Enhanced tunability of thermodynamic stability of complex hydrides by the incorporation of H^- anions

First-principles calculations were employed to investigate hypothetical complex hydrides (M,M′)4FeH8 (M = Na, Li; M′=Mg, Zn, Y, Al). Besides complex anion [FeH₆]^4-, these materials contain two H^- anions, which raise the total anionic charge state from tetravalent to hexavalent, and thereby significantly increasing the number of combinations of countercations. We have determined that similar to complex hydrides (M,M′)2FeH6 containing only [FeH₆]^4-, the thermodynamic stability is tuned by the average cation electronegativity. Thus, the chemical flexibility provided by incorporating H^- enhances the tunability of thermodynamic stability, which will be beneficial in obtaining optimal stability for hydrogen storage materials.