Cuprate superconductors: phase stability of 123 compounds, novel features of $Bi_2Ca_{1-x}Ln_xSr_2Cu_2O_{8+\\delta}$ (Ln=rare earth) and new thallium cuprates

It is shown that $YBa_2Cu_3O_{7.\\delta}$ $(T_c \\sim 90 K)$ is stable and monophasic over the \\delta=0.0-0.2 range; and \\delta=0.25 to 0.5 compositions are monophasic with well-ordered oxygen-deficient structures. The \\delta=0.3-0.4 compositions $(T_c \\sim 60 K)$ seem to be metastable, undergoing transformations on annealing at 470 K for several hours. The 124 and 247 cuprates of a few rare earths have been prepared and characterized by the ceramic method. These cuprates form intergrowths with each other or with the 123 cuprate. Such epitaxial relations may be of significance in using them in materials applications. $Bi_2Ca_{1-x}Ln_xSr_2Cu_2O_{8+\\delta}$ (Ln=Y or rare earth) shows a maximum in the hole concentration as well as Tc around x=0.25. Thallium cuprates of the type $TlSr_{n+1-x}Ln_xCu_nO_{2n+3}$ and $Tl_{1-x}Pb_xSr_{n+1}Cu_nO_{2n+3}$ not containing Ca and Ba have been prepared and characterized for the first time.