Crystal Structure and Coordination of B-Cations in the Ruddlesden–Popper Phases Sr3−xPrx(Fe1.25Ni0.75)O7−δ (0 ≤ x ≤ 0.4)

Abstract

Compounds Sr3−xPrxFe1.25Ni0.75O7−δ with 0 ≤ x ≤ 0.4 and Ruddlesden–Popper n = 2 type structures were synthesized and investigated by X-ray and neutron powder diffraction, thermogravimetry, and Mössbauer spectroscopy. Both samples, prepared at 1300 °C under N2(g) flow and samples subsequently air-annealed at 900 °C, were studied. The structures contained oxygen vacancies in the perovskite layers, and the Fe/Ni cations had an average coordination number less than six. The oxygen content was considerably higher for air-annealed samples than for samples prepared under N2, 7 − δ = ~6.6 and ~5.6 per formula unit, respectively. Mössbauer data collected at 7 K, below magnetic ordering temperatures, were consistent with X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) results. The electrical conductivity was considerably higher for the air-annealed samples and was for x = 0.1~30 S·cm−1 at 500 °C. The thermal expansion coefficients were measured in air between room temperature and 900 °C and was found to be 20–24 ppm·K−1 overall.

Spray-Flame Synthesis of LaFexCo1–xO3 (x = 0.2, 0.3) Perovskite Nanoparticles for Oxygen Evolution Reaction in Water Splitting: Effect of Precursor Chemistry (Acetates and Nitrates)

“…26 The FeO 6 configurations (for perovskite-type structures at room temperature with isomer shifts usually ranging between ≈0.3 and ≈0.4 mm/s) 24 correspond to structures without oxygen vacancies as in the identified LaFe x Co 1−x O 3 perovskite phase in both acetate-and nitrate-based samples. 27,28 As indicated above, the Mossbauer results suggest the presence of iron with a predominant oxidation state of 3+ (Fe 3+ ) in the different analyzed samples and regardless of the metal-precursor type. This observation is supported by the Fe 2p XPS spectra results presented in Figure S1, in which Fe 3+ is also identified as the main oxidation state of the surface iron ions.…”

“…On the other hand, FeO 4 oxygen-defect configurations (tetrahedral coordination with RT isomer shifts typically between ∼0.1 and ∼0.3 mm/s) most likely happen in brownmillerite-type structures (e.g., the identified La 3 (Fe x Co 1– x ) 3 O 8 phase), in which tetrahedral oxygen-deficient BO 4 (B: transition metal ion) layers alternate with octahedral BO 6 layers . The FeO 6 configurations (for perovskite-type structures at room temperature with isomer shifts usually ranging between ≈0.3 and ≈0.4 mm/s) correspond to structures without oxygen vacancies as in the identified LaFe x Co 1– x O 3 perovskite phase in both acetate- and nitrate-based samples. , …”