Using first-principles metadynamics simulation to predict new phases and probe the phase transition of NaAlH4

A first-principles-based metadynamics technique was used to study the solid-solid phase transition of NaAlH4 under normal pressure and at temperatures close to the melting point in order to understand the mechanism of hydrogen release from NaAlH4. Two orthorhombic phases with space symmetry groups of Cmcm and Pbcm were determined based on the structures isolated from the metadynamics trajectories starting from the α phase (I41/a) of NaAlH4. The I41/a→Cmcm→Pbcm transitions can be correlated with the symmetry-lowering process (D2h → C2v → Cs) of the AlH4- complex ions. Thermodynamics analysis revealed that the Cmcm phase forms from the I41/a phase at around the melting point (548 K). Our results are in qualitative agreement with a previous Raman scattering measurement, which showed a transition at ∼503 K. The Cmcm phase is expected to be a key intermediate in hydrogen desorption prior to Na3AlH6 formation. (paper)