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-Tc 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 HfH9 at 200 GPa. HfH9 is composed of H tubes intercalated within Hf-H framework, where two-thirds of the hydrogen atoms are arranged in a tubelike H12 structure located inside channels formed by the remainder HfH3. Each H12 tube gains 0.876 electrons from the HfH3 framework, indicating the ionic character of HfH9. Calculations show that HfH9 is a potential superconductor with an estimated Tc of 110 K at 200 GPa, with the electron-phonon coupling arising mainly from the H12 tube and its interaction with the HfH3 framework. The current results suggest the existence of diverse hydrogen chemistries at high pressure that could be unravelled by future experimental studies.