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

Area of Science:

• Solid-state chemistry
• Materials science
• Computational materials science

Background:

• Sodium-scandium double-cation borohydride NaSc(BH4)4 was previously thought to crystallize in the Cmcm space group.
• This established structure suggested a coordination environment where sodium and scandium atoms are surrounded by six of the other cation.

Purpose of the Study:

• To investigate the dynamic stability of the NaSc(BH4)4 structure.
• To identify more energetically and dynamically favorable structural phases.
• To explore novel structural motifs and their impact on material properties.

Main Methods:

• Ab initio calculations were employed to analyze the dynamic stability of the known NaSc(BH4)4 phase.
• Extensive structural searches using the minima-hopping method were performed to discover new low-energy structures.
• Band gap calculations were conducted to predict the electronic properties of the newly identified phases.

Main Results:

• The previously accepted Cmcm structure of NaSc(BH4)4 was found to be dynamically unstable.
• A more stable phase with C2221 symmetry and a similar X-ray diffraction pattern was identified.
• A class of new low-energy structures with a novel motif, featuring fourfold coordination of sodium and scandium atoms, was discovered.
• These new phases are predicted to be insulators with band gaps ranging from 7.9 to 8.2 eV.

Conclusions:

• The dynamic instability of the Cmcm phase necessitates a re-evaluation of NaSc(BH4)4's crystal structure.
• The discovery of new, stable phases with unique coordination environments opens avenues for further materials design.
• The identified structural changes are expected to influence the hydrogen-storage performance of NaSc(BH4)4, warranting further investigation.