Ultralight Zintl solids assembled by Al₆Na₂ clusters

Abstract

The Al₆Na₂ cluster with 20 valence electrons is a magic structure. The Na⁺ cations are ionically bonded to the Al₆²⁻ anions and the octahedral Al₆ core itself is very stable. Here, we predicted two stable crystals built by Al₆Na₂ based on DFT calculations. The two structures have Pn-3m and Fd-3m space groups. Their binding energies are −18.34 and −18.28 eV per Al₆Na₂ unit, and the interaction energies between the building units are 1.68–1.62 eV. The calculated elastic constants and phonon spectra demonstrate that both the allotropes are mechanically and dynamically stable. The band structures show that these two crystals are semiconductors; the band gaps calculated on PAW-PBE are 0.89 and 1.40 eV. These two allotropes are the first predicted Zintl phase solids formed by simple metals, and it shows semiconductors can be synthesized from all metal atoms by using cluster as building blocks. The densities of the Pn-3m and Fd-3m allotropes are 0.92 and 0.45 cm⁻³. The latter structure forms a porous solid with a pore diameter of 10.35 Å, and its density is less than half of the water. This ultralight porous solid may have many potential applications.

Graphical abstract