Inhomogeneous state of the electron system in BaPb1 − xSbxO3 superconducting perovskites: The 207Pb NMR study

Comprehensive measurements of the ²⁰⁷Pb NMR spectra, Knight shifts ²⁰⁷ K _s , spin-lattice relaxation rate ²⁰⁷ T ₁⁻¹, and the Pb spin echo decay ²⁰⁷ E(2t) in BaPb_{1 − x} Sb _x O₃ superconducting oxides (x ≤ 0.33) have been performed. A considerable increase in the shift distribution width when Sb is substituted for Pb indicates the formation of the inhomogeneous state of the electron system in the oxide conduction band. The shift magnitude proportional to the density of states near the Fermi energy, ²⁰⁷ K s ∝ N(E _F ), reaches its maximum in the compounds with x = 0.18–0.25 with the maximum temperatures T _c of the superconducting transition. In the BaPb_{1 − x} Sb _x O₃ compounds, the Korringa relation is met, indicating that all of the samples are in the metal phase. The data on the Pb spin echo decay and the relevant estimates of the indirect coupling constants of the nuclei of the nearest neighbor Pb atoms clearly demonstrate the development of the electron system state in the oxide metal phase, which is microscopically inhomogeneous over the crystal.