Phase diagrams of iron hydrides at pressures of 100–400 GPa and temperatures of 0–5000 K

The stability of Fe$_4$H, Fe$_2$H, FeH, Fe$_3$H$_5$, FeH$_2$, FeH$_3$, FeH$_4$, Fe$_3$H$_{13}$, FeH$_5$, and FeH$_6$ iron hydrides at temperatures of 0–5000 K and pressures of 100–400 GPa has been analyzed for the first time in the density functional theory using the lattice dynamics method in the quasiharmonic approximation, and the corresponding PT phase diagrams have been obtained. It has been found that heating expands a set of stable stoichiometric compounds, so that a number of structures metastable at room temperature are stabilized at temperatures above 1000 K. The topological analysis of structures of iron hydrides indicates that most of them belong to rare or unique topological types. An increase in the amount of hydrogen in a structure is accompanied by the reduction of the length of an H-H bond, which results in the formation of dumbbell-like hydrogen molecules H$_2$ in FeH$_x$ structures with $x > 6$. However, these structures are thermodynamically unstable and decay into a mixture of FeH$_6$ and solid H.