207Pb and 17O NMR study of the electron density distribution in the metallic phase of BaPb1-xBixO3

The 1 7 O and 2 0 7 Pb NMR spectra are measured in ceramic samples in the metallic phase of BaPb 1 - x Bi x O 3 oxides (0Knight shifts is analyzed in detail. It is shown that Bi atoms, which are randomly incorporated in a BaPbO 3 parent compound, give rise to an increased spin density of carriers within an area which is delimited by its two first cation shells. According to NMR data the percolative overlap of these areas occurs in superconducting compositions, and is accompanied by a sharp growth of the average Knight shift (K s ). The decrease of ( 1 7 K s ) with temperature revealed for x=0.33 evidences that a pseudogap develops near E F even in a metallic phase near the metal-semiconductor transition (x c r =0.35). Our observation supports photoemission results of Uchida et al. [Phase Transit. 8, 95 (1987)] which indicate the opening of pseudogap in superconducting Ba( Pb,Bi)O 3 oxides near x e r .