Direct Evidence of Stacking Disorder in the Mixed Ionic-Electronic Conductor Sr₄Fe₆O_12+δ

Abstract

Determining the structure-to-property relationship of materials becomes particularly challenging when the material under investigation is dominated by defects and structural disorder. Knowledge on the exact atomic arrangement at the defective structure is required to understand its influence on the functional properties. However, standard diffraction techniques deliver structural information that is averaged over many unit cells. In particular, information about defects and order-disorder phenomena is contained in the coherent diffuse scattering intensity which often is difficult to uniquely interpret. Thus, the examination of the local disorder in materials requires a direct method to study their structure on the atomic level with chemical sensitivity. Using aberration-corrected scanning transmission electron microscopy in combination with atomic-resolution electron energy-loss spectroscopy, we show that the controversial structural arrangement of the Fe2O2+δ layers in the mixed ionic-electronic conducting Sr4Fe6O12+δ perovskite can be unambiguously resolved. Our results provide direct experimental evidence for the presence of a nanomixture of "ordered" and "disordered" domains in an epitaxial Sr4Fe6O12+δ thin film. The most favorable arrangement is the disordered structure and is interpreted as a randomly occurring but well-defined local shift of the Fe-O chains in the Fe2O2+δ layers. By analyzing the electron energy-loss near-edge structure of the different building blocks in the Sr4Fe6O12+δ unit cell we find that the mobile holes in this mixed ionic-electronic conducting oxide are highly localized in the Fe2O2+δ layers, which are responsible for the oxide-ion conductivity. A possible link between disorder and oxygen-ion transport along the Fe2O2+δ layers is proposed by arguing that the disorder can effectively break the oxygen diffusion pathways.

Atomic structure and electronic properties of planar defects in SrFeO3−δ thin films

“…The above described ELNES of both the Fe L 2,3 and O K edges are in excellent agreement with the ones previously reported for Sr 4 Fe 6 O 12.74 [33]. Next, we used the differences in the fine structure of the Fe L 2,3 edge to map the Fe oxidation state in the film using the averaged EELS signals from Fig.…”

“…This structure recalls the oxygen-deficient perovskite compound with the general formula Sr 4 Fe 6 O 12+δ [30][31][32][33]. A polyhedral model of the Sr 4 Fe 6 O 12+δ (δ = 0.74) crystal structure [33] (Pn2n space group, a = 38.864 Å, b = 18.938 Å, c = 5.557 Å) viewed along the [001] direction is shown in Fig. 4(a).…”

“…We have compared our experimental spectra with theoretical ELNES to interpret the spectral features appearing in the oxygen K edge spectra obtained at the Fe 2 O 2+α double and the FeO 2 perovskite-type layers. For this purpose, we used the model based on the Sr 4 Fe 6 O 12+α (δ = 0.74) crystal structure [33] [see Fig. 4(a)].…”

Superspace crystallography: a key to the chemistry and properties

“…Although a wide range of oxygen nonstoichiometry has been reported for Sr 4 Fe 6 O 12+ (Avdeev et al, 2002;Solís et al, 2010), the interstitial oxygen atoms remain immobilized in the modulated (Fe 2 O 2+ ) rock-salt block due to strong coupling between their ordering and local strain that causes aperiodic compressive deformation of the (Fe 2 O 2+ ) blocks. Additional detrimental effect on oxygen ion conductivity in this material originates from the abundant antiphase shifts within the (Fe 2 O 2+ ) blocks which cut off the diffusion pathways (Rossell et al, 2013).…”

Atomic Structure of Defects in Anion-Deficient Perovskite-Based Ferrites with a Crystallographic Shear Structure

“…The study was carried out using aberration-corrected scanning transmission electron microscopy (STEM), providing subangstrom resolution. The visualization of atomic columns of both heavy and light elements is possible with high angle annular dark field (HAADF) and annular bright field (ABF) STEM techniques. , The combination of STEM with electron energy-loss spectroscopy (STEM-EELS) or energy-dispersive X-ray (STEM-EDX) spectroscopy allows the acquisition of chemical maps at atomic-column resolution. − Mapping of the iron coordination number is possible by STEM-EELS, through mapping of the iron L 2,3 EELS fine structure. − …”