High-pressure phase transition of hematite, Fe2O3

doi:10.1016/j.jpcs.2003.11.042

Journal of Physics and Chemistry of Solids

Volume 65, Issues 8–9, August–September 2004, Pages 1527-1530

https://doi.org/10.1016/j.jpcs.2003.11.042 Get rights and content

Abstract

Structural phase transitions of iron oxide (Fe₂O₃) have been investigated at pressures up to 70 GPa and temperatures above 2500 K, using a laser-heated diamond anvil cell technique. A phase transition between α-Fe₂O₃ (hematite) and perovskite-type Fe₂O₃ was observed at about 30 GPa. At pressures higher than 50 GPa, we also observed the occurrence of a new high-pressure phase of Fe₂O₃. Diffraction peaks of the new high-pressure phase can be indexed on the basis of orthorhombic or monoclinic symmetries that are denser than other known Fe₂O₃ phases. The volume change from perovskite-type to the new high-pressure phase is about 7%.

Introduction

Iron oxide and iron-bearing compounds exist in significant abundance in the earth's mantle and play an important role in determining the physical properties of the earth's interior. The investigation of hematite, Fe₂O₃, is useful in providing information that can lead to a better understanding of the group of compounds that crystallize in similar structures. In addition, information on the crystal structure of hematite at high-pressure is basic to an understanding of its magnetic and electronic properties under pressure.

Initially, the Hugoniot compression curve of hematite indicated that a phase transition occurs at about 50 GPa accompanied by volume decrease of about 15% [1]. Although many high-pressure experimental studies, using both static compression and shock-wave experiments, have accumulated [2], [3], [4], [5], [6], [7], [8], [9], [10], the phase transition sequence and magnetic properties of Fe₂O₃ remain uncertain. There is a possibility that a transition from high-spin to low-spin states of iron occurs at high pressures [10]. The high-spin state of iron is usually stable in oxides and silicates at ambient pressure. As the ionic radius of the low-spin state is smaller than that of the high-spin state, the spin-pairing transition from the high-spin to low-spin may be induced by increased pressure. High-pressure Mössbauer studies of Fe₂O₃ indicate that the oxidation state of iron transforms from Fe³⁺ at low pressure to a new valence state [4], [8].

In this study, we used a laser-heated diamond anvil cell (LHDAC) that made it possible to acquire precise data on a sample under high-pressures and high-temperatures, using intense X-ray from a synchrotron radiation source. We report on the results of in situ X-ray observations of the high-pressure phases of Fe₂O₃. We also comment on the phase diagram of Fe₂O₃ at high-pressures and high-temperatures.

Section snippets

Experiment

The high-pressure X-ray diffraction experiments were performed using a LHDAC high-pressure apparatus. Synthetic powdered α-type Fe₂O₃, hematite (Wako Pure Chemical Industries, Ltd.; purity 99.9%), was loaded into 50–100 μm diameter hole that was drilled into a rhenium gasket using an excimer laser. Argon and NaCl were used as pressure transmitting mediums to reduce diviatric stress and temperature gradients in the sample. The sample was compressed to the desired pressure and then heated. After

Results and discussion

We used two types of pressure transmitting mediums: argon and NaCl. There were no significant differences in the diffraction patterns of the samples between those prepared with argon or NaCl pressure transmitting mediums. This indicates that the samples did not react with the pressure transmitting mediums at high temperatures during laser heating in this study. In each run, the sample was compressed to the desired pressure and then heated to synthesize the high-pressure phases of Fe₂O₃. X-ray

Acknowledgments

We thank Y. Tatsumi, M. Handler, E. Takahashi, K. Hirose, and T. Iizuka for help during this project, and S.K. Saxena for his invitation to participate to this journal special issue. The synchrotron radiation experiments were performed at the PF, KEK (Proposal No. 2001G222) and at the SPring-8, JASRI (Proposal No. 2002A0106-ND2-np and 2002B0162-ND2-np). This work was also supported by Ministry of Education, Culture, Sport, Science and Technology, Japan.

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High-pressure phase transition of hematite, Fe2O3

Abstract

Introduction

Section snippets

Experiment

Results and discussion

Acknowledgments

Solid State Commun.

Solid State Commun.

Phys. Earth Planet. Inter.

Electrical resistivity and phase transformation of hematite under shock compression

J. Geophys. Res.

A study of the crystal structure of Fe2O3 in the pressure range up to 65 GPa using synchrotron radiation

Phys. Scr.

High-pressure phase transition of hematite, Fe₂O₃

A study of the crystal structure of Fe₂O₃ in the pressure range up to 65 GPa using synchrotron radiation