One-Dimensional Oxygen Diffusion Mechanism in Sr2ScGaO5 Electrolyte Explored by Neutron and Synchrotron Diffraction, 17O NMR, and Density Functional Theory Calculations
Résumé
We investigated moderate-temperature oxygen diffusion
mechanisms in Sr2ScGaO5 with Brownmillerite structure type. From
oxygen isotope 18O−16O exchange experiments we determined that
oxygen mobility sets in above 550 °C. Temperature-dependent neutron
and X-ray (synchrotron) diffraction experiments allowed us to correlate
the oxygen mobility with a subtle phase transition of the orthorhombic
room-temperature structure with I2mb space group toward Imma, going
along with a disorder of the (GaO4)∞-tetrahedral chains. From lattice
dynamical simulations we could clearly evidence that dynamic switching
of the (GaO4)∞-tetrahedral chains from its R to L configuration sets in
at 600 °C, thus correlating oxygen diffusion with the dynamic disorder.
Oxygen ion diffusion pathways are thus constrained along the onedimensional
oxygen vacancy channels, which is a different diffusion
mechanism compared to that of the isostructural CaFeO2.5, where
diffusion of the apical oxygen atoms into the vacant lattice sites are equally involved in the diffusion pathway. The proposed
ordered room-temperature structure in I2mb is strongly supported by 17O, 45Sc, and 71Ga NMR measurements, which indicate
the presence of crystallographically unique sites and the absence of local disordering effects below the phase transition. The
electric field gradient tensor components measured at the nuclear sites are found to be in excellent agreement with calculated
values using the WIEN2k program. The oxygen site assignment has been independently confirmed by 17O{45Sc} transfer of
adiabatic populations double-resonance experiments.