First-Principles Characterization of Amorphous Phases of MB12H12, M = Mg, Ca

Complex borohydrides have been studied as candidate materials for vehicular hydrogen storage. In particular, M(BH4)(2), where M = Mg or Ca, has received considerable attention because of its high gravimetric capacity for H-2. Experimental observations indicate an initial decomposition reaction: 6M(BH4)(2) -> 5MH(2) + MB12H12 + 13H(2), where the MB12H12 species are amorphous. We use first-principles density functional theory to study the structural and energetic properties of MB12H12. The purpose of these calculations is to investigate why these materials form amorphous structures. We have identified two possible reasons for MB12H12 adopting apparently amorphous rather than crystalline structures. First, our calculations reveal a large number of structurally distinct polymorphs having near-degenerate energies. We simulate an X-ray diffraction (XRD) pattern for each polymorph and compute the Boltzmann-averaged XRD patterns for MgB12H12, and CaB12H12. These average patterns appear to be amorphous, having very broad peaks. Therefore, we predict that the amorphous MB12H12 materials observed experimentally may be an intimate mixture of a very large number of structurally distinct crystallites. Second, our first-principles molecular dynamics calculations indicate that cations are highly mobile near room temperature; analysis of cation radial distribution functions gives liquid-like rather than solid-like features that could also result in an experimental observation of an amorphous XRD pattern near room temperature.