Unconventional Stabilization Mechanisms and Emergent Superconductivity in Scandium Polychlorides under Extreme Conditions

Using first-principles evolutionary crystal structure prediction, we systematically investigate scandium polychlorides across 50-300 GPa, predicting multiple thermodynamically stable phases ScCl, ScCl 2 , ScCl 3 , ScCl 5 , and ScCl 7 with unconventional stoichiometries. The exceptional stability of these compounds stems from the mutually compatible crystal orbitals of the Sc and Cl sublattices, strong ionic interactions, and the formation of Cl-Cl homobonds. These factors play critical roles in stabilizing scandium chloride compounds with various unconventional stoichiometries. Notably high-pressure novel ScCl phases with P 6 3 / mmc and Pm-3 m symmetries can be metastable at ambient pressure upon decompression and convert into superconductive electrides. Pm -3-ScCl 7 exhibits significant pressure-modulated superconductivity, featured by the enhancement of Tc to 10.91 K at a low pressure of 75 GPa. In addition, the universal superconductivity found in Pm -3 structured chlorides suggests a promising structural prototype for pressure-tunable superconductors.