207Pb and 17O NMR Study of the Electron Density Distribution in Metal Phase of BaPb_{1-x}Bi_xO_3

The 17 O and 207 Pb NMR spectra were measured in ceramic samples in the metallic phase of BaPb 1 − x Bi x O 3 oxides (0 < x ≤ 0 . 33). The inhomogeneous magnetic broadening which appears due to a distribution of the Knight shifts was analyzed in detail. It is shown that Bi atoms, which are randomly incorporated in BaPbO 3 parent compound give rise to an increased conduction electron spin density 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 it is accompanied by a sharp growth of the average Knight shift < K s > . The decrease of < 17 K s > with temperature revealed for x = 0 . 33 evidences for an opening of the energy gap near E F near the metal-semiconductor transition ( x = 0 . 35).