As novel “post lithium-ion batteries” and promising alternatives to lithium-ion batteries (LIBs) suffering a storage limit of Li source, sodium-ion batteries (SIBs) are emerging and show bright prospect in large-scale...
Based on a combination of the CALYPSO method for crystal structure prediction and first-principles calculations, we explore the crystal structures of VH2 under the pressure range of 0−300 GPa. The cubic Fm-3m phase with regular VH8 cubes is predicted to transform into orthorhombic Pnma structure with fascinating distorted VH9 tetrakaidecahedrons at 47.36 GPa. Both the Fm-3m phase at 0 GPa and the Pnma phase at 100 GPa are mechanically and dynamically stable, as verified with the calculations of elastic constants and phonon dispersions, respectively. Moreover, the calculated electronic band structure and density of states indicate both stable phases are metallic. Remarkably, the analyses of the Poisson’s ratio, electron localization function (ELF) and Bader charge substantiate that both stable phases are ionic crystals on account of effective charges transferring from V atom to H. On the basis of the microscopic hardness model, the Fm-3m and Pnma crystals of VH2 are potentially incompressible and hard materials with the hardness values of 17.83 and 17.68 GPa, respectively.
We report a detailed theoretical study of the electronic structure, phase stability, elastic and mechanical properties of SiB in the pressure range of 0-160 GPa by employing the crystal structure analysis by particle swarm optimization (CALYPSO) method combined with first-principles calculations. Our theoretical predictions reveal that, as the pressure increases, SiB moves through the following sequence of phases: P321 → C2/m → P2/m, and the corresponding transition pressures are computed to be 30 and 64 GPa, respectively. The results of band structures, density of states and electronic localization functions indicate that all three phases act as metallic with strong covalent bonding. The Vickers hardness of C2/m and P2/m phases has been estimated by Gao's, Lyakhov-Oganov's and Šimůnek's models, implying that SiB is a potential hard material with a hardness value of ∼20 GPa. The superconducting critical temperatures of polymeric SiB are also uncovered to be 3.6 K for the C2/m phase at 50 GPa and 5.7 K for the P2/m phase at 100 GPa. Our results enrich the crystal structures of the Si-B system and provide a further understanding of structures and their properties.
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