title: Probing Oxide-Ion Mobility in the Mixed Ionic-Electronic Conductor La2NiO4+δ by Solid-State (17)O MAS NMR Spectroscopy.
abstract: While solid-state NMR spectroscopic techniques have helped clarify the local structure and dynamics of ionic conductors, similar studies of mixed ionic-electronic conductors (MIECs) have been hampered by the paramagnetic behavior of these systems. Here we report high-resolution (17)O (I = 5/2) solid-state NMR spectra of the mixed-conducting solid oxide fuel cell (SOFC) cathode material La2NiO4+δ, a paramagnetic transition-metal oxide. Three distinct oxygen environments (equatorial, axial, and interstitial) can be assigned on the basis of hyperfine (Fermi contact) shifts and quadrupolar nutation behavior, aided by results from periodic DFT calculations. Distinct structural distortions among the axial sites, arising from the nonstoichiometric incorporation of interstitial oxygen, can be resolved by advanced magic angle turning and phase-adjusted sideband separation (MATPASS) NMR experiments. Finally, variable-temperature spectra reveal the onset of rapid interstitial oxide motion and exchange with axial sites at ∼130 °C, associated with the reported orthorhombic-to-tetragonal phase transition of La2NiO4+δ. From the variable-temperature spectra, we develop a model of oxide-ion dynamics on the spectral time scale that accounts for motional differences of all distinct oxygen sites. Though we treat La2NiO4+δ as a model system for a combined paramagnetic (17)O NMR and DFT methodology, the approach presented herein should prove applicable to MIECs and other functionally important paramagnetic oxides.
authors.affiliation: Department of Chemistry, University of Cambridge , Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: David M
authors.initials: DM
authors.lastname: Halat
authors.affiliation: Department of Chemistry, University of Cambridge , Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: Rıza
authors.initials: R
authors.lastname: Dervişoğlu
authors.affiliation: Department of Chemistry, University of Cambridge , Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: Gunwoo
authors.initials: G
authors.lastname: Kim
authors.affiliation: Department of Chemistry, University of Cambridge , Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: Matthew T
authors.initials: MT
authors.lastname: Dunstan
authors.affiliation: Department of Chemistry, University of Cambridge , Lensfield Road, Cambridge CB2 1EW, United Kingdom.|Department of Chemistry, Stephenson Institute for Renewable Energy, University of Liverpool , Crown Street, Liverpool L69 7ZD, United Kingdom.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Blanc
authors.affiliation: Department of Chemistry, University of Warwick , Coventry CV4 7AL, United Kingdom.
authors.forename: Derek S
authors.initials: DS
authors.lastname: Middlemiss
authors.affiliation: Department of Chemistry, University of Cambridge , Lensfield Road, Cambridge CB2 1EW, United Kingdom.
authors.forename: Clare P
authors.initials: CP
authors.lastname: Grey