Thermodynamic Analysis of the V–H Binary Phase Diagram

Abstract

A thermodynamic analysis of the V–H binary system has been performed by combining first-principles calculations with the CALPHAD approach. In order to represent the order-disorder transition between the monoclinic β₁ and body-centered tetragonal β₂ phases arising from the ordering of H atoms located at the octahedral interstitial sites, a (V)_1⁄2(H,Va)_1⁄4(H,Va)_1⁄4-type three-sublattice model was applied. The formation energies of the δ-VH₂ hydride phase and solid solution β phases were calculated in the ground state using the Full-potential Linearized Augmented Plane Wave method, and the estimated values were introduced into a CALPHAD-type thermodynamic analysis, along with some experimental information on the phase boundaries and the hydrogen isotherms. The calculated phase diagram and hydrogen isotherms were in good agreement with these experimental results.