No abstract

We report a systematic study of a series of single-phased LaiBa2Cu307-& samples where the oxygen content was determined by thermogravimetric analysis. The highest zero-resistance T, was 72 K, corresponding to 8 0.25. An increase in 8 reduced T, to 44 K, similar to the behavior reported for Y&Ba2Cu307-&. This is consistent with the idea that, as oxygen is taken away from the planes where Cu-0 chains exist, T, is reduced. For samples where the oxygen content was increased until 8 became negative, T, decreased to a value as low as 23 K, presumably due to increased La-Ba disorder. These samples with low T, and high oxygen content have apparent tetragonal x-ray spectra, suggesting that Cu-0 chain ordering has been further disrupted and that the oxygen occupancy on these planes has increased. Extensive experimental studies have been focused on the high-T, superconductor Y|BazCu307-s. Detailed neutron studies on this'2 compound as well as several structural studies on 1:2:3compounds with Y replaced by rare-earth elements,~~have associated the orthorhombic sequential structure with high zero-resistance T, 's near 90 K. The orthorhombicity is attributed to the ordering of oxygen vacancies in the copper planes between the Ba layers such that ordered Cu-0 chains remain. While the exact role of these Cu-0 chains is unclear, they are thought to be important to the high superconducting transition temperature either directly or through their interaction with the inner Cu-0 sheets. Recently, considerable attention has been focused on the compound LaiBa2Cu307s and its variations where different levels of lanthanum are doped into the barium sites. s '7 X-ray spectra on samples of the La|Ba2Cu307-s compound with low T, 's have been reported to appear tetragonal, although orthorhombic shoulderin~has been observed in samples with T, 's above 60 K. Further x-ray studies on samples of La|BazCu307-s with zero resistances as high as 80 K concluded that the samples with high T, 's were indeed orthorhombic. s7 This strengthens the idea that ordered Cu-0 chains play a very important role in these so-called 1:2:3 superconducting RiBa2Cu307-s (R is Y, La, and other rare-earth elements) materials, and that for these samples with T, 's approaching the 90-K plateau, an orthorhombic structure is required.The importance of the Cu-0 chains has been questioned by the observation that there exist certain LalBa2Cu3-07s samples that possess an apparent tetragonal structure but still become superconductors below 40 K. While several quenching studies on the Y|Ba2Cu307s compound have suggested an empirical relation between the orthorhombic distortion and T, (with the tetragonal phase being nonsuperconducting), a similar relationship has yet to be established in La|Ba2Cu307 s In thispa.per, we report a systematic study of the dependence of T, on the oxygen content (x) in La|Ba2Cu30q-s samples as measured by thermogravimetric analysis (TGA). The samples are predominantly single-phased with T, 's ranging from 23 to 72 K. Our highest zero-resistance transitio...