Probing Cation and Vacancy Ordering in the Dry and Hydrated Yttrium-Substituted BaSnO_3 Perovskite by NMR Spectroscopy and First Principles Calculations: Implications for Proton Mobility

摘要

Hydrated BaSn_(1-x)Y_xO_(3-x/2) is a protonic conductor that, unlike many other related perovskites, shows high conductivity even at high substitution levels. A joint multinuclear NMR spectroscopy and density functional theory (total energy and GIPAW NMR calculations) investigation of BaSn_(1-x)Y_xO_(3-x/2)(0.10 ≤ x ≤ 0.50) was performed to investigate cation ordering and the location of the oxygen vacancies in the dry material. The DFT energetics show that Y doping on the Sn site is favored over doping on the Ba site. The ~(199)Sn chemical shifts are sensitive to the number of neighboring Sn and Y cations, an experimental observation that is supported by the GIPAW calculations and that allows clustering to be monitored: Y substitution on the Sn sublattice is close to random up to x = 0.20, while at higher substitution levels, Y-O-Y linkages are avoided, leading, at x = 0.50, to strict Y-O-Sn alternation of B-site cations. These results are confirmed by the absence of a "Y-O-Y" ~(17)O resonance and supported by the ~(17)O NMR shift calculations. Although resonances due to six-coordinate Y cations were observed by ~(89)Y NMR, the agreement between the experimental and calculated shifts was poor. Five-coordinate Sn and Y sites (i.e., sites next to the vacancy) were observed by ~(119)Sn and ~(89)Y NMR, respectively, these sites disappearing on hydration. More five-coordinated Sn than five-coordinated Y sites are seen, even at x = 0.50, which is ascribed to the presence of residual Sn-O-Sn defects in the cation-ordered material and their ability to accommodate O vacancies. High-temperature ~(119)Sn NMR reveals that the O ions are mobile above 400 ℃, oxygen mobility being required to hydrate these materials. The high protonic mobility, even in the high Y-content materials, is ascribed to the Y-O-Sn cation ordering, which prevents proton trapping on the more basic Y-O-Y sites.