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 pressureinduced 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 (Tc) of ∼2.8 K. The Tc 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.

Cai., Lin, W., Li, L-H., Malliakas, C.D., Zhang, R., Groesbeck, M., Bao, J-K., Zhang, D., Sterer, E., Kanatzidis, M.G., Deemyad, S., Pressure-induced Superconductivity and Flattened Se6 Rings in the Wide Band Gap Semiconductor Cu2I2Se6, J. Am. Chem. Soc. (2019) 141, 38, 15174-15182, DOI : 10.1021/jacs.9b06794

Experimental evidence of pressure-induced superconductivity in Cu2I2Se6.
(a−c) Temperature-dependent resistance at various pressures in compression, decompression, and recompression cycles in run 2. A step-like transition is observed in compression and decompression cycles; see
Supporting Information for more discussion.
(d) Evolution of superconducting temperature Tc in compression−decompression-recompression cycles. A prominent enhancement of Tc was observed in the recompression cycle.
(e) Temperature-dependent resistances at various magnetic fields from 0.0 to 6.0 T. The inset indicates the photograph of Cu2I2Se6 at 25.4 GPa using the four-terminal method for resistivity measurements.
(f) Temperature dependence of the superconducting critical field. The curve is fitted with the formula of Hc2(T) = Hc2(0)(1 − (T/Tc)2)/(1 + (T/Tc)2) based on the Ginzburg−Landau (G-L) theory.