First-principles study on the stability of intermediate compounds of LiBH4

We report the results of the first-principles calculation on the intermediate compounds of LiBH4. The stability of LiB3 H8 and Li2 Bn Hn (n=5-12) has been examined with the ultrasoft pseudopotential method based on the density-functional theory. Theoretical prediction has suggested that monoclinic Li2 B12 H12 is the most stable among the candidate materials. We propose the following hydriding (dehydriding) process of LiBH4 via this intermediate compound: LiBH4 ↔ 1 12 Li2 B12 H12 + 5 6 LiH+ 13 12 H2 ↔LiH+B+ 3 2 H2. The hydrogen content and enthalpy of the first reaction are estimated to be 10 mass % and 56 kJ mol H2, respectively, and those of the second reaction are 4 mass % and 125 kJ mol H2. They are in good agreement with experimental results of the thermal desorption spectra of LiBH4. Our calculation has predicted that the bending modes for the Γ -phonon frequencies of monoclinic Li2 B12 H12 are lower than that of LiBH4, while stretching modes are higher. These results are very useful for the experimental search and identification of possible intermediate compounds.