Effects of oxygen and strontium vacancies on the superconductivity of single crystals of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{CuO}}_{6\mathrm{-}\mathrm{y}}$

Single crystals of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{CuO}}_{6\mathrm{-}\mathrm{y}}$ (2201 phase) were grown from CuO-rich melts. The Sr concentration in the crystals was varied from x=0.1 to 0.5 by adjusting the starting composition of the melt, and the oxygen content of the crystals was reversibly adjusted between y=0 and 0.5 by an appropriate heat treatment of the crystals in a thermogravimetric system. With decreasing Sr content the superconducting transition temperature, $T_c$, of the crystals decreased rapidly from 9 K to below 4.2 K, and the resistivity in the a-b plane changed from metallic (linear in T from 30 to 300 K) to semiconducting. Reducing the oxygen content in the crystals had a similar effect on the resistivity. Only crystals with close to the maximum oxygen content (y=0) were superconducting, and removal of oxygen from previously superconducting crystals resulted in a rapid decrease of $T_c$ and the eventual loss of superconductivity ($T_c$<4.2 K). A bond-valence sum analysis suggests that oxygen is removed from the ${\mathrm{Bi}}_2$ ${\mathrm{O}}_2$ layers.