We have studied the decomposition pathways of both Ca- and Mg-borohydride using density-functional theory (DFT) calculations of the free energy (including vibrational contributions) in conjunction with a Monte Carlo-based crystal-structure prediction method, the prototype electrostatic ground-state (PEGS) search method. We find that a recently proposed CaB2 H 2 intermediate is energetically highly unfavorable and hence very unlikely to form. We systematically search for low-energy structures of CaB 2 Hn compounds with n=2, 4, and 6 using PEGS simulations, refining the resulting structures with accurate DFT calculations. We find that the lowest-energy CaB2 H2 and CaB2 H 4 crystal structures do not lie on the thermodynamically stable decomposition path but rather are unstable with respect to a decomposition pathway involving the previously proposed CaB12 H12 phase. We also predict a CaB2 H6 compound which forms a low-energy intermediate in the calcium borohydride decomposition pathway. This new reaction pathway is practically degenerate with decomposition into the CaB12 H12 phase. Similar calculations for magnesium borohydride show that a recently predicted MgB2 H6 phase does not form a stable intermediate in the decomposition pathway of Mg(BH 4)2.
|Original language||English (US)|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - Nov 9 2010|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics