Investigations into the structure of La3Ni2−xFexO7±δ

Evgeny Alexandrovich Kiselev,Götz Eckold,Vladimir Alexandrovich Cherepanov,Armin Feldhoff,Klaus-Dieter Becker,Piotr Gaczynski

doi:10.15826/chimtech.2019.6.2.03

Evgeny Alexandrovich Kiselev, Götz Eckold + Show 4 more

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https://doi.org/10.15826/chimtech.2019.6.2.03

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Abstract

The room-temperature (RT) 57Fe Mössbauer spectra of the La3Ni2−xFexO7±δ oxide solid solutions of Ruddlesden-Popper-type (x = 0.05, 0.10) reveal two doublets for Fe3+ ions in octahedral coordination by oxygen. The existence of two inequivalent sites for Fe at RT is at variance with the space groups Fmmm and Cmcm (Amam) which have been reported for La3Ni2O7±δ. This unexpected finding is discussed in connection with Patterson analyses and Rietveld refinements of powder XRD data for x = 0, 0.05, and 0.10. Alternative structural models have been proposed which can explain the spectroscopic findings and which are compatible with the results from X-ray diffraction.

Highlights

Layered Ruddlesden-Popper (R-P) lanthanum nickelates of the general formula Lan+1NinO3n+1 have recently been studied for potential application as SOFC cathodes [1,2,3,4,5] for the members with n = 1, 2, 3
It was shown that the stoichiometric parameter δ of La3Ni2O7–δ changes from about 0.057 at 900 °C to 0.072 at 1150 °C which is in good agreement with the results presented in Refs. [1, 11, 12] for the samples prepared by nitrate route, Table 1
In order to reduce the number of space groups, we focused on those which can be deduced from group-subgroup relations keeping both unit cell dimensions and orthorhombic crystal system of the parent group unchanged

Summary

Introduction

As discussed in the Introduction, to date the crystal structure of La3Ni2O7±δ has been assigned to the orthorhombic space groups Fmmm (No 69) [1, 11,12,13] or Cmcm (No 63) [14, 15] According to these structural models, lanthanum cations occupy two different crystallographic positions, but the nickel cations are located on a single site exclusively. This is in striking contrast with the present results of the Mössbauer measurements which clearly indicate the existence of two nonequivalent lattice sites for iron/nickel in the studied La3Ni2–xFexO7±δ solid solutions at RT.

Findings

Discussion

Conclusions