High-temperature superconductivity in the Ti-H system at high pressures

Abstract

Search for stable high-pressure compounds in the Ti-H system reveals the existence of titanium hydrides with new stoichiometries, including $Ibam\text{\ensuremath{-}}{\mathrm{Ti}}_{2}{\mathrm{H}}_{5}$, $I4/m\text{\ensuremath{-}}{\mathrm{Ti}}_{5}{\mathrm{H}}_{13}$, $I\overline{4}\text{\ensuremath{-}}{\mathrm{Ti}}_{5}{\mathrm{H}}_{14}$, $Fddd\text{\ensuremath{-}}{\mathrm{TiH}}_{4}$, $Immm\text{\ensuremath{-}}{\mathrm{Ti}}_{2}{\mathrm{H}}_{13}$, $P\overline{1}\text{\ensuremath{-}}{\mathrm{TiH}}_{12}$, and $C2/m\text{\ensuremath{-}}{\mathrm{TiH}}_{22}$. Our calculations predict $I4/mmm\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}R\overline{3}m$ and $I4/mmm\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}$ $Cmma$ transitions in TiH and ${\mathrm{TiH}}_{2}$, respectively. Phonons and the electron-phonon coupling in all searched titanium hydrides are analyzed at high pressure. It is found that $Immm\text{\ensuremath{-}}{\mathrm{Ti}}_{2}{\mathrm{H}}_{13}$, rather than the hydrogen-richest $C2/m\text{\ensuremath{-}}{\mathrm{TiH}}_{22}$, exhibits the highest superconducting critical temperature ${T}_{c}$. The estimated ${T}_{c}$ of $Immm\text{\ensuremath{-}}{\mathrm{Ti}}_{2}{\mathrm{H}}_{13}$ and $C2/m\text{\ensuremath{-}}{\mathrm{TiH}}_{22}$ are, respectively, 127.4--149.4 K (${\ensuremath{\mu}}^{*}=0.1$--0.15) at 350 GPa and 91.3--110.2 K at 250 GPa, as found by numerically solving the Eliashberg equations.