Influence of Ba content and Ce doping on the structural features of YBa2Cu4O8 superconductor—a neutron study

The structures of A: YBa₂Cu₄O₈ (T _c=80 K), B: Y_0.9Ce_0.1Ba₂Cu₄O₈ (T _c=5K), and C: YBa_2.2Cu₄O₈ (T _c=80 K) have been investigated using the powder neutron technique with a view to assessing the influence of the bridging Cu(2)-O(4) and Cu(1)-O(4) bonds on the superconducting transition temperature. The Cu(2)-O(4) bond for samples A and C (withT _c=80 K) appears to be compressed as compared to the reported data. On the other hand, the substitution of Ce at 10% Y sites elongates the Cu(2)-O(4) bond substantially and also brings theT _cdown to 55 K. The role of Cu(2)-O(4) bond length as well as other parameters expected to influence theT _cbehavior of Y-124 samples are discussed in the light of the present data. The study also shows that the ambient oxygen pressure route, if tried with the starting composition YBa_2.2Cu₄O _y , leads to a quite homogeneous 124-phase.