Dynamics and Thermodynamics of a Novel Phase of NaAlH4

TL;DR

This paper reports the discovery and characterization of a new orthorhombic phase of NaAlH4, revealing its thermodynamic stability and potential role as an intermediate in hydrogen release, supported by first-principles simulations and experimental consistency.

Contribution

It introduces a novel gamma phase of NaAlH4, analyzing its stability and role in dehydrogenation through first-principles molecular dynamics.

Findings

Gamma-NaAlH4 is stable above 320 K due to vibrational entropy.

The gamma phase is structurally similar to alpha-Na3AlH6.

Experimental observations align with the existence of this new phase.

Abstract

We characterize a novel orthorhombic phase (gamma) of NaAlH4, 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 AlH4 rotational modes. The structural similarity of gamma-NaAlH4 to alpha-Na3AlH6 suggests it acts as a key intermediate during hydrogen release. Findings are consistent with recent experiments recording an unknown phase during dehydrogenation.