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

Crystal structure and phonon instability of high-temperature β-Ca(BH4)2
이영수김윤영조영환Shapiro, DainelWolverton, C.Ozolins, V.
Issue Date
Physical review B, Condensed matter and materials physics
VOL 79, 104107-1-104107-9
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 -Ca BH4 2 phase to a high-temperature -Ca BH4 2 phase. The crystal structure of -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 -Ca BH4 2 and assessed its thermodynamic stability in the quasiharmonic approximation. The space-group P42 /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 -Ca BH4 2 and the -Ca BH4 2 phases at T=0 K without vibrations . Using DFT linear-response calculations, we find that the 1 2 1 2 acoustic phonon branch of -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 -Ca BH4 2 structure has a room-temperature vibrational entropy that is 16 J /mol K higher than that of the -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 -Ca BH4 2 and -Ca BH4 2 phases comes from the low-frequency region dominated by translational and rotational phonon modes.
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