Chemical state of Tl in the superconductor ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10}$ studied by x-ray photoelectron spectroscopy

Tl 4f core levels of ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{10}$, ${\mathrm{Tl}}_2$ ${\mathrm{O}}_3$, and ${\mathrm{Tl}}_2$O were measured by x-ray photoelectron spectroscopy in order to elucidate the chemical state of Tl ions in the superconducting oxide. The Tl 4$f_{7/2}$ level is found at 117.4, 118.2, and 118.6 eV for ${\mathrm{Tl}}_2$ ${\mathrm{O}}_3$, ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{10}$, and ${\mathrm{Tl}}_2$O, respectively. This result reveals that the Tl valence is between +3 and +1. The creation of holes in ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{10}$ is discussed in terms of a charge transfer between Tl ions and [Cu-O] layers. Furthermore, the stabilization of double [Tl-O] layers is also discussed from the viewpoint of structural chemistry.