  High-purity and homogeneous YBa2Cu3Oy single crystals with carrier doping
level near the AFM-SC boundary have been obtained in the oxygen content range
between y = 6.340 and 6.370. The crystals are ortho-II phase at room
temperature and undergo the orthorhombic to tetragonal transition at about
140_Degree_C. They show sharp superconducting transitions, with Tc between 4
and 20 K. Tc changes by 0.8 K when the oxygen content y is changed by 0.001,
and is also sensitive to annealing conditions near room temperature, due to the
dependence of doping on oxygen ordering correlation lengths. Crystals with
oxygen content y lower than 6.345 are non-superconducting.
