Predicting the Structural Transition in Medium-Sized Boron

    Recent experimental observation of the first bilayer clusters B48-/0 reveals a new structural domain in boron nanostructures. Inspired by the previously reported bilayer B48, B54, B60, and B62 and based on extensive global-minimum searches and densityfunctional theory calculations, we predict herein a new series of medium-sized bilayer boron nanoclusters C2 B64 (I), D2 B66 (II), D2 B68 (III), C1 B70 (IV), and Ci B72 (V) which all contain an elongated B46 bilayer hexagonal prism at the center with four effective interlayer 2c–2e B-B σ bonds formed between the top and bottom layers and the bilayer to core-shell structural transition at B74 where core-shell species start to dominate in thermodynamics, defining the up-limit of the bilayer boron nanoclusters at B72. The newly obtained bilayer C2 B64 (I), D2 B68 (III), and C1 B70 (IV) appear to be systematically more stable than the previously reported cage-like D4d B64, core-shell C1 B68, and quasi-planar C3v B70, respectively. Detailed bonding analyses indicate that these bilayer species follow the universal bonding pattern of σ+π double delocalization, rendering three-dimensional aromaticity to the systems. The IR, Raman, and UV-vis spectra of the concerned bilayer species are computationally simulated to facilitate their future characterizations.