Phase Stability, Phase Mixing, and Phase Separation in Fluorinated Alkaline Earth Hydrides

Abstract

Alkaline-earth hydrides (AH2) are considered as potential hydrogen storage material. Due to high decomposition temperature and slow sorption kinetics, these hydrides cannot be used for energy storage applications. As fluorine is more electronegative than hydrogen, substitution of hydrogen by fluorine will bring anisotropic bonding interaction, and hence, it may improve the hydrogen storage properties. Hence, the structural stability, electronic structure, and chemical bonding of AH2 and fluorinated AH2 (AH2–xFx) are delineated using ab initio calculations. From the calculated enthalpy of formation we have predicted that AH2–xFx are relatively more stable than the corresponding pure hydrides. The positive and very low value of enthalpy of mixing for AH2–xFx imply that single-phase of AH2–xFx may form at reasonable temperatures. The band structure and density of states (DOS) calculations reveal that AH2–xFx are insulators. Partial DOS, charge density, electron localization function, and crystal orbital Hamiltonian population analyses conclude that these compounds are governed mainly by ionic bonding. The calculated H site energy increases as the fluorination increases and thus fluorination bring extra stability in the lattice. The present results suggest that hydrogen closer to fluorine can be removed more easily than that far away from fluorine. Hence, the fluorination brings disproportionation in the bonding between the constituents.