Abstract:A novel high pressure polymorph of iron sesquioxide, m-Fe 2 O 3 , has been identified by means of singlecrystal synchrotron X-ray diffraction (XRD). Upon compression of a single crystal of hematite, α-Fe 2 O 3 , in a diamond anvil cell, the transition occurs at pressure of about 54 GPa and results in ∼10% volume reduction. The crystal structure of the new phase was solved by the direct method (monoclinic space group P2 1 /n, a = 4.588(3), b = 4.945(2), c = 6.679(7) Å and β = 91.31(9) • ) and refined to R 1 ∼11… Show more
“…On the other hand, the value of the quadratic elongation of the octahedral unit around Fe(3) is around 1.008 and remains almost unalterable with compression along the stability range of this phase. This parameter is quite similar to those observed in other polymorphs such as γ-Fe 2 O 3 (with λ equal to 1.0024 for the octahedron and 1.000 for the tetrahedron) 41 , the high pressure monoclinic-Fe 2 O 3 (with two out of the three octahedral units with a λ of 1.0025 and 1.0021, respectively) 42 , and ζ-Fe 2 O 3 (an octahedron with λ of 1.0109) 43 .Fig. 3Analysis of the volume and the quadratic elongation of the polyhedral units under pressure.…”
Iron oxides are among the major constituents of the deep Earth’s interior. Among them, the epsilon phase of Fe2O3 is one of the less studied polymorphs and there is a lack of information about its structural, electronic and magnetic transformations at extreme conditions. Here we report the precise determination of its equation of state and a deep analysis of the evolution of the polyhedral units under compression, thanks to the agreement between our experiments and ab-initio simulations. Our results indicate that this material, with remarkable magnetic properties, is stable at pressures up to 27 GPa. Above 27 GPa, a volume collapse has been observed and ascribed to a change of the local environment of the tetrahedrally coordinated iron towards an octahedral coordination, finding evidence for a different iron oxide polymorph.
“…On the other hand, the value of the quadratic elongation of the octahedral unit around Fe(3) is around 1.008 and remains almost unalterable with compression along the stability range of this phase. This parameter is quite similar to those observed in other polymorphs such as γ-Fe 2 O 3 (with λ equal to 1.0024 for the octahedron and 1.000 for the tetrahedron) 41 , the high pressure monoclinic-Fe 2 O 3 (with two out of the three octahedral units with a λ of 1.0025 and 1.0021, respectively) 42 , and ζ-Fe 2 O 3 (an octahedron with λ of 1.0109) 43 .Fig. 3Analysis of the volume and the quadratic elongation of the polyhedral units under pressure.…”
Iron oxides are among the major constituents of the deep Earth’s interior. Among them, the epsilon phase of Fe2O3 is one of the less studied polymorphs and there is a lack of information about its structural, electronic and magnetic transformations at extreme conditions. Here we report the precise determination of its equation of state and a deep analysis of the evolution of the polyhedral units under compression, thanks to the agreement between our experiments and ab-initio simulations. Our results indicate that this material, with remarkable magnetic properties, is stable at pressures up to 27 GPa. Above 27 GPa, a volume collapse has been observed and ascribed to a change of the local environment of the tetrahedrally coordinated iron towards an octahedral coordination, finding evidence for a different iron oxide polymorph.
“…For the novel monoclinic phase (space group P21/n) proposed by Bykova et al [20,21] at 54 GPa, only the expected average Fe-O distance is in agreement with that of EXAFS (Tab. I, 4th row); in contrast, the value of σ 2 indicates a small, but not negligible, Fe-O distortion and therefore we cannot validate this HP structure.…”
supporting
confidence: 73%
“…However this conclusion cannot be regarded as definitive because not based on x-ray "single-crystal" diffraction data, that provides the most accurate structural refinement, and moreover because certain aspects have been challenged by Badro and co-workers [2]. In confirmation of this, a recent synchrotron x-ray single-crystal diffraction study proposes that, in the mixed state above 50 GPa, Fe 2 O 3 forms a novel monoclinic phase with space group P2 1 /n and, above 67 GPa, compression triggers the transition to a different HP phase with orthorhombic unit cell and space group Aba2 [20,21]. Another important and controversial point refers to the nature of the phase-transition at ∼50 GPa: is it the structural transition that drives the electronic transition or viceversa?…”
The pressure evolution of the local structure of Fe2O3 hematite has been determined for the first time by extended x-ray absorption fine structure up to ∼79 GPa. The comparison to the different high-pressure forms proposed in the literature suggests that the orthorhombic structure with space group Aba2 is the most probable. The crossover from Fe high-spin to low-spin states with pressure increase has been monitored from the pre-edge region of the Fe K-edge absorption spectra. The "simultaneous" comparison with the local structural changes allows us to definitively conclude that it is the electronic transition that drives the structural transition and not viceversa.The high-pressure behavior of hematite (α-Fe 2 O 3 ) has raised much debate in the scientific community over the past decades. At ambient conditions, hematite crystallizes in the rhombohedral corundum-type structure, space group R3c, and is a wide-band antiferromagnetic insulator. By increasing pressure at room temperature, the corundum structure of hematite is progressively distorted and, above ∼50 GPa, a series of physical changes occur [1][2][3][4][5][6][7][8][9][10][11]: the unit cell volume drops down by about 10 %, the crystal symmetry changes completely, the electrical resistivity decreases drastically due to the breakdown of the d-electron correlation (Mott insulator-metal transition), the magnetic moments collapse [transition of iron ions from high-spin (HS) to low-spin (LS) state] and the long-range magnetic order disappears. Besides being interesting from the viewpoint of solid-state physics, the phenomena are also important in geophysics for modeling materials behavior in deep Earth's mantle [12][13][14][15][16].
“…For instance, at room temperature cubic Fe 1-x O wüstite with the rocksalt structure transforms to a rhombohedral lattice above 20 GPa1, while at high temperatures the rocksalt structure of Fe 1-x O is stable to at least 60 GPa12. High temperature-assisted phase transitions in Fe 3 O 4 from cubic spinel to an orthorhombic phase3435, and from corundum-type α -Fe 2 O 3 to an orthorhombic Rh 2 O 3 (II)-type or to other phases45678 were also observed in some studies already above 20–25 GPa, although these phase transitions are still hotly debated. These observations suggest that all conventional iron oxides ( α -Fe 2 O 3 , Fe 3 O 4 , and Fe 1-x O) become unstable with respect to structural transformations in approximately similar pressure ranges that might be related to similar shortening of Fe-O bond lengths in their structures.…”
Section: Discussionmentioning
confidence: 99%
“…Many studies have been devoted to investigations of various properties of iron oxides at conditions relevant to the Earth’s interior, i.e., at high pressure or at high pressure combined with high temperature (HP-HT). These studies reported a number of remarkable findings for the three basic iron oxides, Fe 1-x O wüstite12, α -Fe 2 O 3 hematite345678, and Fe 3 O 4 magnetite91011. Meanwhile, several experimental and theoretical studies indicated that the chemistry of iron oxides at extreme conditions of high pressure and high temperature may extend to intriguing behavior beyond these three well-known oxides12131415.…”
Iron oxides are fundamentally important compounds for basic and applied sciences as well as in numerous industrial applications. In this work we report the synthesis and investigation of a new binary iron oxide with the hitherto unknown stoichiometry of Fe7O9. This new oxide was synthesized at high-pressure high-temperature (HP-HT) conditions, and its black single crystals were successfully recovered at ambient conditions. By means of single crystal X-ray diffraction we determined that Fe7O9 adopts a monoclinic C2/m lattice with the most distorted crystal structure among the binary iron oxides known to date. The synthesis of Fe7O9 opens a new portal to exotic iron-rich (M,Fe)7O9 oxides with unusual stoichiometry and distorted crystal structures. Moreover, the crystal structure and phase relations of such new iron oxide groups may provide new insight into the cycling of volatiles in the Earth’s interior.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
