Elsevier

Physica C: Superconductivity

Volume 156, Issue 4, 1 November 1988, Pages 629-634
Physica C: Superconductivity

The influence of oxygen on the physical properties of the superconducting series Bi2.1(CaχSr1−χ)n+1Cun+4+δ

https://doi.org/10.1016/0921-4534(88)90033-0Get rights and content

Abstract

The sensitivity of Tc to oxygen sorption and the Ca to Sr ratio has been studied for the homologous series Bi2.1(CaχSr1−χ)n+1CunO2n+4+δ for n=1, 2 and 3. The n=0 and n=∞ members have been produced as single phase samples but are found to be semiconducting. While single phase n=2 material has been prepared, the n=1 member always displays a small n=2 contribution to the X-ray diffraction pattern. n=3 is only found in multiphase samples. Single phase superconducting samples were equilibrated under a fixed partial pressure of oxygen at temperatures between 300 and 850°C and quenched in liquid nitrogen. It is shown that Tc can be controlled in a reversible and systematic way by varying the oxygen partial pressure and the annealing temperature and consequently the oxygen stoichiometry. The site energy per oxygen sorbed for n=2 is 3.24 eV, only a little less than that for YBa2Cu3O7. However, the volume change on sorption is ten times smaller than that for YBa2Cu3O7, and is manifested as a changing c-axis with the a-axis invariant. This can be interpreted as implying that oxygen sorption occurs within the Bi-O layers. Tc is found to be maximised for a Ca to Sr ratio of 1 to 2 for both n=1 and 2. Thermal expansion data for n=2 material is also reported to illustrate that oxygen sorption does effect the physical properties of these materials.

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