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Graphical abstract Highlights • Different with previous research, the structure of cubic NaB 6 is stable. • NaB 6 has lower formation energy than the synthesized KB 6 and may be prepared. • NaB 6 and KB 6 may be potential superconducting materials at ambient pressure. Abstract The structural stability, electronic structure, and superconductivity of cubic sodium hexaboride NaB 6 are calculated from first principles using density functional theory (DFT). The calculations of phonon dispersion spectra, formation energy, and elastic constants indicate that the cubic NaB 6 is dynamically, thermodynamically, and mechanically stable, respectively. The cubic NaB 6 has lower formation energy compared with the experimentally synthesized KB 6. The calculated band structure and PDOS demonstrate the metallicity of NaB 6. The superconducting transition temperatures at ambient pressure for NaB 6 and KB 6 are predicted to be 15.7 K with λ = 0.60 and 14.6 K with λ = 0.57, respectively. [ABSTRACT FROM AUTHOR]

Diamond-like B C compounds exhibit the favorable properties of high hardness and superconductivity. The explorations for novel B C phases are kept on going and it was desirable to explore whether the diamond-like structure can still exist stably or not when the content of boron increases. Herein we predicted a nondiamond-like structure t 2 -BC took place the lowest energy position of the diamond-like structure, which is completely different from the BC x (x ≥ 2) system. Based on elastic constants and phonon dispersion calculations, the mechanical and dynamic stability of t 2 -BC were confirmed. Furthermore, hardness and electron–phonon calculations reveal that t 2 -BC is superhard (H v = 55.4 GPa) and superconductive with superconducting critical temperature reaching 6.4 K. These results have practical significance for the experimental synthesis of diamond and nondiamond like B C compounds. Unlabelled Image • A novel low-energy BC phase named t 2 -BC with a triclinic structure is predicted. • The nondiamond-like structure t 2 -BC has lower energy than diamond-like BC structure, which is completely different from the other BC x (x ≥ 2) system. • The theoretical Vickers hardness of t 2 -BC is estimated to be 55.4 GPa. • The electron-phonon calculations reveal that t 2 -BC structure is superconductive with superconducting critical temperature reaching 6.4 K. [ABSTRACT FROM AUTHOR]

The clear proof of superconductivity in transition-metal diborides was rarely reported. In the current work, the recently developed particle swarm optimization structural search method was utilized to propose three ZrB 2 structures, namely, I 41/ amd -ZrB 2 , P 42/ mmc -ZrB 2 , and P 4/ nmm -ZrB 2. The structural stability of the three novel ZrB 2 phases was confirmed on the basis of elastic constant and phonon dispersion calculations. At ambient pressure, the mechanical properties of the I 41/ amd -ZrB 2 and P 42/ mmc -ZrB 2 phases are comparable to those of AlB 2 –ZrB 2. Electron–phonon coupling (EPC) calculations revealed that the P 4/ nmm -ZrB 2 phase is predicted to be a potential high- T c superconductor with a calculated T c of 12.7 K at 20 GPa. Moreover, significant pressure-induced EPC enhancement can also be found in the P 4/ nmm -ZrB 2 phase. The maximum EPC constant λ and T c under 600 GPa are 1.05 and 34.4 K, respectively. [ABSTRACT FROM AUTHOR]

Driven by the advances of porous materials in hydrogen storage, ion filtration/separation, and catalysis, three novel 3D porous frameworks of BC 2 N compounds were designed in this study. They are characterized by a porous structure with low density formed by hexagonal arrays of carbon and boron nitride in alternating patterns. These structures contain sp 2-and sp 3-hybridized B–C, C–C, and C–N bonds. Structural stability is rigorously evaluated by means of energy, elastic constants, and phonon dispersion, confirming the thermodynamic, mechanical, and kinetic stability of the designed BC 2 N- x frameworks. First-principles calculations reveal their semiconducting nature, which is characterized by a narrow direct bandgap ranging from 0.140 eV to 0.422 eV. The mechanical properties are studied with tensile strength calculations, and results show these structures are as strong as diamond with tensile strength value of 97∼102 GPa along <010> direction. Therefore, we suggest that these BC 2 N- x structures are promising for optoelectronic device and catalysts. [ABSTRACT FROM AUTHOR]