Type-I clathrate calcium hydride and its hydrogen-vacancy structures at high pressure
Decheng An, Defang Duan, Zihan Zhang, Qiwen Jiang, Tiancheng Ma, Zihao Huo, Hao Song, Tian Cui
Abstract
Recently, the first predicted metal clathrate hydride ${\mathrm{CaH}}_{6}$ was successfully synthesized experimentally and shown to have a superconducting critical temperature ${T}_{\mathrm{c}}$ exceeding 200 K. However, experimental findings suggest the existence of additional unidentified phases besides ${\mathrm{CaH}}_{6}$. In this study, we performed an extensive structure search of the $\mathrm{Ca}--\mathrm{H}$ system at 200 GPa, revealing a thermodynamically stable type-I clathrate hydride, ${\mathrm{Ca}}_{8}{\mathrm{H}}_{46}$, along with two analogous structures featuring hydrogen vacancies. ${\mathrm{Ca}}_{8}{\mathrm{H}}_{46}$ exhibits a high ${T}_{\mathrm{c}}$ range of 210--223 K at 150 GPa, and the presence of hydrogen vacancies shifts the ${T}_{\mathrm{c}}$. Our simulated x-ray-diffraction patterns, calculated equation of state, and ${T}_{\mathrm{c}}$'s for ${\mathrm{Ca}}_{8}{\mathrm{H}}_{x}$ $(x=44\ensuremath{-}46)$ agree with the experimental results, confirming their presence in multiple synthesized phases. Our findings serve to revise the Ca--H phase diagram under high pressure and offer valuable insights into the effects of hydrogen vacancies in ${\mathrm{Ca}}_{8}{\mathrm{H}}_{46}$, which explain the anomalous decrease in ${T}_{\mathrm{c}}$ observed in experiments.