承認日 Date of Approval 2017/03/06
承認者 Approver Takanori Hattori
Date of Report 2017/03/06

Experimental Report
Project No.
2016B0017
Title of experimentHydrogen volume in iron hydride at high pressure and temperature
Name of principal investigator
Eiji Ohtani

Takanori Hattori
Name of Instrument/ BL No.
ATSUHIME 6-axis press at Planet
beamline (BL11)
Date of Experiment
January 14-19

Affiliation
Graduate school of Science, Tohoku University

Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1.
Name of sample(s) and chemical formula, or compositions including physical form.
The starting material was powdered sample of iron (99.9% purity, Wako pure chemical industries, Ltd) with agrain size about 300 um.

The powdered Fe was compressed by uniaxial hand press to the pressure of 100-200

MPa, and a Fe disc with 3 mm diameter and 2.5 mm length was prepared for the starting material.

2.
Experimental method and results.

If you failed to conduct experiment as planned, please describe reasons.

Experimental method:
A cubic high-pressure apparatus (Atsuhime) installed at BL11 was used for high pressure generation. An Fe discspecimen (3mm in diameter and 2.5mm in height) is placed at the center of a hydrogen-sealing capsule made ofNaCl (5.5mm in diameter and 8mm in height) with internal hydrogen sources of NH3BH3 pellets above andbelow. The NaCl capsule was inserted in a cylindrical graphite heater and embedded in a pressure-transmitting
medium made of ZrO2 (17-mm-edge cube). High pressure high temperature diffraction experiments wasconducted at around 5-9 GPa in the stability field of fcc-iron and 300-1200 K. Neutron diffraction data wascorrected at high pressure and temperature. Neutron-diffraction profiles were accumulated during the heatingprocess. The temporal evolution of the diffraction profile was monitored at several fixed temperatures above thehydrogenation temperature of NH3BH3. The composition and site occupancy of H atom in the fcc-anddhcp-lattice of FeHx were determined for the equilibrium state at a fixed pressure. The temperature was kept
constant and the temporal evolution of the diffraction profile was monitored to confirm that the solid solution of1

Experimental method and results (continued)

2.

FeHx reached equilibrium with the surrounding H2 fluid. The structure of FeHx and the content of hydrogen weredetermined by Rietveld refinement.
Results and discussion
We have successfully determined the hydrogen atomic volume in iron hydride lattice at various pressure andtemperature for the first time. Thephase transition and hydrogenation ofiron were observed by dhcp-FeHx andfcc-FeHx phase, which are highpressure and high temperature phasesof FeHx. We have conducted two
separate runs. Run 1 was made in thepressure range around 6 GPa using thepressure cell for the cubic anvils with
10 mm truncated edge length. Run 2
was made at around 8 GPa using theanvils with 7 mm truncated edge
length. The pressure-temperature pathsof our experiments are shown in Figure 1. The phase boundaries of FeHx compounds determined here are
significantly different from those reported previously as shown in this figure; stability field of dhcp-FeHx iswider as shown in this figure. We have successfully conducted Rietveld analyses for six runs, and the atomic
fraction of hydrogen, x, was determined for fcc and dhcp-FeHx. We found that x for fcc-FeHx varies from 0.45
to 0.75 in the pressure and temperature range studied, whereas x for dhcp-FeHx is close to unity. We determined
the hydrogen volume in iron hydride lattice based on our Rietveld analyses and previous equation of state of ironat high pressure and temperature. The atomic volume of hydrogen is greater than those estimated previously (e.g.,Fukai et al., 2003) as was determined for the deuterium volume in FeDx (Machida et al., 2014). Our data provide
important clues for evaluation of the compression behavior of hydrogen atom in metallic iron lattice and for morereliable estimation of the hydrogen content in the Earth’s core by comparing with the density of the Earth’s corederived from seismological studies.
We have studied the SEM-EBSD analysis of the recovered iron samples which was transformed to bcc-Fe
after release of hydrogen during decompression and recovery. The recovered bcc-Fe shows characteristic micro–textures with large lattice strain and oriented micro-cracks which might be formed during release of hydrogenfrom the solid iron hydride.
References:
Fukai et al., Jour. Alloy Compound, 348, 105-109 (2003).
Machida et al., Nature communications, 5:5063 doi: 10.1038/ncoms6063 (2014)2