The crystal and electronic structures, and thermodynamic stability of hypothetical complex hydride Li4FeH6 were examined using first-principles calculations. We found that the hydride occurs in a K4CdCl6-type structure with space group R-3c (No. 167), which is the same as that reported for Na4RuH6. The calculated enthalpy change of the reaction, 4LiH+Fe+H2→Li4FeH6, is −54 kJ/mol H2. Given that Fe does not form any stable binary compounds with both Li and H, Li4FeH6 could be the most stable phase among the compounds consisting of Li, Fe and H. Using a calculated entropy change of −115 J/mol H2•K, the equilibrium pressure under 773 K is estimated to be 24 MPa. The electronic structure illustrates that four Li atoms donate a total of four electrons per formula unit to the FeH6 octahedron, forming an ionic configuration of Li+4[FeH6]4−. The complex hydride reported here has the higher hydrogen gravimetric density, a value of 6.7 mass%, than other Fe-based complex hydrides ever reported, and thus would be of interest in terms of hydrogen storage.
