Prediction of unexpected BnPn structures: promising materials for non-linear optical devices and photocatalytic activities

Abstract

In the present work, a modern method of crystal structure prediction, namely USPEX conjugated with density functional theory (DFT) calculations, was used to predict the new stable structures of B_nP_n (n = 12, 24) clusters. Since B₁₂N₁₂ and B₂₄N₂₄ fullerenes have been synthesized experimentally, it motivated us to explore the structural prediction of B₁₂P₁₂ and B₂₄P₂₄ clusters. All new structures were predicted to be energetically favorable with negative binding energy in the range from −4.7 to −4.8 eV per atom, suggesting good experimental feasibility for the synthesis of these structures. Our search for the most stable structure of B_nP_n clusters led us to classify the predicted structures into two completely distinct structures such as α-B_nP_n and β-B_nP_n phases. In α-B_nP_n, each phosphorus atom is doped into a boron atom, whereas B atoms form a B_n unit. On the other hand, each boron atom in the β-phase was bonded to a phosphorus atom to make a fullerene-like cage structure. Besides, theoretical simulations determined that α-B_nP_n structures, especially α-B₂₄P₂₄, show superior oxidation resistance and also, both α-B_nP_n and β-B_nP_n exhibit better thermal stability; the upper limit temperature that structures can tolerance is 900 K. The electronic properties of new compounds illustrate a higher degree of absorption in the UV and visible-region with the absorption coefficient larger than 10⁵ cm⁻¹, which suggests a wide range of opportunities for advanced optoelectronic applications. The β-B_nP_n phase has suitable band alignments in the visible-light excitation region, which will produce enhanced photocatalytic activities. On the other hand, α-B_nP_n structures with modest band gap exhibit large second hyperpolarizability, which are anticipated to have excellent potential as second-order non-linear optical (NLO) materials.