Phase stability and elasticity of C15 transition-metal intermetallic compounds

First-principles quantum mechanical calculations based on the local-density-functional theory have been performed to study the electronic, physical, and metallurgical properties of C15 intermetallics MV2 (M = Zr, Hf, or Ta), The elastic constants of C15 HfV2 + Nb were measured by the resonant ultrasound spectroscopy technique, The phase stability of C15 HfV2 + Nb was studied by specific heat measurements and by TEM in a low-temperature specimen holder, The total energies and their lattice volume dependence were used to obtain the equilibrium lattice constants and bulk modulus, The band structures at the X-point near the Fermi level were employed to understand the anomalous temperature dependence of the shear modulus of the C15 intermetallics. It was found that the double degeneracy with a linear dispersion relation of electronic levels at the X-point near the Fermi surface is mainly responsible for the C15 anomalous elasticity. The density of states at the Fermi level, N(E-F), and the Fermi surface geometry were obtained to understand the low-temperature phase instability of C15 HfV2 and ZrV2 and the stability of C15 TaV2, It is proposed that the large N(E-F) and Fermi surface nesting are the physical reasons for the structural instability of the C15 HfV2 and ZrV2 at low temperatures, The relationship between the anomalous elasticity and structural instability of C15 HfV2 and ZrV2 are also discussed.