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

The Ruddlesden–Popper (RP) phases La2Ni1 − xFexO4 + δ (x = 0.02, 0.1) and La4Ni3 − xFexO10 − 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 La2Ni0.9Fe0.1O4 + δ, approximately 5–10% of the higher order RP phase La4Ni2.1Fe0.9O10 − δ were detected in backscattered electron contrast imaging, whereas La2Ni0.98Fe0.02O4 + δ exhibits no foreign phases. This was also confirmed by transmission electron microscopy (TEM). Furthermore, the oxygen permeation performance and electrical conductivity of La2Ni1 −xFexO4 + δ were found to decrease slightly with increasing iron content. The Mössbauer spectra of La2Ni1 −xFexO4 + 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.