Sulfur effect on superconducting properties in YBaCuO ceramics

Influence of sulfur substitution for oxygen on superconducting and structural properties of YBa2Cu3OxSy ceramics was investigated. Sulfur was introduced to tetragonal YBa2Cu3O6 lattice. Mixture of YBa2Cu3O6 and sulfur powders (y=0-0.8) was annealed at π=350 deg C in vacuum, then - in the air at the same temperature during several hours. The samples remained tetragonal and nonsuperconducting after the first stage of heat treatment. According to RPA data, lattice parameters of ceramics with different y didn't change; sulfur probably, doesn't enter the crystal lattice of YBaCuO. Samples became rhombic and superconducting after the second stage of heat treatment. Some improvement of superconducting properties of YBa2Cu3O6Sy ceramics was observed for small y (Y<0.15), as compared to y=0. This can be related with the fact, that small amounts of sulfur stabilize the orthophase, being the catalyst of synthesis processes and promote more rapid oxygen diffusion in YBaCuO lattice over the whole sample volume