First-Principles Study of Experimental and Hypothetical Mg(BH<sub>4</sub>)<sub>2</sub> Crystal Structures Bing Dai David S. Sholl J. Karl Johnson 10.1021/jp710154t.s004 https://acs.figshare.com/articles/dataset/First_Principles_Study_of_Experimental_and_Hypothetical_Mg_BH_sub_4_sub_sub_2_sub_Crystal_Structures/2950090 We have used first-principles density functional theory to relax the experimentally reported crystal structures for the low- and high-temperature phases of Mg(BH<sub>4</sub>)<sub>2</sub>, which contain 330 and 704 atoms per unit cell, respectively. The relaxed low-temperature structure was found to belong to the <i>P</i>6<sub>1</sub>22 space group, whereas the original experimental structure has <i>P</i>6<sub>1</sub> symmetry. The higher symmetry identified in our calculations may be the <i>T</i> = 0 ground-state structure or may be the actual room-temperature structure because it is difficult to distinguish between <i>P</i>6<sub>1</sub> and <i>P</i>6<sub>1</sub>22 with the available powder diffraction data. We have identified several hypothetical structures for Mg(BH<sub>4</sub>)<sub>2</sub> 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 <i>P</i>6<sub>1</sub>22 Mg(BH<sub>4</sub>)<sub>2</sub> structure and have compared this with charges in the much simpler Mg(AlH<sub>4</sub>)<sub>2</sub> 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 <i>P</i>6<sub>1</sub>22 structure are in good agreement with experimental Raman spectra for the low-temperature Mg(BH<sub>4</sub>)<sub>2</sub> 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. 2008-03-20 00:00:00 Bader charge analysis powder diffraction data P 6 1 22 P 6 1 22 space group formula unit P 6 1 symmetry 2 structure P 6 1 22 structure crystal structures 2 Crystal Structures P 6 1