Tl_2-xCd_xBa₂CuO_6+δ: Varition of T_c with cadmium substitution

The compositions Tl_2-xCd_xBa₂CuO_6+δ have been prepared in the range of x=0 to 0.7 using either BaO or BaO₂ as sources of barium and oxygen. The materials are superconducting with T_c varying smoothly as x increases. When the materials is prepared using BaO₂ as the Ba source, T_c falls progressively from 92 K to 0 K as x proceeds from 0 to 0.5. When BaO is the Ba source, however, T_c rises from 65 K (x=0) to 90 K (x=0.2) before falling to 0 K at x=0.7. These behaviors indicate 1) that T_c is very sensitive to the average Cu oxidation state (hole concentration), 2) that the hole concentration can be adjusted either by oxygen excess (δ) or by cation doping (x). Metallic behavior is observed in the region x>62;0.6. High resolution electron microscopy, X-ray microanalysis and a structural refinement based on neutron powder diffraction data for Tl_1.75Cd_0.25Ba₂CuO_6+δ (14/mmm a=3.851(1) Å, c=23.313(8) Å) confirm Cd²⁺ substitutes exclusively for thallium. A comparison with the structure of Tl₂Ba₂CuO_6+δ shows a decrease in the Cu-O bond length correlating with oxidation of the CuO₂ sheet. An increase in the c-axis results from the substitution of the larger Cd²⁺ for Tl³⁺. X-ray absorption measurements suggest this substitution has no effect on the average oxidation state of the thallium in these materials.