Crystal structure and high-temperature properties of the Ruddlesden-Popper phases Sr_3-xY_x(Fe_1.25Ni_0.75)O_7-δ (0≤x≤0.75)

Abstract Ruddlesden-Popper n=2 member phases Sr_3-xY_xFe_1.25Ni_0.75O_7-δ, 0≤x≤0.75, have been investigated by X-ray and neutron powder diffraction, thermogravimetry and Mössbauer spectroscopy. Both samples as-prepared at 1300 C under N₂(g) flow and samples subsequently air-annealed at 900 C were studied. The as-prepared x=0.75 phase is highly oxygen deficient with δ=1, the O1 atom site being vacant, and the Fe³⁺/Ni²⁺ ions having a square pyramidal coordination. For as-prepared phases with lower x values, the Mössbauer spectral data are in good agreement with the presence of both 5- and 4-coordinated Fe³⁺ ions, implying in addition a partial occupancy of the O3 atom sites that form the basal plane of the square pyramid. The air-annealed x=0.75 sample has a δ value of 0.61(1) and the structure has Fe/Ni ions in both square pyramids and octahedra. Mössbauer spectroscopy shows the phase to contain only Fe³⁺, implying that all Ni is present as Ni³⁺. Air-annealed phases with lower x values are found to contain both Fe³⁺ and Fe⁴⁺. For both the as-prepared and the air-annealed samples, the Y³⁺ cations are found to be mainly located in the perovskite block. The high-temperature thermal expansion of as-prepared and air-annealed x=0.75 phases were investigated by high-temperature X-ray diffraction and dilatometry and the linear thermal expansion coefficient determined to be 14.4 ppm K^-1. Electrical conductivity measurements showed that the air-annealed samples have higher conductivity than the as-prepared ones.