First-principles study of experimental and hypothetical Mg(BH4)2 crystal structures

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 P6(1)22 space group, whereas the original experimental structure has P6(1) 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 P6(1) and P6(1)22 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.