Type-I clathrate calcium hydride and its hydrogen-vacancy structures at high pressure

Recently, the first predicted metal clathrate hydride ${\mathrm{CaH}}_6$ was successfully synthesized experimentally and shown to have a superconducting critical temperature $T_c$ 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 $\mathrm{Ca}–H$ system at 200 GPa, revealing a thermodynamically stable type-I clathrate hydride, ${\mathrm{Ca}}_8{H}_{46}$, along with two analogous structures featuring hydrogen vacancies. ${\mathrm{Ca}}_8{H}_{46}$ exhibits a high $T_c$ range of 210–223 K at 150 GPa, and the presence of hydrogen vacancies shifts the $T_c$. Our simulated x-ray-diffraction patterns, calculated equation of state, and $T_c$'s for ${\mathrm{Ca}}_8{H}_x$ $(x=44-46)$ 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 ${\mathrm{Ca}}_8{H}_{46}$, which explain the anomalous decrease in $T_c$ observed in experiments.