First-principles investigation of metal-hydride phase stability: The Ti-H system

Various factors that affect metal-hydride phase stability are investigated from first principles. As a particular example, we consider hydride stability in the Ti-H system, exploring the role of configurational degrees of freedom, zero-point vibrational energy as well as coherency strains. The tetragonal $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Ti}\mathrm{H}$ phase is predicted (within generalized gradient approximation) to be unstable relative to hcp Ti ($\ensuremath{\alpha}$ phase) and the fcc based $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Ti}{\mathrm{H}}_{2}$. Zero-point vibrational energy significantly affects the formation energies in this system and makes the $\ensuremath{\gamma}$ phase even less stable relative to hcp Ti and $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Ti}{\mathrm{H}}_{2}$. The effect of stress and strain on the stability of the $\ensuremath{\gamma}$ phase is also investigated showing that coherency strains between hydride precipitates and the hcp Ti matrix stabilize $\ensuremath{\gamma}\text{\ensuremath{-}}\mathrm{Ti}\mathrm{H}$ relative to $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ti}$ and $\ensuremath{\delta}\text{\ensuremath{-}}\mathrm{Ti}{\mathrm{H}}_{2}$. We also find that hydrogen prefers octahedral sites at low hydrogen concentration and tetrahedral sites at high concentration. Both harmonic vibrational as well as electronic origins for the cubic to tetragonal phase transformation of $\mathrm{Ti}{\mathrm{H}}_{2}$ are investigated, and we argue that anharmonic vibrational degrees of freedom are likely to play an important role in stabilizing cubic $\mathrm{Ti}{\mathrm{H}}_{2}$.