PROPERTIES OF THE CHEMICALLY CHARACTERIZED THALLIUM CUPRATE SUPERCONDUCTORS

Thallium cuprates have been chemically characterized by adopting simple wet-chemical procedures for the determination of Tl and oxygen contents. The oxidation of the CuO2 sheets in the Tl2-yBa2Can-1CunO2n+4-x family is primarily due to (i) an overlap of the Tl-6s band with the conduction bond of the CuO2 sheets for smaller y≈0.0 and (ii) Tl vacancies for larger y≈0.5. In the intermediate region 0<y<0.5, the Tl-6s overlap decreases sensitively with increasing y. The unusual situation of a partially filled Tl-6s band causes an extrusion of Tl from the lattice as Tl2O3 upon exposure to air above 70°C. This remarkable observation will have important consequences for processing. In contrast, the Tl1-yBa2Can-1CunO2n+3-x family does not have a partially filled Tl-6s band is stable to oxidation without any Tl extrusion. Thallium cuprates have an oxygen deficiency in the Tl2O2 layers unlike the analogous bismuth cuprates, which have excess oxygen in the Bi2O2+x layer. The origin of the orthorhombicity, which appears after low-temperature Tl extrusion in the Tl2-yBa2CuO6-x system, appears to be due to an ordered extrusion of "TlO" units that may be stabilized by a cooperative tilting of CuO6 octahedra. © 1990.