First-principles predicted low-energy structures of NaSc(BH4)4

TL;DR

This study uses first-principles calculations to identify more stable low-energy structures of NaSc(BH4)4, revealing new phases with unique motifs and assessing their potential impact on hydrogen storage.

Contribution

The paper discovers new low-energy structures of NaSc(BH4)4 with novel motifs using ab initio calculations and structural searches, challenging previous crystallographic interpretations.

Findings

Previous $Cmcm$ structure is dynamically unstable.

New $C222_1$ and other low-energy phases are more stable.

All predicted phases are insulators with band gaps of 7.9-8.2 eV.

Abstract

According to previous interpretations of experimental data, sodium-scandium double-cation borohydride NaSc(BH$_4$)$_4$ crystallizes in the crystallographic space group $Cmcm$ where each sodium (scandium) atom is surrounded by six scandium (sodium) atoms. A careful investigation of this phase based on \textit{ab initio} calculations indicates that the structure is dynamically unstable and gives rise to an energetically and dynamically more favorable phase with $C222_1$ symmetry and nearly identical x-ray diffraction pattern. By additionally performing extensive structural searches with the minima-hopping method we discover a class of new low-energy structures exhibiting a novel structural motif in which each sodium (scandium) atom is surrounded by four scandium (sodium) atoms arranged at the corners of either a rectangle with nearly equal sides or a tetrahedron. These new phases are all predicted to be insulators with band gaps of $7.9-8.2$ eV. Finally, we estimate the influence of these structures on the hydrogen-storage performance of NaSc(BH$_4$)$_4$.