Hole concentration and critical temperature in Tl2 –y Ba2 –zLazCuO6 –x
Samples of the system Tl2 –y Ba2 –zLazCuO6 –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 Tc, 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 Tcversus the hole concentration, p, per Cu atom in the CuO2 sheets, but for Δp= 0.10, ΔTc=–52 °C, showing that Tc drops sharply with increasing p. We also conclude that Tl extrusion to form Tl2O3proceeds by an ordered removal of Tl until the Fermi energy EF falls below a mobility edge in the Tl 6s band.