Lee, Young-Su Kim, Yoonyoung Cho, Young Whan Shapiro, Daniel Wolverton, Chris Ozolins, Vidvuds 2024-01-20T22:00:32Z 2024-01-20T22:00:32Z 2021-09-03 2009-03 1098-0121 https://pubs.kist.re.kr/handle/201004/132705 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. English AMER PHYSICAL SOC HYDROGEN STORAGE MAGNESIUM BOROHYDRIDE CALCIUM BOROHYDRIDE ELECTRON-GAS CA(BH4)(2) DIFFRACTION LIBH4 MG(BH4)(2) DYNAMICS NEUTRON Crystal structure and phonon instability of high-temperature beta-Ca(BH4)(2) Article 10.1103/PhysRevB.79.104107 1 PHYSICAL REVIEW B, v.79, no.10 PHYSICAL REVIEW B 79 10 scie scopus 000264768600034 2-s2.0-63249110680 Materials Science, Multidisciplinary Physics, Applied Physics, Condensed Matter Materials Science Physics Article HYDROGEN STORAGE MAGNESIUM BOROHYDRIDE CALCIUM BOROHYDRIDE ELECTRON-GAS CA(BH4)(2) DIFFRACTION LIBH4 MG(BH4)(2) DYNAMICS NEUTRON