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 $x\ensuremath{\sim}2$ 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 ${\mathrm{FeH}}_{\ensuremath{\sim}2}$ 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, ${\mathrm{FeH}}_{\ensuremath{\sim}2}$, and ${\mathrm{FeH}}_{3}$.