doi: 10.1038/ncomms10661.
Structural complexity of simple Fe2O3 at high pressures and temperatures
E Bykova 1 2, L Dubrovinsky 1, N Dubrovinskaia 2, M Bykov 1 2, C McCammon 1, S V Ovsyannikov 1, H-P Liermann 3, I Kupenko 1 4, A I Chumakov 4, R Rüffer 4, M Hanfland 4, V Prakapenka 5
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- PMID:26864300
- PMCID: PMC4753252
- DOI: 10.1038/ncomms10661
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Structural complexity of simple Fe2O3 at high pressures and temperatures
E Bykova et al. Nat Commun..
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Abstract
Although chemically very simple, Fe2O3 is known to undergo a series of enigmatic structural, electronic and magnetic transformations at high pressures and high temperatures. So far, these transformations have neither been correctly described nor understood because of the lack of structural data. Here we report a systematic investigation of the behaviour of Fe2O3 at pressures over 100 GPa and temperatures above 2,500 K employing single crystal X-ray diffraction and synchrotron Mössbauer source spectroscopy. Crystal chemical analysis of structures presented here and known Fe(II, III) oxides shows their fundamental relationships and that they can be described by the homologous series nFeO·mFe2O3. Decomposition of Fe2O3 and Fe3O4 observed at pressures above 60 GPa and temperatures of 2,000 K leads to crystallization of unusual Fe5O7 and Fe25O32 phases with release of oxygen. Our findings suggest that mixed-valence iron oxides may play a significant role in oxygen cycling between earth reservoirs.
Figures
Building blocks are octahedra (brown) and trigonal prisms (blue). The prisms in Fe5O7, Fe25O32 and η-Fe2O3 have one or two additional apices. Hematite (a) consists of FeO6 octahedra connected in a corundum-like motif, namely each octahedron connects with three neighbours via edges in honeycomb layers, and layers are interconnected through common triangular faces of octahedra. The
structure (b) is built of only FeO6 octahedra but each two octahedra are connected through a common triangular face; such units pack in a herringbone pattern and layers pack with a shift along thec-direction having common edges. In distorted perovskite ζ-Fe2O3 (c) octahedra connect through common vertices and prisms share only common edges. θ-Fe2O3 (e) adopts the packing motif from but instead of octahedra it consists of FeO6 prisms. Post-perovskite (d) and Fe5O7 (f) are members of the homologous seriesnFeO·mFe2O3 (see also Fig. 4), where prisms are connected through common triangular faces, while octahedra connect only via shared edges. In addition to triangular face-shared prisms and edge-shared octahedra, Fe25O32 (g) has edge-shared one-capped prisms; therefore it belongs neither to the homologous series nor adopts any other known structural motif.
structure (b) is built of only FeO6 octahedra but each two octahedra are connected through a common triangular face; such units pack in a herringbone pattern and layers pack with a shift along thec-direction having common edges. In distorted perovskite ζ-Fe2O3 (c) octahedra connect through common vertices and prisms share only common edges. θ-Fe2O3 (e) adopts the packing motif from but instead of octahedra it consists of FeO6 prisms. Post-perovskite (d) and Fe5O7 (f) are members of the homologous seriesnFeO·mFe2O3 (see also Fig. 4), where prisms are connected through common triangular faces, while octahedra connect only via shared edges. In addition to triangular face-shared prisms and edge-shared octahedra, Fe25O32 (g) has edge-shared one-capped prisms; therefore it belongs neither to the homologous series nor adopts any other known structural motif.
(a) ⋄-
hematite (α-Fe2O3), Δ-
distorted perovskite (ζ-Fe2O3), ○-Aba2 (θ-Fe2O3, probably metastable), □-Cmcm post-perovskite (η-Fe2O3) and × -Rh2O3-II type phase (). The boundary between hematite α-Fe2O3 and is defined according to ref. . The geotherm is defined according to refs , .
hematite (α-Fe2O3), Δ- distorted perovskite (ζ-Fe2O3), ○-Aba2 (θ-Fe2O3, probably metastable), □-Cmcm post-perovskite (η-Fe2O3) and × -Rh2O3-II type phase (). The boundary between hematite α-Fe2O3 and is defined according to ref. . The geotherm is defined according to refs , .
Spectra collected during compression (a–d) and after heating (e). In hematite (a) iron atoms have a HS state (at ∼24 GPa CS=0.306(4) mm s-1), and spectra are split due to magnetic ordering (M). After the first transition at 49 GPa (b) a new non-magnetic (NM) component appears with CS of 0.074(5) mm s-1 corresponding to a LS state. During further compression a fraction of the magnetic component decreases (c) and it disappears completely after the second transition to the θ-Fe2O3 phase (d) that has only one non-magnetic position of LS iron atoms in the crystal structure. After heating above 1,600(50) K (e) a transformation to η-Fe2O3 occurs. The crystal structure has two HS-iron positions (both CS are ∼0.45 mm s-1), where one position is magnetically ordered and the other is non-magnetic.
The structures may be described as assembled from two building blocks, FeO6 octahedra and trigonal prisms (prisms could be two-capped but they are not shown for simplicity). Prisms connect to each other through triangular faces, while octahedra share edges, so that they form parallel columns of face-shared prisms and edge-shared octahedra arranged in different motifs as seen in the figure with structures viewed from the top of the columns. Increasing Fe2+ content favours octahedral packing over mixed octahedral and prismatic packing. This requires denser packing of FeO6 octahedra and as a result columns of octahedra condense in slabs by sharing common edges. In particular, η-Fe2O3 has ordinary columns of prisms and octahedra with a chequerboard-like arrangement; Fe5O7 has ordinary and doubled columns of octahedra; the HP-Fe3O4 (ref. 41) possesses only doubled columns; Fe7O9 (ICSD reference number CSD-430601) has doubled columns and tripled columns organized in zigzag slabs; Fe4O5 (ref. 20) possesses only tripled and Fe5O6 (ref. 21) only quadruple zigzag slabs. The end-member of the homologous series wüstite (FeO) consists of octahedra with a maximum (12) number of edge-shared neighbours.
References
- Dobson D. P. & Brodholt J. P. Subducted banded iron formations as a source of ultralow-velocity zones at the core-mantle boundary. Nature 434, 371–374 (2005). - PubMed
- Olsen J. S., Cousins C. S. G., Gerward L., Jhans H. & Sheldon B. J. A study of the crystal structure of Fe2O3 in the pressure range up to 65 GPa using synchrotron radiation. Phys. Scr. 43, 327–330 (1991).
- Pasternak M. et al. Breakdown of the Mott-Hubbard state in Fe2O3: A first-order insulator-metaltransition with collapse of magnetism at 50 GPa. Phys. Rev. Lett. 82, 4663–4666 (1999).
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