First-principles investigation of the structure and thermodynamic properties of titanium hydrides.

The formation of titanium hydrides is closely related to the hydrogen embrittlement in titanium and its alloy. In this work, a comprehensive study on the stable and metastable structures of TiH x (x=0.5, 1, 1.5 and 2) has been performed by means of first-principles calculations in combination with the evolutionary algorithm. The structural features, dynamical stability and elastic properties of these searched titanium hydrides have been systematically investigated. The influence of temperature on the structure stability and phase transition of the Ti 2 H, TiH, Ti 2 H 3 and TiH 2 phases is considered using the quasi-harmonic approximation theory. Our calculations predict that Ti 2 H, Ti 2 H 3 and TiH 2 undergo temperature-driven phase transitions. The reason that the theoretical calculation overestimates the critical temperature of the tetragonal to cubic phase transition in TiH 2 within the quasi-harmonic approximations approach is also explored and discussed. The thermodynamic stability of these TiH x (x=0.5, 1, 1.5 and 2) structures is further analyzed over the temperature range of 0–1100 K. TiH 2 is the only stable structure in the Ti–H system and Ti 2 H, TiH, and Ti 2 H 3 are thermodynamically metastable within 900 K. [Display omitted] [ABSTRACT FROM AUTHOR]