Fundamental mechanism of tetragonal transitions in titanium hydride

•Tetragonal transition of TiHx is divided into two types in terms of energy pathway.•The intrinsic composition range of the δ→ε transition is 1.5≤x≤2.•Mechanical instability causes δ→ε and internal symmetry breaking induces δ→γ.•Poisson ratio of δ phases between x and z axes is proposed for tetragonal transitions.•The present results could clarify the controversies about tetragonal transitions.

First principles calculation reveals that the tetragonal transitions of TiHx (1≤x≤2) could be divided into two types in terms of energy pathway, and that the intrinsic composition range of the δ→ε transition is 1.5≤x≤2. Calculation also indicates that the fundamental reasons for the δ→ε and δ→γ transitions are quite different from each other, i.e., mechanical instability causes the δ→ε transition and internal symmetry breaking of cubic structures induces the δ→γ transition. In addition, the Poisson ratio of δ phases between the x and z axes is proposed to provide a deeper understanding of intrinsic natures of tetragonal transitions.