Structures and properties of Ca-Xe compounds at extreme pressure and temperature

Calcium, one of the most abundant elements in the Earth's mantle, does not react easily with noble gases (e.g., He and Xe) under ambient conditions. However, high pressure can alter electron configurations in atoms, leading to the formation of unconventional compounds. In this study, we systematically investigate Ca-Xe compounds across pressures of 0-150 GPa using calypso structure prediction methods combined with first-principles calculations. We identify four novel Ca-Xe compounds Pm3̄m-CaXe, P4/mmm-CaXe2, I4/m-Ca3Xe, and P4/mmm-Ca2Xe3 that demonstrate stability over a wide pressure range from 37.5 to 150 GPa. All these compounds exhibit metallic properties and are dynamically stable, as indicated by the absence of imaginary frequencies in their phonon dispersion spectra. Ionic bonding between Ca and Xe is observed due to electron transfer from Ca to Xe. Ab initio molecular dynamics simulations show that Pm3̄m-CaXe, P4/mmm-CaXe2, and P4/mmm-Ca2Xe3 remain solid up to pressures of 135 GPa and temperatures of 4000 K. In contrast, I4/m-Ca3Xe undergoes a transition from solid to liquid at temperatures above 3500 K due to weakened Ca-Xe bonds. The findings suggest that these Ca-Xe compounds could potentially be synthesized experimentally under high-pressure conditions. The results offer theoretical guidance for discovering new high-pressure Xe compounds and provide valuable insights into Xe chemistry.