Single phase charge ordered stoichiometric CaFe3O5 with commensurate and incommensurate trimeron ordering

Simon J Cassidy,Pascal Manuel,Simon J Clarke,Fabio Orlandi

doi:10.1038/s41467-019-13450-5

Simon J Cassidy, Pascal Manuel + Show 2 more

Open Access

https://doi.org/10.1038/s41467-019-13450-5

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Abstract

Mixed-valent transition metal compounds display complex structural, electronic and magnetic properties which can often be exquisitely tuned. Here the charge-ordered state of stoichiometric CaFe3O5 is probed using neutron powder diffraction, Monte Carlo simulation and symmetry analysis. Magnetic ordering is dominated by the formation of ferromagnetic Fe3+–Fe2+–Fe3+ trimers which are evident above the magnetic ordering transition. Between TN = 289 K and 281 K an incommensurate magnetically ordered phase develops due to magnetic frustration, but a spin Jahn-Teller distortion lifts the frustration and enables the magnetic ordering to lock in to a charge-ordered commensurate state at lower temperatures. Stoichiometric CaFe3O5 exhibits single phase behaviour throughout and avoids the phase separation into two distinct crystallographic phases with different magnetic structures and Fe valence distributions reported recently, which likely occurs due to partial Fe2+ for Ca2+ substitution. This underlines the sensitivity of the magnetism and chemistry of these mixed-valent systems to composition.

Highlights

Mixed-valent transition metal compounds display complex structural, electronic and magnetic properties which can often be exquisitely tuned
Bond valence sums for the Fe sites suggested that the minority phase remains charge averaged (CA) below the magnetic ordering transition, while the majority phase is charge ordered (CO) with the Fe1 site characterised as Fe3+ and the Fe2 site as Fe2+, reflecting the expectation of the work of Gerardin et al.[12]
Aside from the phase separation evident in sample B, no peak splitting or superstructure reflections were observed as a function of temperature and Rietveld refinements were consistent with the structure maintaining Cmcm symmetry above and below the magnetic transition

Summary

Introduction

Mixed-valent transition metal compounds display complex structural, electronic and magnetic properties which can often be exquisitely tuned. Stoichiometric CaFe3O5 exhibits single phase behaviour throughout and avoids the phase separation into two distinct crystallographic phases with different magnetic structures and Fe valence distributions reported recently, which likely occurs due to partial Fe2+ for Ca2+ substitution. This underlines the sensitivity of the magnetism and chemistry of these mixedvalent systems to composition. The n = 3 member of the series, CaFe5O7, exhibits a complex structure[6], stacking faults that are synthesis dependent[7,8], complex long range magnetic order and Charge Order (CO), an important phenomenon in mixed-valent compounds[9]. Show in their analysis of bond lengths that the CO phase meets the requirements for trimeron formation by means of localising the minority spin in Fe1-Fe2-Fe1 units[2,13]

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