First-principles calculated decomposition pathways for LiBH(4) nanoclusters

We analyze thermodynamic stability and decomposition pathways of LiBH(4) nanoclusters using grand-canonical free-energy minimization based on total energies and vibrational frequencies obtained from density-functional theory (DFT) calculations. We consider (LiBH(4))(n) nanoclusters with n = 2 to 12 as reactants, while the possible products include (Li)(n), (B)(n), (LiB)(n), (LiH)(n), and Li(2)B(n)H(n); off-stoichiometric Li(n)B(n)H(m) (m ≤ 4n) clusters were considered for n = 2, 3, and 6. Cluster ground-state configurations have been predicted using prototype electrostatic ground-state (PEGS) and genetic algorithm (GA) based structural optimizations. Free-energy calculations show hydrogen release pathways markedly differ from those in bulk LiBH(4). While experiments have found that the bulk material decomposes into LiH and B, with Li(2)B(12)H(12) as a kinetically inhibited intermediate phase, (LiBH(4))(n) nanoclusters with n ≤ 12 are predicted to decompose into mixed Li(n)B(n) clusters via a series of intermediate clusters of Li(n)B(n)H(m) (m ≤ 4n). The calculated pressure-composition isotherms and temperature-pressure isobars exhibit sloping plateaus due to finite size effects on reaction thermodynamics. Generally, decomposition temperatures of free-standing clusters are found to increase with decreasing cluster size due to thermodynamic destabilization of reaction products.