Ultralight Zintl solids assembled by Al6Na2 clusters
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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.
KeywordsSolid State and Materials
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