New Iron Hydrides under High Pressure

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 ${\mathrm{FeH}}_x$ compounds, with $x\sim 2$ 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 ${\mathrm{FeH}}_{\sim 2}$ and as simple cubic with space group $Pm\overline{3}m$ for ${\mathrm{FeH}}_3$. Large metastability regimes are observed that allowed to measure the $P(V)$ equation of state at room temperature of FeH, ${\mathrm{FeH}}_{\sim 2}$, and ${\mathrm{FeH}}_3$.

Figure 1

Phase diagram of the Fe-H system. The black boundary lines for FeH have been taken from Refs. [8] and [9]. Colored rectangles depict the $P\text{−}T$ regions where laser heating has been performed and the stability domains of dhcp-FeH, ${\mathrm{FeH}}_2$, and ${\mathrm{FeH}}_3$ are indicated. Horizontal solid and dashed lines indicate the pressure range over which these phases are stable and metastable, respectively, at 298 K. Inset: photograph of a typical sample configuration. The sample is embedded in hydrogen and sits on $c\text{−}\mathrm{BN}$ grains, the small black dots which can also be seen around it.

Figure 2

X-ray diffraction patterns of FeH, ${\mathrm{FeH}}_2$, and ${\mathrm{FeH}}_3$ (right: image plates, left: integrated patterns). Bottom: dhcp-FeH at 64 GPa. Black dots mark the diffraction peaks from insulating material KCl. The significant broadening of the (102) peak relative to the others is attributed to random stacking faults [23]. Middle: ${\mathrm{FeH}}_2$ at 67 GPa, described as a tetragonal $I4/mmm$ ($a=2.479\AA$, $c=8.415\AA$, $Z=4$) unit cell, Fe in positions $4e$ (0,0,0.853). The Rietveld refinement is shown with a Bragg $R$ factor $R_B=12.1\%$. Top: Simple cubic $Pm\overline{3}m$ ($a=2.437\AA$, $Z=1$) phase of ${\mathrm{FeH}}_3$ at 87 GPa. The Rietveld refinement with Fe in positions 1a (0,0,0) is shown with a Bragg $R$ factor $R_B=8.4\%$.

Figure 3

Molar volume per formula unit as a function of pressure for iron hydrides. Experimental data for dhcp-FeH, ${\mathrm{FeH}}_{\sim 2}$, and ${\mathrm{FeH}}_3$ are represented together with literature data for FeH [22, 23]. Error bars do not exceed the size of the data points. The Vinet fits [24] of the data are full lines and correspond to the parameters given in Table 1. Dashed lines are results from the ab initio calculations with the most stable magnetic order (see text). Dash-dotted lines depict the molar volumes of ideal mixtures of iron and hydrogen with H:Fe ratio of 1, 2, and 3 using the equations of state of Refs. [26, 27].

Figure 4

(a) Calculated enthalpies as a function of pressure for the Fe-H structures proposed to match the experimental data. The magnetic phase of ${\mathrm{FeH}}_3$ has been taken as reference so that $\mathrm{\Delta }\mathrm{H}=\mathrm{H}({\mathrm{FeH}}_{3-n})+(n/2)\mathrm{H}({\mathrm{H}}_2)\text{−}\mathrm{H}({{\mathrm{FeH}}_3}^{\text{FM}})$. (b) Representation of the structures of FeH, ${\mathrm{FeH}}_2$, and ${\mathrm{FeH}}_3$. Large blue and small red spheres represent the iron and hydrogen atoms, respectively.