In-situ Mössbauer studies of 57Fe-doped Ruddlesden–Popper type lanthanum nickel oxides
Abstract The Ruddlesden–Popper (RP) phases La 2 Ni 1 −  x Fe x O 4 + δ ( x  = 0.02, 0.1) and La 4 Ni 3 −  x Fe x O 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 La 2 Ni 0.9 Fe 0.1 O 4 + δ , approximately 5–10% of the higher order RP phase La 4 Ni 2.1 Fe 0.9 O 10 − δ were detected in backscattered electron contrast imaging, whereas La 2 Ni 0.98 Fe 0.02 O 4 + δ exhibits no foreign phases. This was also confirmed by transmission electron microscopy (TEM). Furthermore, the oxygen permeation performance and electrical conductivity of La 2 Ni 1 − x Fe x O 4 + δ were found to decrease slightly with increasing iron content. The Mossbauer spectra of La 2 Ni 1 − x Fe x O 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 Mossbauer measurements at high temperatures and different oxygen partial pressures. This is most likely related to ordering phenomena of oxygen.