The structural stability and mechanical properties of ZrHx from first-principles calculations

The structural, thermodynamic and elastic properties of zirconium hydrides have been systematically investigated from first principles. ZrH2 is the only stable structure in Zr-H system and other zirconium hydrides are thermodynamically metastable. The enthalpy of formation of P4(2)/mmc-ZrH, P4(2)/nnm-Zr2H3 (a>c) and P (4) over bar 2m-Zr4H7 (a>c) is just a few meV/atom above the convex-hull. To determine the overall stability of the material, the dynamical and mechanical stabilities of the zirconium hydrides, including the thermodynamically stable and metastable phases, are evaluated by calculating their phonon dispersion and elastic constants, respectively. Except for P4(2)/mcm-Zr2H, P (4) over bar 3m-Zr4H3 and P (4) over bar 3m-ZrH, the other investigated ZrHx phases are all dynamically stable. The mechanical instability of cubic and tetragonal zirconium hydrides is induced by their negative tetragonal shear moduli. The mechanical properties (bulk modulus, shear modulus, Young's modulus, Poisson's ratio and hardness) of zirconium hydrides and their dependence on the hydrogen concentration are calculated and discussed. The bulk modulus almost monotonically increases with H concentration. Compared with hydrogen-rich zirconium, the calculated values of mechanical properties, including elastic modulus, Poisson's ratio and hardness of subhydrides (Zr4H, Zr2H and Zr4H3) are closer to those of the pure zirconium. Among these hydrides, newly discovered metastable P (4) over bar 2m-Zr4H5 is the hardest and has a Vickers hardness of 4.3 GPa.