In-situ Mössbauer studies of ⁵⁷Fe-doped Ruddlesden–Popper type lanthanum nickel oxides

Abstract

The Ruddlesden–Popper (RP) phases La₂Ni_{1 − x}Fe_xO_{4 + δ} (x = 0.02, 0.1) and La₄Ni_{3 − x}Fe_xO_{10 − δ} (x = 0.3, 0.9) were successfully synthesized via the sol–gel route. Even though X-ray diffraction did not show any secondary phases for La₂Ni_0.9Fe_0.1O_{4 + δ}, approximately 5–10% of the higher order RP phase La₄Ni_2.1Fe_0.9O_{10 − δ} were detected in backscattered electron contrast imaging, whereas La₂Ni_0.98Fe_0.02O_{4 + δ} exhibits no foreign phases. This was also confirmed by transmission electron microscopy (TEM). Furthermore, the oxygen permeation performance and electrical conductivity of La₂Ni_{1 −x}Fe_xO_{4 + δ} were found to decrease slightly with increasing iron content. The Mössbauer spectra of La₂Ni_{1 −x}Fe_xO_{4 + δ} (x = 0.02, 0.1) are dominated by a quadrupole-split doublet. With increasing temperature, however, a minority iron species is detected. The dominating doublet reveals an unusual high-temperature behavior of quadrupolar interactions, which was observed by in-situ Mössbauer measurements at high temperatures and different oxygen partial pressures. This is most likely related to ordering phenomena of oxygen.