The Crystal Structure and Surface Energy of NaAlH₄:  A Comparison of DFT Methodologies

Abstract

This paper presents a comparison of the bulk structure, cleavage energies, and local densities of states of solid NaAlH 4 calculated using several different density functional theory methodologies. Good agreement is obtained for the bulk crystal structure. Larger differences become apparent for the calculated surface energies and local densities of states. The (001) NaAlH 4 surface is clearly identified as the most stable surface, followed by the (112) and (101) surfaces, with the (100) surface being the least stable. We present an analysis of the local density of states of atoms in the exposed NaAlH 4 surface.

Surfaces and Clusters of Mg(NH2)2 Studied by Density Functional Theory Calculations

“…It is clear that the most stable slab is the (112)-terminated one, with a cleavage energy only half of the second most stable slabssthe (110)-and (012)-terminated ones. It is also clear that the cleavage energy is correlated with the number of broken bonds per square length unitsconsistent with previous studies on complex aluminohydrides 28,29 sand with the structural changes ∆r av and the energy changes ∆E resulting from the surface relaxations. In addition to exhibiting the lowest cleavage energy, the (112) slab has distinctly lower ∆r av and ∆E than any of the other slabs.…”

“…In this work, most of the slab calculations have been performed on slabs containing three formula units (except the (112) slab, which contained four formula units, because of the special symmetry of this face). From previous calculations on another complex hydride (NaAlH 4 , ref 29), we believe that the thickness is large enough to avoid interactions between the two surfaces so that the cleavage energies are converged.…”

The importance of vibrations in modelling complex metal hydrides

“…Though not all DFT implementations were created equal [4,5], DFT in general is capable of accurately describing the structure and energetics of complex metal hydrides and their decomposition products.…”

Dynamics and Thermodynamics of a Novel Phase ofNaAlH4

“…As such, the material has been widely studied as a template for developing novel hydrogen storage solutions. Several recent first-principles studies [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20] have addressed the structure and thermodynamics of the sodium alanate system. Nevertheless, important questions regarding the precise mechanism of dehydrogenation and the nature of the key phase transition from NaAlH 4 to Na 3 AlH 6 remain unanswered.…”