OXIDATION-STATE OF MERCURY AND LEAD IN HG0.66PB0.34BA2CA2CU3O8-RAY PHOTOELECTRON-SPECTROSCOPY(DELTA STUDIED BY X)

We have carried out the X-ray photoelectron spectroscopic (XPS) measur ements on several samples of Hg0.66Pb0.34Ba2Ca2Cu3O8+delta superconduc tor. 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 ioni c configuration. These results suggest that the Pb atom acts as an ele ctron reservoir and plays an important role in the superconductivity o f 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-delta and Cu-O layers through their apical oxygen. Furthermore, it is also found that in the (Hg,Pb)-1223 superconductor the hole densit y is approximately uniformly shared between the three CuO2 planes.