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 TiHx (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 Ti2H, TiH, Ti2H3 and TiH2 phases is considered using the quasi-harmonic approximation theory. Our calculations predict that Ti2H, Ti2H3 and TiH2 undergo temperature-driven phase transitions. The reason that the theoretical calculation overestimates the critical temperature of the tetragonal to cubic phase transition in TiH2 within the quasi-harmonic approximations approach is also explored and discussed. The thermodynamic stability of these TiHx (x=0.5, 1, 1.5 and 2) structures is further analyzed over the temperature range of 0–1100 K. TiH2 is the only stable structure in the Ti–H system and Ti2H, TiH, and Ti2H3 are thermodynamically metastable within 900 K.