Dynamics and Thermodynamics of a Novel Phase of NaAlH[subscript 4]

Abstract

We characterize a novel orthorhombic phase (γ) of NaAlH[subscript 4], discovered using first-principles molecular dynamics, and discuss its relevance to the dehydrogenation mechanism. This phase is close in energy to the known low-temperature structure and becomes the stabler phase above 320 K, thanks to a larger vibrational entropy associated with AlH[subscript 4] rotational modes. The structural similarity of γ-NaAlH[subscript 4] to α-Na[subscript 3]AlH[subscript 6] suggests it acts as a key intermediate during hydrogen release. Findings are consistent with recent experiments recording an unknown phase during dehydrogenation.