Valence charge fluctuations in Tl_2-x-zBa₂Ca_2+xCu₃O_10-y system

Abstract

Systematic XPS core level studies and wet chemical analysis in the Tl_2-x-zBa₂Ca_2+xCu₃O_10-y system have been carried out for the "as synthesized" samples. Broadening of Ca 2p core level and ionic radii considerations suggest that calcium substitutes at the Tl site. The valence state of thallium in Tl2223 is between +3 and +1, and substitution of calcium at thallium site oxidizes thallium to Tl(III) to optimize the hole concentration and maximize T_c. Origin of holes in this system is discussed in terms of charge transfer between Tl (or Tl, Ca) and CuO₂ layers. The Cu 2p_3/2 core level shows that the relative intensity of satellite, the energy separation between main line and the satellite and binding energy of main line varies in a systematic way. The observed variations are explained with a simplified configuration interaction model. An increase in charge transfer energy and a decrease in hybridization strength is predicted for higher Ca substitution.