecules with atomic diameter of 2.6 A and the molecule concentration n_B = 25.6 10^20 cm^-3. The Bose-Einstein condensation (BEC) temperature T_cB ~ 400K is calculated for the electron effective mass value m*=7.5m_e, where m_e is the isolated electron mass. Three-dimensional networks of Na2 chains form in Na solutions in NH3 and WO3 as well (respectively, with n_B=4.71 10^20 cm^-3; T_c sim 180K; m*=5.0m_e and n_B=4.74 10^20 cm^-3; T_c sim 91K; m*=10m_e). Close magnitudes of the Ag2 and Na2 chains parameters respectively in NH3 and WO3 favors the opinion that all these structures have a composite structure and similar mechanisms of the high temperature superconductivity.
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