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.
Highlights
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
A recent series of high-pressure high-temperature (HPHT) studies using single crystal X-ray diffraction methods discovered and identified a number of novel iron oxides, e.g., Fe4O512,13, Fe5O614,15, Fe13O1916, Fe5O717, Fe25O3217, FeO218–21, Fe7O922, and a new monoclinic polymorph of Fe2O3 which can be stable at ambient conditions[23]
The crystal structure of Fe4O5 comprises chains of both trigonal prisms Fe1 with shared triangular faces, occupied by Fe2+ cations, and an octahedral network occupied by mixed Fe2+/Fe3+ cations and consisting of (i) single chains of octahedra sharing opposite edges, and (ii) double chains of octahedra composed of two chains similar to the above Fe2 ones, but attached together side-by-side via two other octahedron edges[12]
Summary
Phase III and two options for its crystal structure. In all experiments, we start from normal conditions (Fe4O5-I phase). With further temperature decrease and pressure increase, these reflections become progressively stronger and clearer (Fig. 3c). We verify that these reflections do not belong to the earlier-reported charge-ordered Fe4O5-II phase[32], and label this phase as Fe4O5-III. Upon heating at about 18 GPa, we can follow the superlattice reflections of Fe4O5-III up to at least 270 K (Fig. 2). We can index the single-crystal diffraction patterns of Fe4O5-III in either orthorhombic or monoclinic unit cells (Fig. 3d). We determine two candidate crystal structures, namely, Fe4O5-IIIa with a monoclinic C2/m lattice (Fig. 4) and Fe4O5-III-b with an orthorhombic C2221 unit cell (Fig. 5), which can well fit the experimental X-ray diffraction patterns
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