First-Principles Prediction of Possible Synthesis of Li-Fe Based Complex Hydride Li₄FeH₆

Abstract

  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.