Hence, the formation of Fe polyhydrides can be
viewed as an ideal mixing of Fe and atomic metal
H and associated, for a large fraction of metal H
(25), with the formation of a thicker atomic H
slab in the structure of the polyhydride.
The electron-density distribution (Fig. 3) gives
a qualitative indication of the bonding in FeH5.
The valence electron density is nonuniform in
space (Fig. 3A). The higher electron density be-
tween Fe and H suggests bonding between them.
Thirteen H atoms bond to each Fe atom, 12 of them
bridging two Fe atoms, and the remaining one
has a single Fe–H bond parallel to c (“top” H). The
ABAB stacking of FeH3 units allows a compact
arrangement of “top” H atoms. Many hydrogen
atoms can be bound to a transition metal atom (31),
although only FeH, FeH2, and FeH3 molecules
could be isolated in matrices at ambient pressure
(32). We visualized the electron density distribu-
tion in the plane parallel to the stacking direction,
also showing the H–Fe bonding (Fig. 3B). The
electron density in the plane of the hydrogen atoms
is more uniform (Fig. 3C), with a minimum value
3.6 times lower (0.3 el/Å3) than that along Fe–H.
It clearly shows that no H–H bonding exists. The
topology of the electron density seems to give a
different representation of the FeH5 structure than
the geometrical one, more as a stacking of alter-
nate FeH5 layers reminiscent of two-dimensional
(2D) van der Waals materials. The reality is prob-
ably in between, with the atomic H slab inter-
acting with the Fe atoms to stabilize dense atomic
H at a much lower pressure than in pure hydrogen,
in the spirit of the chemical “precompression”
proposed by Ashcroft (3).
We also calculated the band structure and the
electronic densities of states (DOS) for FeH5 (Fig. 4).
The DOS has a clear steplike character at the bot-
tom of the occupied band, indicating that elec-
tronically FeH5 is also a 2D metal. In this range,
the DOS is constituted by H 1s and Fe 4s orbitals.
Yet, there is a clear depletion of the DOS near the
Fermi level, so FeH5 should be considered as a
weak metal. The electronic band structure, as
drawn in Fig. 4B, shows that the bandgap closure
is only effective for a few points at the Brillouin-
The series of Fe polyhydrides, from FeH to FeH5,
present an example of how hydrogen can com-
bine with transition metals as pressure increases.
Remarkably, the structures of these polyhydrides
are built of atomic hydrogen only. By increasing
pressure, hence the H content, slabs of atomic hy-
drogen are stabilized, giving, as in an FeH5 struc-
ture, structures resembling bulk atomic hydrogen.
The next step would now be to quantify in FeH5
some of the intriguing quantum effects of atomic
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384 28 JULY 2017 • VOL 357 ISSUE 6349 sciencemag.org SCIENCE
Fig. 3. Electron-density distribution at
147 GPa. (A) Structure of FeH5 showing
the isosurface for the electron density of
0.8 el/Å3. (B) Electron density map in the
plane y = 0 as shown on the truncated
structure. The large hidden spheres at the
bottom around x = –2 and 2 bohrs are iron
atoms. (C) Electronic density map in a
plane perpendicular to the c axis as shown
on the truncated structure. The hydrogen
atom in position (0, 0, 0.23) is taken as the
reference position (0, 0).
Fig. 4. Band structure and electronic densities of state of FeH5
at 147 GPa. (Left) Electronic density of states of FeH5. (Right) Electronic
band structure plotted on a path in the kz = 0 plane. (Inset) View of
the Brillouin zone of the conventional unit cell, with the path used to plot
the band structure (red arrows). The Fermi level is set at 0 eV in both