A study of the crystal structure of Fe₂O₃in the pressure range up to 65 GPa using synchrotron radiation

Abstract

High-pressure X-ray powder diffraction has been performed on corundumtype aFe,O, (hematite) in the pressure range up to 65 GPa using synchrotron radiation and a diamond anvil cell. A structural phase transformation has been observed with a transition pressure of about 55 GPa. The high-pressure form of Fe,O, is orthorhombic. The lattice parameters at 60GPa are a = 4.59(3)& b = 4.97( 3)& c = 6.68( 5)& Z = 4. The crystal structure is most probably a GdFe0,-type perovskite with space group Pbnm. The volume per formula unit in the orthorhombic phase is 10% smaller than in aFe,O,. It is suggested that the volume change is due to a combination of a valence change and a high-spin/low-spin transition. The bulk modulus, E,, and its pressure derivative, E& evaluated at zero pressure for aFe,O, are E, = 230(5)GPa and EA = 3.5(6).

Electronic and magnetic structures of the postperovskite-type Fe 2 O 3 and implications for planetary magnetic records and deep interiors

“…At 70 GPa and 300 K, our XRD confirms a complete transformation to the RhII phase, consistent with previous studies (20). We found a very simple Mössbauer spectrum for the RhII phase (Fig.…”

“…Several intriguing changes have been observed in Fe 2 O 3 at 40 ≤ P ≤ 60 GPa: a large increase in density (14%) and a drastic drop in resistivity (17)(18)(19), a phase transition to a Rh 2 O 3 -II (RhII) type structure (20), disappearance of magnetic hyperfine fields during the hematite → RhII transition (19), and the HS → LS transition of Fe (21).…”

X-ray diffraction investigations of CaF2 at high pressure

“…Dandekar & Jamieson (1969) report a similar value (10%) and Seifert (1966) a slightly smaller value (8.1%). Volume changes of the same order are also observed for the B1 to B2 transition in compounds with the rock-salt structure (Gerward, Olsen, Steenstrup, Benedict & DabosSeignon, 1990) or corundum structure (Olsen, Cousins, Gerward, Jhans & Sheldon, 1991).…”

Intriguing sequence of GaFeO3structures and electronic states to 70 GPa

“…Therefore, in contrast to the Pc2 1 n structure, the octahedral framework can readily transform between the two structures by rotating them as rigid units, without breaking any bonds. The V 0 of the R3c phase is ~ 5% smaller than that of the Pc2 1 n phase but its bulk modulus K 0 remains almost the same and is practically identical to that of hematite [4,34,35]. At recompression the first-order structural phase transition takes place again at P~50 GPa …”

“…What has triggered the interest on the HP properties of these antiferromagnetic Mott insulators was the discovery [3] of the electronic mechanism responsible for the unusual ~ 10% volume shrinkage observed in Fe 2 O 3 (hematite) at ~50 GPa [4]. In that case [5], a pressure-induced progressive distortion of the corundum like hematite structure was observed, preceding the transition and signifying the increasing asymmetry of the FeO 6 octahedra, which culminated in a first-order structural phase transition from a corundum to a Rh 2 O 3 (II)-type structure.…”