Charge-ordering transition in iron oxide Fe4O5 involving competing dimer and trimer formation

Abstract

Phase transitions that occur in materials, driven, for instance, by changes in temperature or pressure, can dramatically change the materials' properties. Discovering new types of transitions and understanding their mechanisms is important not only from a fundamental perspective, but also for practical applications. Here we investigate a recently discovered Fe4O5 that adopts an orthorhombic CaFe3O5-type crystal structure that features linear chains of Fe ions. On cooling below ∼150 K, Fe4O5 undergoes an unusual charge-ordering transition that involves competing dimeric and trimeric ordering within the chains of Fe ions. This transition is concurrent with a significant increase in electrical resistivity. Magnetic-susceptibility measurements and neutron diffraction establish the formation of a collinear antiferromagnetic order above room temperature and a spin canting at 85 K that gives rise to spontaneous magnetization. We discuss possible mechanisms of this transition and compare it with the trimeronic charge ordering observed in magnetite below the Verwey transition temperature.

Stability of iron-bearing carbonates in the deep Earth’s interior

“…The average Fe–O distances in Fe(1)O7 and Fe(2)O8 polyhedra are similar to one another (∼1.96 Å and ∼2.02 Å, respectively, at 97(2) GPa) and longer than expected for low-spin ferric or ferrous iron26. Similarity in the sizes of iron polyhedra may indicate that Fe cations are in a mixed valence state (intermediate between +2 and +3) as proposed for high-pressure iron oxides2627.…”

Long range electronic phase separation in CaFe3O5

“…Here, following recent studies of spin and charge ordering in non-spinel M Fe n –1 O n +1 ( n  ≥ 4) ferrites with M  = Fe 6,7 , Mn 8,9 and Ca 10,11 , we investigate n  = 4 CaFe 3 O 5 and discover long range electronic phase separation between a magnetite-like charge ordered phase and a charge averaged phase.…”

Effect of Fe3+ on Phase Relations in the Lower Mantle: Implications for Redox Melting in Stagnant Slabs

“…The obtained electron diffraction pattern is consistent with that of a previous study (Myhill et al, ). The EELS spectra show a broad Fe L 2,3 ‐edge, which may be due to electron charge transfer between Fe 2+ and Fe 3+ (Ovsyannikov et al, ). Although it is not straightforward to obtain the Fe 3+ /total Fe ratio because of the broadness, the sample spectra indicate a high Fe 3+ /total Fe ratio (van Aken et al, ).…”