Abstract
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.
Highlights
Based on a combination of the Crystal Structure Analysis by Particle Swarm Optimization (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 orthorhombic Pnma structure is favored over the Fm-3m phase above 46.37 GPa, which remains stable in the ground state up to at least 300 GPa
We concentrate on the relevant properties of the Fm-3m phase at 0 GPa and the Pnma phase at 100 GPa, which are most stable and accessible to experiment at the corresponding pressures
Summary
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. They reported another theoretical investigation on HfH2 that it undergoes pressure-induced structural phase transition I4/mmm → Cmma → P21/m at 180 and 250 GPa, respectively, and HfH2 is classified as a ionic crystal with the charges transferring from Hf atom to H23.
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