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

Abstract The structural, electronic, dynamic and thermodynamic properties of Zr 1- x Hf x H 2 (where x is the concentration of constituent element Hf, which changes in the range from 0 to 1 with step size Δ x  = 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 A to 3.474 A with Hf substitute ratio increasing and the variation is quite small. The electronic density of states (DOS) of Zr 1- x Hf x H 2 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 Zr 1- x Hf x -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 (ZrH 2 ) to 25.48 THz (HfH 2 ), and the phonon density of states gets wider. The calculated thermodynamic properties indicate stronger thermodynamic stability of ZrH 2 than HfH 2 .