Transport, Structural, and Thermogravimetric Studies of LANTHANUM(1) - BARIUM(2) - COPPER(3) - OXIDE(7-DELTA): Correlations to the Structure and Empirical Superconducting Mechanism.

In this study, the different high temperature superconductor structures are discussed in detail, particularly the differences in the Cu-O structure. These structures are compared to the structure of La_1Ba _2Cu_3O _{7-delta}. The report by this investigation, of tetragonal La_1Ba _2Cu_3O _{7-delta} superconductors at the April (1987) meeting of the Materials Research Society, was one of the first reports of a tetragonal 1:2:3 structure (presumably without CuO "chains") in the superconducting state. This report indicated that the superconducting mechanism was directly inherent to the Cu-O square pyramidal "plane" structure, i.e. the superconducting mechanism was likely two-dimensional. The further structural study on the La_1Ba_2Cu _3O_{7-delta } series, provided information that the Cu -O "chains" were important for obtaining the highest values of T_{rm c} in the 1:2:3 structure. These structural studies were also important in ruling out the "3:3:6" structure that had been proposed as existing in this superconductor series. Changes in processing involved two different preparation methods (the coprecipitation and solid state reaction methods), and changes in processing variables such as temperature, ambient atmosphere (oxygen, air, argon), and time. The range of zero-resistance transition temperatures (T_ {rm c}'s) was markedly different between the coprecipitation and solid state reaction methods. The use of oxygen, air, or argon during the heating process also had substantial effects upon the transport properties and structure. In particular, oxygen diffusion or distribution in the structure was found to be very important for the superconducting state.