Crystal structure and phonon instability of high-temperature beta-Ca(BH4)(2)

Abstract

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 alpha-Ca(BH4)(2) phase to a high-temperature beta-Ca(BH4)(2) phase. The crystal structure of beta-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 beta-Ca(BH4)(2) and assessed its thermodynamic stability in the quasiharmonic approximation. The space-group P4(2)/m 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 beta-Ca(BH4)(2) and the alpha-Ca(BH4)(2) phases at T=0 K (without vibrations). Using DFT linear-response calculations, we find that the [1/21/2 xi] acoustic phonon branch of beta-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 beta-Ca(BH4)(2) structure has a room-temperature vibrational entropy that is 16 J/mol K higher than that of the alpha-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 alpha-Ca(BH4)(2) and beta-Ca(BH4)(2) phases comes from the low-frequency region dominated by translational and rotational phonon modes.