Elsevier

Solid State Ionics

Volume 176, Issues 1–2, 14 January 2005, Pages 97-102
Solid State Ionics

Local interactions and electronic phenomena in substituted LaFeO3 perovskites

https://doi.org/10.1016/j.ssi.2004.06.001Get rights and content

Abstract

Effects of electronic charge disproportionation were observed by 57Fe Mössbauer spectroscopy at low temperatures in substituted LaFeO3 perovskites as (La1−xCax)FeO3 and La(Fe1−xMgx)O3 (x≤0.5). The presence of both Fe3+ and Fe5+ species was evidenced in both series of compounds. Direct evidence for disproportionation was supported by correlating the Mössbauer results with the chemical determination of tetravalent iron at RT. The evolution of hyperfine parameters vs. substitution degree was comparatively analysed for Fe3+ and Fe5+ ions in relation to their electronic microscopic origin. Electron delocalisation processes are discussed in terms of ion distribution and valence state.

Introduction

The perovskite oxides of general formula ABO3 (where A is a rare earth and B a 3d transition metal) are able to stabilise cations with unusually high oxidation states; the anionic sublattice can accommodate a high concentration of vacancies. The presence of elements with different oxidation states (mainly transition elements) on both A and B sites does not affect the very stable structure in any substantial way. The substitutions induce higher valence states and/or vacancies of oxygen to preserve charge neutrality. In this respect, investigation of these structures is important, both for basic knowledge and for applications in domains in which materials involving electron transfer are of great interest. For example, they may be successfully used as catalysts for the complete oxidation of hydrocarbons and CO, replacing the very expensive noble metals Pt and Pd [1]. In the last few years, many studies have focused on substituted perovskites of type AFeO3, in which the high ionisation state of iron gives rise to better yields of methane oxidation. The presence of Sr, Gd [2], Sm, Nd and La [3] has been reported on the A sites. The crystal structure is known to be orthorhombic with Pnma space group symmetry [2], [3], [4], [5]. In the peculiar cases of SrFeO3 and LaFeO3, the lattice symmetry changes from orthorhombic to nearly cubic. A charge disproportionation of tetravalent iron is assumed to take place at low temperatures in such compounds, according to: 2Fe4+→Fe3++Fe5+ [6], [7], [8], [9], [10], [11], [12]. However, in spite of its relatively simpler structure, SrFeO3 does not respect this reaction. Initially [13], [14], electron delocalisation occurring in a partially filled π* band leading to metal-like conductivity down to 4 K was considered in this case. More recent results obtained on Ti substituting Fe in SrFeO3 have shown that the increase in the amount of Ti4+ leads to stronger localisation of the σ* (Fe 3d–O 2p) electrons, without any change in the disproportionation of Fe4+ [15]. Therefore, SrFeO3 does not provide cations in even higher oxidation states, which would be really useful for catalytic processes. Due to their nearly cubic symmetry and possible induced charge disproportionation, substituted LaFeO3 perovskites may be more suitable for study of the intimate electronic interactions responsible for catalytic activity.

This presentation focuses on local electronic phenomena and magnetic interactions in substituted LaFeO3-type systems, studied especially at low temperature using Mössbauer spectroscopy. Special attention was paid to the effects induced by the presence of bivalent ions on both rare earth and transition metal sites.

Section snippets

Experimental

The reference LaFeO3 sample and other compounds from the series (La1−xCax)FeO3 with x=0.1, 0.2, 0.3, 0.4, 0.5 and La(Fe1−xMgx)O3 with x=0.1, 0.3, 0.5 were prepared by the citrate method and subsequently annealed at 1073 K in order to improve their crystalline structure. Structural data (phase analysis, lattice parameters, particle sizes) as well as temperature programmed reduction (TPR) experiments providing the relative amount of Fe4+ at room temperature (RT) have been reported previously [3],

Results and discussion

It is worth noting that the substitutional ions were bivalent and, having lower valence than the substituted trivalent La3+ and Fe3+, the electroneutrality of the compound imposes either a higher oxidation state of iron or a decrease in oxygen content. One of the aims of the present study was to describe these induced phenomena in detail. Additional information about the local coordination and magnetic state of iron was also obtained by Mössbauer spectroscopy. Spectra collected at room

Conclusions

A clear charge disproportionation was induced by substitutions of lower valence ions (Ca2+ and Mg2+) for La3+ and Fe3+ respectively, in substituted LaFeO3 perovskites.

Low temperature Mössbauer measurements revealed the existence of Fe3+ and Fe5+ in both systems. The behaviour of two main parameters, hyperfine magnetic field and isomer shift, related to Fe3+ ions, was interpreted in terms of 3d electron screening induced by both substitutional atoms and anion vacancies in the lattice. The effect

Acknowledgements

The authors thank the Education and Research Ministry of Romania and the Ministry of Foreign Affairs of Italy for their financial support, within the framework of the XIV Protocol for Scientific and Technologic Cooperation between Romania and Italy. V.K. and G.F. acknowledge the financial support of the National Romanian Core Program “New Directions in Solid State and Material Physics”. G. Walton revised the English text.

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