Electron density distribution in Ba[Pb.sub.1-x][Sb.sub.x][O.sub.3] superconducting oxides studied by double nuclear magnetic resonance methods

The effect of charge disorder on the formation of an inhomogeneous state of the electron system in the conduction band in Ba[Pb.sub.1-x][Sb.sub.x][O.sub.3] superconducting oxides is investigated experimentally by NMR methods. The NMR spectra of [sup.17]O are measured systematically, and the contributions from [sup.17]O atoms with different cation nearest surroundings are identified. It is found that microscopic regions with an elevated spin density of charge carriers are formed within two coordination spheres near antimony ions. Nuclei of the superconducting phase of the oxide (regions with an elevated antimony concentration) microscopically distributed over the sample are detected in compounds with x = 0.25 and 0.33. Experiments in which a double resonance signal of the spin echo of [sup.17]O-[sup.207]Pb and [sup.17]O-[sup.121]Sb are measured in the metal phase of Ba[Pb.sub.1-x][Sb.sub.x][O.sub.3] oxides are carried out for the first time. The constants of indirect heteronuclear spin--spin [sup.17]O-[sup.207]Pb interaction are determined as functions of the local Knight shift [sup.207][K.sub.s]. The estimates of the constants of the indirect interaction between the nuclei of the nearest neighbors (O--Pb and Pb-- Pb atoms) and analysis of evolution of the NMR spectra of [sup.17]O upon a change in the antimony concentration are convincing evidence in favor of the development of a microscopically inhomogeneous state of the electron system in the metal phase of Ba[Pb.sub.1-x][Sb.sub.x][O.sub.3] oxides.