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Boron is an intriguing element due to its electron deficiency and the ability to form multicenter bonds in allotropes and borides, exhibiting diversified structures, unique chemical bonds, and interesting properties. Using swarm-intelligence structural prediction driven by a machine learning potential, we identified a boron phase with a 24-atom cubic unit cell, called c-B 24 , consisting of a B 6 octahedron in addition to well-known B 2 pairs and B 12 icosahedra at ambient pressure. There appear unusual four-center-two-electron (4c-2e) bonds in the B 12 icosahedron, originating from the peculiar bonding pattern between the B 2 pair and B 12 icosahedron, which is in sharp contrast with the 3c-2e and 2c-2e bonds in α-B 12 . More interestingly, c-B 24 is a metal with a superconducting critical temperature of 13.8 K at ambient pressure. The predicted Vickers hardness (23.1 GPa) indicates that c-B 24 is a potential hard material. Notably, it also has a good shear/tensile resistance (48.9/29.3 GPa). Our work not only enriches the understanding of the chemical properties of boron, but also sparks efforts on trying to synthesize this particular compound, c-B 24 .

Finding allotropes with novel structures and intriguing properties is of great interest from both fundamental and applicable standpoints. Here, we propose a hitherto unknown boron allotrope, possessing intrinsic superconductivity and...

The design of metal-bearing carbon-based materials with unique structures and intriguing properties is highly desirable in the fields of physics, chemistry, and materials science.Here, within swarm-intelligence structure search and first-principles computations, we uncovered several hitherto unknown sodium carbides (i.e., Na 4 C, Na 3 C 2 , NaC, Na 2 C 3 , and NaC 2 ) under high pressure. Intriguingly, the C atom arrangement reveals multiple structure evolution behavior with increased carbon content, from isolated anions in Na 4 C, tetramers in Na 3 C 2 , extended chains in NaC, pentagonal rings in Na 2 C 3 , to eventually hexagonal rings in NaC 2 . Among predicted phases, the superconducting critical temperature T c of NaC 2 could approach ∼42 K at 80 GPa, which is slightly higher than the T c of 39 K in the highest phonon-mediated superconductivity of MgB 2 at ambient pressure. This work offers insights into the reaction of carbides containing alkali metals and paves the way for the future investigation of high superconductivity in metal carbide systems.

Transition metal nitrides have attracted great interest due to their unique crystal structures and applications. Here, we predict two N-rich iridium nitrides (IrN4 and IrN7) under moderate pressure through first-principles swarm-intelligence structural searches. The two new compounds are composed of stable IrN6 octahedrons and interlinked with high energy polynitrogens (planar N4 or cyclo-N5). Balanced structural robustness and energy content result in IrN4 and IrN7 being dynamically stable under ambient conditions and potentially as high energy density materials. The calculated energy densities for IrN4 and IrN7 are 1.3 kJ/g and 1.4 kJ/g, respectively, comparable to other transition metal nitrides. In addition, IrN4 is predicted to have good tensile (40.2 GPa) and shear strengths (33.2 GPa), as well as adequate hardness (20 GPa). Moderate pressure for synthesis and ambient pressure recoverability encourage experimental realization of these two compounds in near future.

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Fluorine chemistry was demonstrated to show the importance of stretching the limits of chemical synthesis, oxidation state, and chemical bonding at ambient conditions. Thus far, the highest fluorine stoichiometry of...