Oxidation state of mercury and lead in Hg0.66Pb0.34Ba2Ca2Cu3O8+δ studied by X-ray photoelectron spectroscopy

Abstract We have carried out the X-ray photoelectron spectroscopic (XPS) measurements on several samples of Hg 0.66 Pb 0.34 Ba 2 Ca 2 Cu 3 O 8+δ superconductor. To our surprise and contrary to general expectations, the spectra from the fresh surfaces of fractured samples indicate that the Pb ion exists in both of +2 and +4 valence states, and its average oxidation state is higher than +2; while the Hg ion maintains almost at +2 ionic configuration. These results suggest that the Pb atom acts as an electron reservoir and plays an important role in the superconductivity of the (Hg,Pb)-1223 phase. Finally, the creation of charge carriers in (Hg,Pb)-1223 is discussed in terms of a charge transfer between (Hg,Pb) O σ and Cu O layers through their apical oxygen. Furthermore, it is also found that in the (Hg,Pb)-1223 superconductor the hole density is approximately uniformly shared between the three CuO 2 planes.