Mechanical and structural stability of zirconium dihydride

First-principles calculation reveals that the ZrHx phases (x = 1, 1.25, 1.5, 1.75, and 2) with the cubic fluorite-type (fcc, δ phase) and face-centered-tetragonal (fct: ɛ phase, c/a < 1; γ phase, c/a > 1) structures are all energetically favorable with negative heats of formation of −30 to −56 kJ/(mol H) and very small structural energy differences, while mechanical stability plays a more important role in determining the existence of various ZrHx phases. Calculation also shows that the intrinsic composition range of the δ → ɛ transition of ZrHx phases is x ≥ 1.5, and that the fundamental mechanism of this transition is mechanical unstableness of the δ phase which will spontaneously transform into ɛ phase by means of the {110}<110> shear. Moreover, electronic structures show that the co-function of van Hove singularities and degenerate bands along several directions brings about the high level of density of states at or near the Fermi level and fundamentally induces the mechanical unstableness of the δ phase.