Structural, electronic, dynamic and thermodynamic properties of Zr1-xHfxH2 hydride alloys: A first-principles study based on the virtual crystal approximation

The structural, electronic, dynamic and thermodynamic properties of Zr1-xHfxH2 (where x is the concentration of constituent element Hf, which changes in the range from 0 to 1 with step size Δx = 0.1) are investigated using first-principle calculations. These are done using the density-functional theory (DFT) and density functional perturbation theory (DFPT) within Generalized Gradient Approximation (GGA) and employing virtual-crystal approximation (VCA) method. The lattice constant gradually decreases from 3.527 Å to 3.474 Å with Hf substitute ratio increasing and the variation is quite small. The electronic density of states (DOS) of Zr1-xHfxH2 gradually expands when x value varies from 0 to 1 and all of these compounds show metallic nature and the metallicity increases. The analyses of charge density and the charge density differences indicate that the Zr1-xHfx-H interactions in the hydride are primarily metallic with a small ionic component, and the metallic nature enhances as x increases. Moreover, with Hf substitute ratio increasing, the optical branch and the acoustical branch of the phonon spectrum have a larger separation, which grows from 23.24 THz (ZrH2) to 25.48 THz (HfH2), and the phonon density of states gets wider. The calculated thermodynamic properties indicate stronger thermodynamic stability of ZrH2 than HfH2.