Ag-doping-induced coordination incompatibility and its effect on superconductivity in YBCO

A series of samples of YBa2Cu3-xAgxO7-y with 0 less than or equal to x less than or equal to 0.12 composition was studied to probe into the Ag substitution effect on oxygen stoichiometry, lattice parameters, and superconducting properties. With the samples prepared at a relatively lower sintering temperature, Ag could be doped in the grains rather than precipitate at grain boundaries. Thus, unlike in the case of YBCO+Ag composites or in doped systems. annealed at high temperatures where Ag occupies mostly the grain boundary, the present system showed a drastic change in T-c, oxygen stoichiometry, and lattice parameters with Ag concentration, indicating the substitution of Ag at the Cu(1) sites in the grains. The stable 2-fold oxygen coordination of Ag substituting Cu(1)'s explains the observed variation of oxygen deficiency with Ag. A crystallochemical analysis has been made to reveal the crucial role of Ag-substitution-induced coordination incompatibility and charge state instability on carrier concentration and T-c.