Photon Factory Activity Report 2014 #32 (2015) B
AR-NE1A/2013G079

Relationship between structural variation and spin transition of hcp-Fe under highpressures and high temperatures
Shigeaki Ono1,*1
Research and Development Center for Ocean Drilling Science, Japan Agency for Marine-Earth
Science and Technology (JAMSTEC), Yokosuka 237-0061, Japan

2 Experiment
The high-pressure and high-temperature X-ray
diffraction experiments were performed using a laserheated diamond anvil cell high-pressure apparatus. Fe
powder was loaded into a 30-100 μm diameter holes
drilled into a rhenium gasket, which was preindented to a
thickness of 20-70 μm. The sample was sandwiched
between pellets of NaCl powder to reduce any residual
nonhydrostatic stresses. The sample was probed using
angle-dispersive X-ray diffraction, employing the NE1A
synchrotron beamline at KEK. The angle-dispersive Xray diffraction patterns were obtained on an image platesystem. The spectra were collected for about 5-10 min.
The observed intensities on the imaging plates wereintegrated as a function of 2θ using the ESRF Fit2d
software code to obtain conventional, one-dimensional
diffraction profiles.

The structural variation in the c/a ratio of hcp-Fe wasinvestigated up to 180 GPa and 2000 K. Prior to laser
heating, we compressed the α-Fe starting material to thedesired pressure at room temperature, and found that it
transformed to the hcp structure. The room-temperature
data were in general agreement with those reported in ourprevious study. The c/a ratio decreased from 1.61 to 1.60
as the pressure increased to 60 GPa, and remained
approximately constant at 1.60 after that. At hightemperatures, the c/a ratio decreased at 2000 K withincreasing pressure. At pressures above 140 GPa, the ratio
deviated noticeably from the decreasing trend. Theseresults show that the spin transition of Fe occurred at~140 GPa at 2000 K [1]. The gradient of dP/dT of thespin transition boundary was positive. The boundary
shown in Fig. 1 is represented by the linear equation:
P (GPa) = 51 + 0.045 x T (K).

8000

Temperature (K)1 Introduction
The high-pressure spin state of Fe is key to theunderstanding of magnetic, electronic, and structuralproperties of 3d electrons and of the structure of theEarth’s or other planets’ interiors. It is known fromexperiments that the pressure-induced transition frombody-centered cubic (bcc) to hexagonal close-packed(hcp) is accompanied by a simultaneous magnetictransition from the ferromagnetic to nonmagnetic states,
because Mössbauer effect experiments have neverdetected the presence of a hyperfine magnetic field in the
hcp structure. However, some experimental observationsof the structural variation, x-ray emission spectroscopy, orRaman mode splitting indicated that hcp-Fe could haveremnant magnetism. According to the structural variation,
first-principles calculations predicted that the c/a ratio of
the hcp structure in nonmagnetic Fe increases as thepressure increases. In contrast, recent experimental
observations showed that the c/a ratio of hcp-Fe
decreased with increasing pressure, indicating that hcp-Fe
has the antiferromagnetic state. As the pressure increases,
the magnetic transition from the antiferromagnetic to thenonmagnetic state occur, and is accompanied by spin
collapse at ~60 GPa. This spin transition from the high- to
the low-spin states was predicted by the first-principles
calculations and was confirmed from the structuralvariation using the high-pressure experiments. In thisstudy, we investigated the pressure-induced spintransition at high temperatures using the structuralvariation of hcp-Fe.

6000

Melt4000

fcc
00bcc

100

200300400

Pressure (GPa)Fig. 1: Schematic phase diagram of iron at high pressuresand temperatures. The dashed line represents theboundary between the high-spin (HS) and low-spin (LS)states in hcp-Fe.

Acknowledgement
We thank the PF staffs for their helps in carrying out
the experiments.
References[1] S. Ono, Solid State Communications 203, 1-4 (2015).
* sono@jamstec.go.jp

3 Results and Discussion

LS-hcpHS-hcp2000