First-principles insight into the degeneracy of ground-state LiBH4 structures

Recently, a number of ground-state structures of LiBH4 have been proposed, both from experimental and computational works. The results show controversy between computational and experimental ground-state crystal structures of LiBH4. In order to determine which is truly the lowest in energy, we study LiBH4 in a variety of crystal structures using density functional theory (DFT) calculations of the free energy (T = 0 K total energy plus vibrational thermodynamics), employing a variety of DFT methods and exchange-correlation functionals. Our calculations show that the experimentally observed structures are lowest in energy in DFT. However, multiple LiBH4 structures are degenerate with the experimental ground-state crystal structure and there exists a relatively flat potential energy landscape between them. These degenerate structures include the recently theoretically predicted LiBH4 structure [Tekin, Caputo, and Zuettel, Phys. Rev. Lett. 104, 215501 (2010)], which the authors claimed to be 9.66 kJ/(mol LiBH4) (or similar to 100 meV/fu) lower in energy than the experimentally XRD determined LiBH4 structure [Soulie, Renaudin, Cerny, and Yvon, J. Alloys Compd. 346, 200 (2002)]. Our calculations do not support these previous claims, and hence resolve this discrepancy between DFT and experiment.