Superconducting hydrogen tubes in hafnium hydrides at high pressure

Compressing hydrogen-rich hydrides is an effective method to search for exotic properties such as high-${T}_{\text{c}}$ superconductivity. Here we show that high pressure and high temperature stabilize unique hydrogen tubes in hafnium hydrides. A combination of structural searches and first-principle calculations predict a metastable stochiometric ${\mathrm{HfH}}_{9}$ at 200 GPa. ${\mathrm{HfH}}_{9}$ is composed of H tubes intercalated within Hf-H framework, where two-thirds of the hydrogen atoms are arranged in a tubelike ${\mathrm{H}}_{12}$ structure located inside channels formed by the remainder ${\mathrm{HfH}}_{3}$. Each ${\mathrm{H}}_{12}$ tube gains 0.876 electrons from the ${\mathrm{HfH}}_{3}$ framework, indicating the ionic character of ${\mathrm{HfH}}_{9}$. Calculations show that ${\mathrm{HfH}}_{9}$ is a potential superconductor with an estimated ${T}_{\text{c}}$ of 110 K at 200 GPa, with the electron-phonon coupling arising mainly from the ${\mathrm{H}}_{12}$ tube and its interaction with the ${\mathrm{HfH}}_{3}$ framework. The current results suggest the existence of diverse hydrogen chemistries at high pressure that could be unravelled by future experimental studies.