Hole concentration and critical temperature in Tl2 –y Ba2 –zLazCuO6 –x

Abstract

Samples of the system Tl_{2 –y} Ba_{2 –z}La_zCuO_{6 –x} with starting values of y= 0.00 and 0.35 for 0 ⩽z⩽ 0.2 have been synthesized at 860–880 °C in sealed gold tubes and then quenched in liquid N₂. The lattice parameters and critical temperature T_c, were measured, and chemical analysis was made for y and x of the single-phase products before a further anneal at 350 °C in 1 atm O₂. During the O₂ anneal, samples with a starting value of y= 0.00 extruded thallium to form Tl₂O₃ and transformed from tetragonal to orthorhombic; samples with a starting value of y= 0.35 did not. The data indicate that the superconductive samples are p-type at the high oxidation side of the range of superconductor solid solutions and that all samples contain Tl 6s electrons. Without a quantitative knowledge of the concentration of Tl 6s electrons, it was not possible to obtain a quantitative plot of T_cversus the hole concentration, p, per Cu atom in the CuO₂ sheets, but for Δp= 0.10, ΔT_c=–52 °C, showing that T_c drops sharply with increasing p. We also conclude that Tl extrusion to form Tl₂O₃proceeds by an ordered removal of Tl until the Fermi energy E_F falls below a mobility edge in the Tl 6s band.