STOICHIOMETRY AND T(C) OF BI-2201 DOPED WITH LA, PR, ND AND PB

The T(c) of rare-earth doped Bi-2201 depends on the post-sintering treatment, with a change of behaviour occuring around x=0.40 in the formula Bi1.95Sr2.05-xRxCuOq, R= La, Pr and Nd. For x less-than-or-equal-to 0.4, samples prepared in air are overdoped and T(c) is increased by annealing in argon. For a given x, T, is independent of the magnetic moment and size of the rare-earth ion. T(c) increases with x and passes through a maximum of 36 K at x=0.40. For x>0.4, samples prepared in air are underdoped and T(c) can be increased by annealing in oxygen; superconductivity is found for x less-than-or-equal-to 0. 80 but so far has not been attained at higher x. Solubility limits are x= 1.30 (La), 0.80 (Pr) and 0.60 (Nd). The effect of Sr deficiency, starting from the composition x = 0.40, is to decrease T(c); the decrease is greater after O2 annealing treatment, indicating that Sr deficient samples are overdoped. Substitution of Pb for Bi in the composition x= 0.40 does not affect T(c) but does lead to an increase in sample crystallinity, as shown by the sharpness of X-ray powder diffraction peaks and to an increase in the superconducting volume fraction, as shown by an increase in the diamagnetic signal at T< T(c).