High-Temperature Superconductivity in the Ti--H System at High Pressures

Search for stable high-pressure compounds in the Ti–H system reveals the existence of titanium hydrides with new stoichiometries, including Ibam-Ti${}_2$H${}_5$, I4/m-Ti${}_5$H${}_{13}$, I$\bar{4}$-Ti${}_5$H${}_{14}$, Fddd-TiH${}_4$, Immm-Ti${}_2$H${}_{13}$, P$\bar{1}$-TiH${}_{12}$, and C2/m-TiH${}_{22}$. Our calculations predict I4/mmm $\to$ R$\bar{3}$m and I4/mmm $\to$ Cmma transitions in TiH and TiH${}_2$, respectively. Phonons and the electron–phonon coupling of all searched titanium hydrides are analyzed at high pressure. It is found that Immm-Ti${}_2$H${}_{13}$ rather than the highest hydrogen content C2/m-TiH${}_{22}$, exhibits the highest superconducting critical temperature T${}_c$. The estimated T${}_c$ of Immm-Ti${}_2$H${}_{13}$ and C2/m-TiH${}_{22}$ are respectively 127.4–149.4 K (${\mu }^{\ast }$=0.1-0.15) at 350 GPa and 91.3–110.2 K at 250 GPa by numerically solving the Eliashberg equations. One of the effects of pressure on T${}_c$ can be attributed to the softening and hardening of phonons with increasing pressure.