Photon Factory Activity Report 2005 #23 Part B (2006)
High Pressure Science

13A/2005G122

Phase transitions of FeSi and FeS at high pressures and high temperatures
Shigeaki ONO*1
Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science andTechnology (JAMSTEC), Yokosuka, Kanagawa 237-0061, Japan

Introduction
Iron silicide and iron sulfide are the probablecandidates of the origin of the ultra-low velocity zone at
the base of the Earth’s mantle, which was reported byseismic studies. Iron-bearing materials may appear at thecore-mantle boundary, because the liquid iron coexistingwith the solid silicates. In order to understand the reaction
between liquid core and the solid mantle, there have been
a number of studies for the phase relations of iron-bearing
materials. It is known that iron silicide and iron sulfide
have some high-pressure forms as pressure increases.
However, it has not investigated reasonably by highpressure experiments that what structures are stable atextreme high pressures and high temperatures
corresponding to the Earth’s core-mantle boundary. In
this study, therefore, the high-pressure phases of ironsilicate and iron sulfide were investigated using a laserheated diamond anvil cell combined with a synchrotron
X-ray diffraction method.
Experimental
High-pressure X-ray diffraction experiments wereperformed using a laser-heated diamond anvil cell highpressure apparatus. Synthetic powdered FeSi and FeS
were loaded into a 100 ȝm diameter holes that weredrilled into a rhenium gasket. NaCl was used as pressure
calibrant [1]. The samples were heated with a YAG laser
to overcome any potential kinetic effects on possible
phase transitions. The samples were probed using anangle-dispersive X-ray diffraction technique at thesynchrotron beam lines BL13A, Photon Factory in Japan[2]. A monochromatic incident X-ray beam with awavelength of about 0.42 angstrom was used. The X-raybeams were collimated to a diameter of 15-30 ȝm, andthe angle-dispersive X-ray diffraction patterns wereobtained on an imaging plate (Rigaku).
Results and Discussion
FeSi was compressed up to 67 GPa. A phase transition
in FeSi to cubic was observed at 40 GPa after the laserheating. Before the heating, a strain–broadening of thediffraction peaks of the sample was observed, because alarge differential stress was induced in the diamond anvil
cell experiments as pressure increased. Next, the sample
was heated to about 1500-2000 K to relax the differential
stress and to overcome potential kinetic effects on
possible phase transitions. After the heating, some newpeaks appeared in the diffraction pattern. This implies
that the starting material transformed to a new highpressure phase. The transition pressure could not been

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Users' Report

determined. The lattice parameters at 41 GPa and 300 K,for example, are a = 2.954(1) Å with a unit cell volume of25.77(2) Å3 for the cubic. A pressure-volume relation isshown in Fig. 1. The isothermal bulk modulus of cubic
phase was determined to be about 230 GPa. This value isin generally agreement with that predicted by thetheoretical calculations reported by previous study.
FeS was compressed up to 110 GPa. Some phasetransitions in FeS were observed. Before the heating, astrain–broadening of the diffraction peaks of the sample
was also observed. At about 55 GPa, a new orthorhombic
phase was observed, after the laser-heating. Next, thesample was compressed to 100 GPa. Some new peaks
appeared in the diffraction pattern after the heating. Thisimplies that a new high-pressure phase was stable atpressures higher than 100 GPa. This new high-pressure
phase has cubic symmetry.
According to our preliminary results, both iron silicide
and iron sulfide have the cubic symmetry at the coremantle boundary. These materials are likely to contributeto decrease seismic velocities of both Vp and Vs at thebase of the lower matnle.

References[1] Ono et al., Solid State Commun. 137, 517 (2006).
[2] Ono et al., J. Appl. Phys. 97, 073523 (2005).
* sono@jamstec.go.jp
20Volume (A3)

11918

1730

405060Pressure (GPa)7080

Figure 1. Pressure-volume data for high-pressure phaseof iron silicide at 300 K. Circles are volume data of ironsilicide. Dashed curves are Birch-Murnaghan equation
fits.