Environment-dependent phase stabilities of titanium hydrides: First-principles prediction

Various phases of TiHx (x=1-2) were constructed, with all possible H-atom configurations being considered. The first-principles energetics calculations were performed to evaluate their formation preference and mechanical and thermodynamic stabilities. All the results were then combined to construct the stable and metastable phase stability diagrams of TiHx. It is suggested that many stable and metastable TiHx phases have very comparable formation energies and thus are likely to co-exist. Their relative stabilities are strongly depended on temperature (T) and the partial pressure of hydrogen (p(H2)). Over the entire T and p(H2) range of interest, gamma-TiH and gamma-TiH2 are the only two stable phases. epsilon-TiH, gamma-TiH1.5, epsilon- and delta-TiH1.75, and epsilon-TiH2 can possibly present as metastable phases. These metastable phases are all mechanically stable, and have a thermodynamic tendency to continuously absorb or release H until reaching the equilibrium phases of gamma-TiH2 or gamma-TiH.