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, ScCl₂, ScCl₃, ScCl₅, and ScCl₇ 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 P6₃/mmc and Pm-3m symmetries can be metastable at ambient pressure upon decompression and convert into superconductive electrides. Pm-3-ScCl₇ exhibits significant pressure-modulated superconductivity, featuring an enhancement of T_c to 10.91 K at a low pressure of 75 GPa. In addition, the universal superconductivity found in the Pm-3 structured chlorides suggests a promising structural prototype for pressure-tunable superconductors.