%0 Generic
%A Dai, Bing
%A Sholl, David S.
%A Johnson, J. Karl
%D 2008
%T First-Principles Study of Experimental and Hypothetical Mg(BH4)2 Crystal Structures
%U https://acs.figshare.com/articles/dataset/First_Principles_Study_of_Experimental_and_Hypothetical_Mg_BH_sub_4_sub_sub_2_sub_Crystal_Structures/2950090
%R 10.1021/jp710154t.s004
%2 https://acs.figshare.com/ndownloader/files/4648960
%K Bader charge analysis
%K powder diffraction data
%K P 6 1 22
%K P 6 1 22 space group
%K formula unit
%K P 6 1 symmetry
%K 2 structure
%K P 6 1 22 structure
%K crystal structures
%K 2 Crystal Structures
%K P 6 1
%X We have used first-principles density functional theory to relax the experimentally reported crystal structures
for the low- and high-temperature phases of Mg(BH4)2, which contain 330 and 704 atoms per unit cell,
respectively. The relaxed low-temperature structure was found to belong to the P6122 space group, whereas
the original experimental structure has P61 symmetry. The higher symmetry identified in our calculations
may be the T = 0 ground-state structure or may be the actual room-temperature structure because it is difficult
to distinguish between P61 and P6122 with the available powder diffraction data. We have identified several
hypothetical structures for Mg(BH4)2 that have calculated total energies that are close to the low-temperature
ground-state structure, including two structures that lie within 0.2 eV per formula unit of the ground-state
structure. These alternate structures are all much simpler than the experimentally observed structure. We
have used Bader charge analysis to compute the charge distribution in the P6122 Mg(BH4)2 structure and
have compared this with charges in the much simpler Mg(AlH4)2 structure. We find that the B−H bonds are
significantly more covalent than the Al−H bonds; this difference in bond character may contribute to the
very different crystal structures for these two materials. Our calculated vibrational frequencies for the P6122
structure are in good agreement with experimental Raman spectra for the low-temperature Mg(BH4)2 structure.
The calculated total energy of the high-temperature structure is only about 0.1 eV per formula unit higher in
energy than the low-temperature structure.
%I ACS Publications