New Iron Hydrides under High Pressure

Abstract

The Fe-H system has been investigated by combined x-ray diffraction studies and total energy calculations at pressures up to 136 GPa. The experiments involve laser annealing of hydrogen-embedded iron in a diamond anvil cell. Two new FeHx compounds, with x∼2 and x=3, are discovered at 67 and 86 GPa, respectively. Their crystal structures are identified (unit cell and Fe positional parameters from x-ray diffraction, H positional parameters from ab initio calculations) as tetragonal with space group I4/mmm for FeH(∼2) and as simple cubic with space group Pm3m for FeH3. Large metastability regimes are observed that allowed to measure the P(V) equation of state at room temperature of FeH, FeH(∼2), and FeH3.

High‐Pressure Geophysical Properties of Fcc Phase FeHX

“…Nearly stoichiometric iron hydride (FeH X , X∼1) has been shown to result from either the reaction of Fe and hydrous silicates (Yagi & Hishinuma, ) or the reaction of Fe and water at lower mantle conditions (Ohtani et al, ). Within the known iron hydride phase diagram, the face‐centered cubic ( fcc ) structure is considered the most stable structure at high temperatures, rendering it more important for extrapolation to core P‐T conditions than the bcc or hcp structures (Pépin et al, ). Despite its importance, experimental and ab initio efforts have yet to agree on the equation of state parameters needed to describe the compressional behavior of FeH X (Badding et al, ; Fukai et al, ; Hirao et al, ; Pépin et al, ).…”

“…Within the known iron hydride phase diagram, the face‐centered cubic ( fcc ) structure is considered the most stable structure at high temperatures, rendering it more important for extrapolation to core P‐T conditions than the bcc or hcp structures (Pépin et al, ). Despite its importance, experimental and ab initio efforts have yet to agree on the equation of state parameters needed to describe the compressional behavior of FeH X (Badding et al, ; Fukai et al, ; Hirao et al, ; Pépin et al, ). To constrain the extent to which hydrogen contributes to the density deficit of the Earth's core, it is necessary to establish the compressional wave velocity‐density ( V P ‐ ρ ) relationship of iron hydrides.…”

Synthesis of lithium polyhydrides above 130 GPa at 300 K

“…The transition from RhH to RhH 2 is associated with the change of interstitial hydrogen from octahedral to tetrathedral sites (12). Two novel iron hydrides, FeH 2 and FeH 3 , have been discovered under pressure, in the sequence FeH-FeH 2 -FeH 3 that follows the expected drastic increase of hydrogen content upon pressure increase (13). Moreover, FeH 2 and FeH 3 adopt intriguing structures with layers of atomic hydrogen.…”

Electron impact ionisation cross sections of iron hydrogen clusters

“…The frequency of its vibrational bending mode is 226 cm −1 , characteristic of a very floppy angular bending [14], while the asymmetric Fe-H stretching frequency at 1674 cm −1 indicates bonds of medium strength [15]. In addition, FeH 2 is normally only stable as a diluted gas or in matrices, although recently [16] a solid high-pressure phase with the stoichiometry of FeH 2 has been synthesised. All other FeH n clusters from FeH 3 onwards can be thought to consist of FeH or FeH 2 with H 2 molecules bound to them.…”