THE INFLUENCE OF COMPOSITION ON POLYMORPHISM, STRUCTURE AND CHARGE-CARRIER CONCENTRATION IN TL-2201

Structural and superconducting variations caused by changing the stoic hiometry in Tl-2201 have been studied by X-ray and neutron powder diff raction, susceptibility measurements and NMR spectroscopy. All samples have been prepared by a new method with the possibility to control th allium losses. The effects of changing thallium and oxygen content, su bstituting the Ba site with Sr and La, substituting the Cu site with C o and Fe and moderate heat treatments have been investigated. The phas e transition between orthorhombic and tetragonal form of Tl-2201 can b e induced by changing the oxygen and/or thallium stoichiometry and by substitute the Pa site by Sr. The orthorhombic form can also be stabil ised by solely mild heat treatment. This results in less disordered Tl 2O4 layers.