One-Dimensional Oxygen Diffusion Mechanism in Sr₂ScGaO₅ Electrolyte Explored by Neutron and Synchrotron Diffraction, ¹⁷O NMR, and Density Functional Theory Calculations

Abstract

We investigated moderate-temperature oxygen diffusion mechanisms in Sr 2 ScGaO 5 with Brownmillerite structure type. From oxygen isotope 18 O− 16 O 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 (GaO 4 ) ∞ -tetrahedral chains. From lattice dynamical simulations we could clearly evidence that dynamic switching of the (GaO 4 ) ∞ -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 CaFeO 2.5 , 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 17 O, 45 Sc, and 71 Ga 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 17 O{ 45 Sc} transfer of adiabatic populations double-resonance experiments.

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

“…When synthesizing Sr2ScGaO5 by classical solid state reaction at 1200 °C, the thermodynamically stable phase obtained shows a brownmillerite framework with I2mb space group at room temperature and unit cell parameters a = 5.91048(5) Å, b = 15.1594(1) Å, and c = 5.70926(4) Å [18]. This structure is characterized by two important details: (i) the B-cation shows a specific order with Sc selectively on octahedral and Ga on tetrahedral sites; and (ii) the (GaO4)∞-tetrahedral chains show long range order for the zigzag arrangement, as shown in Figure 1.…”

“…The temperature for oxygen mobility to set in can be easily determined by 18 O/ 16 O isotope exchange behaviour and analysing related mass loss by thermogravimetric analysis. Both compounds show only very low electronic conductivity, but their oxygen mobility sets in at around 450 °C and 500 °C for Ca2Fe2O5 and Sr2ScGaO5, respectively.…”

“…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.…”

“…For a 100% enrichment, the theoretical mass loss is 2.65% and 3.67% for c-SSGO and CFO, respectively. The 18 Both title compounds show oxygen deficiency, and are thus interesting as oxygen ion conductors, with applications as membranes or electrolytes. The temperature for oxygen mobility to set in can be easily determined by 18 O/ 16 O isotope exchange behaviour and analysing related mass loss by thermogravimetric analysis.…”

“…This is the case comparing Ca2Fe2O5 with Sr2ScGaO5. While Sr2ScGaO5 shows a phase transition towards the cubic perovskite phase above 1400 °C [17,18], Ca2Fe2O5 does not. The case of Sr2ScGaO5 is peculiar, as the high temperature perovskite phase, once obtained, is kinetically stable-even during furnace cooling down to room temperature.…”