Pressure-Induced Superconductivity and Flattened Se₆ Rings in the Wide Band Gap Semiconductor Cu₂I₂Se₆

The two major classes of unconventional superconductors, cuprates and Fe-based superconductors, have magnetic parent compounds, are layered, and generally feature square-lattice symmetry. We report the discovery of pressure-induced superconductivity in a nonmagnetic and wide band gap 1.95 eV semiconductor, Cu₂I₂Se₆, with a unique anisotropic structure composed of two types of distinct molecules: Se₆ rings and Cu₂I₂ dimers, which are linked in a three-dimensional framework. Cu₂I₂Se₆ exhibits a concurrent pressure-induced metallization and superconductivity at ∼21.0 GPa with critical temperature (T_c) of ∼2.8 K. The T_c monotonically increases within the range of our study reaching ∼9.0 K around 41.0 GPa. These observations coincide with unprecedented chair-to-planar conformational changes of Se₆ rings, an abrupt decrease along the c-axis, and negative compression within the ab plane during the phase transition. DFT calculations demonstrate that the flattened Se₆ rings within the CuSe layer create a high density of states at the Fermi level. The unique structural features of Cu₂I₂Se₆ imply that superconductivity may emerge in anisotropic Cu-containing materials without square-lattice geometry and magnetic order in the parent compound.