Lattice Dynamics To Trigger Low Temperature Oxygen Mobility in Solid Oxide Ion Conductors

Abstract

SrFeO(2.5) and SrCoO(2.5) are able to intercalate oxygen in a reversible topotactic redox reaction already at room temperature to form the cubic perovskites Sr(Fe,Co)O(3), while CaFeO(2.5) can only be oxidized under extreme conditions. To explain this significant difference in low temperature oxygen mobility, we investigated the homologous SrFeO(2.5) and CaFeO(2.5) by temperature dependent oxygen isotope exchange as well as by inelastic neutron scattering (INS) studies, combined with ab initio (DFT) molecular dynamical calculations. From (18)O/(16)O isotope exchange experiments we proved free oxygen mobility to be realized in SrFeO(x) already below 600 K. We have also evidence that low temperature oxygen mobility relies on the existence of specific, low energy lattice modes, which trigger and amplify oxygen mobility in solids. We interpret the INS data together with the DFT-based molecular dynamical simulation results on SrFeO(2.5) and CaFeO(2.5) in terms of an enhanced, phonon-assisted, low temperature oxygen diffusion for SrFeO(3-x) as a result of the strongly reduced Fe-O-Fe bond strength of the apical oxygen atoms in the FeO(6) octahedra along the stacking axis. This dynamically triggered phenomenon leads to an easy migration of the oxide ions into the open vacancy channels and vice versa. The decisive impact of lattice dynamics, giving rise to structural instabilities in oxygen deficient perovskites, especially with brownmillerite-type structure, is demonstrated, opening new concepts for the design and tailoring of low temperature oxygen ion conductors.

Symmetry‐Driven Atomic Rearrangement at a Brownmillerite–Perovskite Interface

“…[23,24,36] A similar observation was found for the well-studied oxide ion conducting brownmillerite, Ba 2 In 2 O 5 , which verifies the universal nature of anisotropic oxygen diffusion through brownmillerites. [37,38] The region highlighted in pink illustrates the distinct inhomogeneity of the interfacial atomic spacing that was visible in the strain-field region shown in Figure 1b.…”

Influence of Phase Transformations on Crystal Growth of Stoichiometric Brownmillerite Oxides: Sr2ScGaO5 and Ca2Fe2O5

“…The applied growth conditions reported in [28] yielded a Ca2Fe2O5 single-crystal (CFO) of 6 mm in diameter and 120 mm in length, showing a shiny silver-black surface with metallic lustre, as shown in Figure 7. Powder diffraction carried out on crushed parts of the as-grown single crystals yielded lattice parameters of a = 5.4272(4) Å, b = 14.7640(1) Å, and c = 5.59724(4) Å, in very good agreement with previous reports [12,28,29]. The corresponding XRD pattern is shown in Figure 8a.…”

“…Simultaneous to the change of the mass loss, the 18 O/ 16 O exchange was monitored by mass spectroscopy. The more rapid isotope exchange for Ca2Fe2O5 is probably related to the higher catalytic surface activity in the case of Fe 3+ compared to the d 0 /d 10 configuration of Sc/Ga, but also to the fact that FeOx polyhedra are more flexible [16], and not as rigid as reported for ScO6 octahedra or GaO4 tetrahedra [12,18]-even in an average cubic symmetry. A similar tendency was already observed comparing Ca2Fe2O5 and Ca2FeAlO5 [3], where the pure CFO showed a much higher conductivity.…”

“…Ca 2 Fe 2 O 5 is congruently melting [27,28], which significantly helps for the single crystal growth. The applied growth conditions reported in [28] [12,28,29]. The corresponding XRD pattern is shown in Figure 8a.…”

Inelastic neutron scattering applied to materials for energy

“…A microscopic diffusion mechanism, in which a low-lying phonon mode assists the migration of ions, has been recently developed [8]. This "phonon assisted diffusion" model allowed understanding on an atomic scale how oxygen ion diffusion can be triggered in solid oxides down to ambient temperature.…”