Pressure tuning of charge ordering in iron oxide

Abstract

A Verwey-type charge-ordering transition in magnetite at 120 K leads to the formation of linear units of three iron ions with one shared electron, called trimerons. The recently-discovered iron pentoxide (Fe4O5) comprising mixed-valent iron cations at octahedral chains, demonstrates another unusual charge-ordering transition at 150 K involving competing formation of iron trimerons and dimerons. Here, we experimentally show that applied pressure can tune the charge-ordering pattern in Fe4O5 and strongly affect the ordering temperature. We report two charge-ordered phases, the first of which may comprise both dimeron and trimeron units, whereas, the second exhibits an overall dimerization involving both the octahedral and trigonal-prismatic chains of iron in the crystal structure. We link the dramatic change in the charge-ordering pattern in the second phase to redistribution of electrons between the octahedral and prismatic iron chains, and propose that the average oxidation state of the iron cations can pre-determine a charge-ordering pattern.

A Room‐Temperature Verwey‐type Transition in Iron Oxide, Fe5O6

“…This charge order in Fe 4 O 5 could be realized due to a compromise between the high Fe 2.67+ average charge in the octahedra, which is optimal for the formation of trimers (Fe 3+ −Fe 2+ −Fe 3+ ), and a too short d (Fe−Fe) oct =2.891 Å distance that is more suitable for the formation of dimers (Figure d). Remarkably, an applied pressure was found to suppress this charge‐ordered phase of Fe 4 O 5 , to squeeze out the excess charge from the octahedra to the prisms, and to stabilize a purely dimeric order with the strongly enhanced T CO . Thus, Fe 5 O 6 with its shorter d (Fe−Fe) oct =2.877 Å distance, optimally charged octahedral ions (Fe 2.5+ on average), and with the high T CO =275 K temperature further enhanced under pressure fits well to the trend found for Fe 4 O 5 (Figure d).…”

A Room‐Temperature Verwey‐type Transition in Iron Oxide, Fe5O6

“…d) Dependence of the charge‐ordering transition temperature ( T CO ) for Fe 5 O 6 (this work), Fe 4 O 5 , MnFe 3 O 5 , Fe 3 O 4 , and CaFe 3 O 5 on the minimal Fe−Fe distances in their octahedral iron chains. The arrows indicate the pressure‐induced changes found for Fe 3 O 4 and Fe 4 O 5 …”

“…A variety of the charge‐ordering features observed in octahedral networks of iron oxides studied to date suggests the existence of several competing mechanisms of these phenomena. In general, one expects that the type of charge ordering is mainly influenced by the electron count and spin arrangement, should the magnetic ordering precede the charge‐ordering transition.…”

Synthesis of Ilmenite-type ε-Mn2O3 and Its Properties

“…Some of these polymorphs can be quenched from HP-HT and stabilized at ambient conditions, opening up prospects for their academic investigations and potential industrial use. Note, for example, that the most common mixed-valence iron oxides, Fe 4 O 5 and Fe 5 O 6 , were found to undergo spectacular charge-ordering-related phase transitions. − These findings along with the well-known Verwey transition in magnetite studied before, , contributed to a better understanding of physical properties of transition-metal oxides.…”