Stability and superconductivity of hexagonal prism-structured polyhydrides X2MgH18 (X = Li, Na, K, Rb, Cs) under moderate pressure†

The ultra-high pressure required to maintain hydride superconductors currently limits their further development and practical application, making it urgent to explore stable high-temperature hydride superconductors that can operate at moderate pressures. Here, we conducted an extensive search for various structures with the chemical formula X₂MgH₁₈ (X = Li, Na, K, Rb, Cs) and ultimately identified a hexagonal prism hydrogen structure belonging to the space group C2/m. Then, we investigated the stability and superconductivity of these structures. K₂MgH₁₈, Rb₂MgH₁₈, and Cs₂MgH₁₈ were found to be unstable, whereas the structure of Li₂MgH₁₈ is dynamically stable and demonstrates superconductivity under pressures of 20 GPa (with a transition temperature of about 110 K) and 235 GPa (with a transition temperature of about 122 K). Additionally, we found that the Na₂MgH₁₈ structure, also in the C2/m space group, achieves a superconducting transition temperature of about 105 K at 20 GPa and about 147 K at 235 GPa. We have conducted a comprehensive calculation and analysis of these structures, revealing that the vibrations of hydrogen atoms primarily contribute to the superconducting behavior, and finding that the distortion of the hexagonal prism structure leads to the weakening of the H–H covalent bonds, the strengthening of the metallic bonds, and ultimately results in an increase in the superconducting transition temperature. This work significantly enriches our understanding of the chemistry and superconducting properties of high-pressure hydrides, providing a new idea for further exploration in the field of moderate-pressure hydrides.