Glassy Distribution of Bi3+/Bi5+ in Bi1-xPbxNiO3 and Negative Thermal Expansion Induced by Intermetallic Charge Transfer

The valence distribution and local structure of Bi1-xPbxNiO3 (x <= 0.25) were investigated by comprehensive studies of Rietveld analysis of synchrotron X-ray diffraction (SXRD) data, X-ray absorption spectroscopy (XAS), hard X-ray photoemission spectroscopy (HAXPES), and pair distribution function (PDF) analysis of synchrotron X-ray total scattering data. Disproportionation of Bi ions into Bi3+ and Bi5+ was observed for all the samples, but it was a long-ranged one with distinct crystallographic sites in the P (1) over bar triclinic structure for x <= 0.15, while the ordering was short-ranged for x = 0.20 and 0.25. An intermetallic charge transfer between Bi5+ and Ni2+, leading to large volume shrinkage, was observed for all the samples upon heating at similar to 500 K.