Boron Ion Doping Effects on Superconductivity of YBa₂Cu_3-xB_xO_7-y Ceramics

Impurity effects of small ionic size, light atomic weight boron ions on the superconductivity of YBa₂Cu₃O_7-y are investigated. The doped boron content x is from 0.00 to 0.52 in YBa₂Cu_3-xB_xO_7-y as a nominal composition, and the ceramic samples fired in air or annealed in high-pressure oxygen gases are examined with X-ray diffraction, dc electrical resistivity, and ac magnetic susceptibility. The crystal lattice is found to shrink due to the boron doping and to deform from orthorhombic to tetragonal form for values of x of more than about 0.2. Superconductivity is found to be modified by the partial B³⁺ replacement for Cu(1) ions so as to lower the critical temperature T_c and to broaden the transition width. Change from metallic behavior to semiconductive behavior is accompanied by the boron incorporation of x above 0.30 in the normal electrical conduction states. Steep transition is revealed on the high-pressure oxygen gas annealed samples.