First-principles calculations ofC14-type Laves phase Ti-Mn hydrides

We have performed first-principles calculations of hexagonal $C14\ensuremath{-}\mathrm{type}$ Laves phase Ti-Mn metals and their hydrides, namely, ${\mathrm{Ti}}_{1.0}{\mathrm{Mn}}_{2.0}{\mathrm{H}}_{3},$ ${\mathrm{Ti}}_{1.25}{\mathrm{Mn}}_{1.75}{\mathrm{H}}_{3},$ and ${\mathrm{Ti}}_{1.5}{\mathrm{Mn}}_{1.5}{\mathrm{H}}_{3},$ to understand their hydrogen storage properties, particularly focusing on materials trend of the heat of formation. We employed the ultrasoft pseudopotential method within the generalized gradient approximation to density functional theory; in calculations, we neglected effects of magnetic ordering and assumed that all hydrogen atoms are located on the 12k sites of the space group ${P6}_{3}/mmc.$ We have found that the calculated heat of formation reproduces the experimental trend, and it involves two dominant contributions: (i) from an energy cost by volume expansion, and (ii) from an energy gain by hydrogen insertion. We thus emphasize that an equilibrium volume and a bulk modulus of the host metal are the keys to describe the heat of formation for these systems.