Abstract
Neutron powder diffraction profiles were collected for iron deuteride (FeDx) while the temperature decreased from 1023 to 300 K for a pressure range of 4–6 gigapascal (GPa). The ε′ deuteride with a double hexagonal close-packed (dhcp) structure, which coexisted with other stable or metastable deutrides at each temperature and pressure condition, formed solid solutions with a composition of FeD0.68(1) at 673 K and 6.1 GPa and FeD0.74(1) at 603 K and 4.8 GPa. Upon stepwise cooling to 300 K, the D-content x increased to a stoichiometric value of 1.0 to form monodeuteride FeD1.0. In the dhcp FeD1.0 at 300 K and 4.2 GPa, dissolved D atoms fully occupied the octahedral interstitial sites, slightly displaced from the octahedral centers in the dhcp metal lattice, and the dhcp sequence of close-packed Fe planes contained hcp-stacking faults at 12%. Magnetic moments with 2.11 ± 0.06 μB/Fe-atom aligned ferromagnetically in parallel on the Fe planes.
Highlights
Iron (Fe) reacts with hydrogen (H) to form solid solution FeHx or stoichiometric monohydride FeH1.0 at hydrogen pressures in a gigapascal (GPa) range
The most recent X-ray diffraction measurements revealed a substantial reduction in volume at high temperatures near the dhcp–fcc phase boundary, and the partial release of dissolved H atoms or the formation of the solid solution was suggested in this regard[16]
The structure of dhcp FeD1.0 contained the off-central displacement of D atoms on octahedral interstitial sites and stacking faults in the dhcp sequence of Fe planes, consistent with the early results[6]
Summary
Iron (Fe) reacts with hydrogen (H) to form solid solution FeHx or stoichiometric monohydride FeH1.0 at hydrogen pressures (hereafter referred to as pressure) in a gigapascal (GPa) range. The phase diagram of the Fe–H system extending to 3000 K and 120 GPa indicates the ε′ phase as the only stable phase at pressures greater than 20 GPa, presumably maintaining the stoichiometric composition of FeH1.012 Such unique phase stability allows for the investigation of the structural and magnetic properties over a wide T–P range. Ferromagnetic–paramagnetic transition has been experimentally investigated at ambient temperature and pressure up to 80 GPa by Mössbauer (MB) and X-ray magnetic circular dichroism (XMCD) spectroscopies[21,22,23] These results showed that the magnetic moment of dhcp FeH1.0 continuously deceased with pressure and eventually disappeared at roughly 30 GPa at 300 K. The magnitude and alignment of the magnetic moments were in agreement with those theoretically calculated for ferromagnetic dhcp FeH1.026
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