Abstract
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. undergoes a polymorphic transformation around 400–440 K from a low-temperature phase to a high-temperature phase. The crystal structure of 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 and assessed its thermodynamic stability in the quasiharmonic approximation. The space-group gives an excellent agreement between experiment and theory, confirming the result of a recent study [Buchter et al., J. Phys. Chem. B 112, 8042 (2008)]. Using density-functional theory (DFT), we obtained a value of 10.9 kJ/mol for the static total-energy difference between the and the phases at (without vibrations). Using DFT linear-response calculations, we find that the acoustic phonon branch of is dynamically unstable on the Brillouin-zone boundary at the 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 structure has a room-temperature vibrational entropy that is higher than that of the phase, qualitatively consistent with the observed stabilization of the former at elevated temperatures. The main contribution to the entropy difference between the and phases comes from the low-frequency region dominated by translational and rotational phonon modes.
- Received 26 November 2008
DOI:https://doi.org/10.1103/PhysRevB.79.104107
©2009 American Physical Society