Abstract
A relation between T c and the density of holes per CuO 2 plane is presented for Bi 2Sr 2CaCu 2O 8+δ. The hole concentration was varied by replacing calcium in Bi 2Sr 2CaCu 2O 8+δ partly by yttrium, and by changing the oxygen content of samples of Bi 2Sr 2CaCu 2O 8+δ, Bi 2Sr 1.5Ca 1.5Cu 2O 8+δ and Bi 2Sr 2Ca 0.9Y 0.1Cu 2O 8+δ. The oxygen content of samples of Bi 1.7Pb 0.3Sr 2Ca 0.7Y 0.3Cu 2O 8+δ could not be changed, which indicates stabilization of the crystal structure as a result of bismuth being replaced by lead. The hole concentration was derived from determinations of the oxidizing power. The phase diagram of Bi 2Sr 2CaCu 2O 8+δ shows an unambiguous relation between T c and the density of holes per CuO 2 plane. Superconductivity does not occur at doping levels up to about 0.07 holes/Cu per CuO 2 plane. With increasing hole content T c shows a broad maximum of about 90 K at about 0.2 holes/Cu per CuO 2 plane. At higher doping levels T c starts to decrease. The phase diagram presented is comparable to that reported for YBa 2Cu 3O 6+ y and La 2− x Sr x CuO 4, indicating that these phase diagrams are a universal property of p-type high- T c cuprate superconductors.
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