Ca (BH4) 2 is an interesting candidate for high-density hydrogen storage since it contains a large amount of hydrogen by weight and volume, and has been shown to reversibly release and absorb hydrogen, albeit at moderately high temperatures. Ca (BH4) 2 undergoes a polymorphic transformation around 400-440 K from a low-temperature α-Ca (BH4) 2 phase to a high-temperature β-Ca (BH4) 2 phase. The crystal structure of β-Ca (BH4) 2 has only recently been resolved, and its thermodynamic phase stability is still not well understood. Using a combined experimental and theoretical approach, we have independently determined the structure of β-Ca (BH4) 2 and assessed its thermodynamic stability in the quasiharmonic approximation. The space-group P 42 /m gives an excellent agreement between experiment and theory, confirming the result of a recent study. Using density-functional theory (DFT), we obtained a value of 10.9 kJ/mol for the static total-energy difference between the β-Ca (BH4) 2 and the α-Ca (BH4) 2 phases at T=0 K (without vibrations). Using DFT linear-response calculations, we find that the [1 2 1 2 ξ] acoustic phonon branch of β-Ca (BH4) 2 is dynamically unstable on the Brillouin-zone boundary at the T=0 K lattice parameters predicted from static DFT calculations. This phonon branch is very sensitive to the lattice parameters and can be stabilized by including lattice expansion due to zero-point vibrational contributions in the quasiharmonic approximation. This expanded stable β-Ca (BH4) 2 structure has a room-temperature vibrational entropy that is 16 J/molK higher than that of the α-Ca (BH4) 2 phase, qualitatively consistent with the observed stabilization of the former at elevated temperatures. The main contribution to the entropy difference between the α-Ca (BH4) 2 and β-Ca (BH4) 2 phases comes from the low-frequency region dominated by translational and rotational phonon modes.
|Original language||English (US)|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - Mar 3 2009|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics