Crystal structure and coordination of B-cations in the ruddlesden-popper phases Sr_3-xPr_x(Fe_1.25Ni_0.75)O_7-δ (0 ≤ x ≤ 0.4)

Compounds Sr_3-xPr_xFe_1.25Ni_0.75O_7-δ with 0 ≤ x ≤ 0.4 and Ruddlesden-Popper n = 2 type structureswere 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.