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

Abstract

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 115 K 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, Hf 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(E _F) 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.