First-Principles Study of BN Cluster-Assembled Porous Nanomaterials.

Owing to the similar valence electron structures between the B-N bond and the C-C bond, boron nitride, similar to carbon, can form abundant polymorphs with different frameworks, which possess rich mechanical and electronic properties. Using the hollow, cage-like BN cluster as building blocks, here, we established three new BN polymorphs with low-density porous structures, termed Cub-BN, Tet-BN, and Ort-BN, which have cubic (P4¯3m), tetragonal (4/), and orthomorphic () symmetries, respectively. Our density functional theory (DFT) calculations indicated that the existence of porous structure Cub-BN, Tet-BN, and Ort-BN were not only energetically, dynamically, thermally and mechanically stable, they were even more stable than some known phases, such as sc-BN and Hp-BN. The obtained Pugh's ratio showed that the Cub-BN and Tet-BN structures were brittle materials, but Ort-BN was ductile. The analysis of ideal strength, Young's moduli, and shear moduli revealed that the proposed new phases all exhibited sizable mechanical anisotropy. Additionally, the calculation of electronic band structures and density of states showed that they were all semiconducting with a wide, indirect band gap (~3 eV). The results obtained in this work not only identified three stable BN polymorphs, they also highlighted a bottom-up way to obtain the desired materials with the clusters serving as building blocks.