Phase stability of C15 MV2 (M = Zr, Hf or Ta):: an electronic structure investigation

The phase stability of HfV2-based C15 Laves phases has been studied by specific heat measurements and by in-situ transmission electron microscopy at low temperatures. The results indicate that HfV2 undergoes a structural transformation at approximately 115K and that Nb additions reduce the transformation temperature, eventually stabilizing the C15 structure. First-principles quantum-mechanical calculations based on the local-density functional theory have been performed to study the electronic structure and physical properties of C15 Laves phases MV2 (M = Zr, I-If or Ta). The density of states at the Fermi level N(E-F) and the Fermi surface geometry were obtained in order to understand the low-temperature phase instability of HfV2 and ZrV2 and the corresponding low-temperature stability of TaV2. We propose that a large N(EF) and Fermi surface nesting, which give rise to phonon softening, are the physical reasons for the structural instability of HfV2 and ZrV2 at low temperatures. The relationships between the anomalous elasticity and structural instability of HfV2 and ZrV2 are also discussed.