Pressure-induced band-gap closure and metallization in two-dimensional transition metal halide CdI2

Abstract In this letter, we present the pressure-induced evolution of the band-gap, conductance, and crystal structure of layered transition metal dihalide (TMH), CdI2, an insulator at ambient conditions, using electrical transport measurement, UV–vis absorption spectroscopy, X-ray diffraction, and Raman scattering spectroscopy. We found that the band gap shrinks gradually following a sharp drop at 34.5 GPa. Meanwhile, the temperature-dependent resistance indicated an insulator-to-semiconductor and then metal transition occurred at 36 and 62 GPa, respectively. Both X-ray diffraction and Raman scattering measurements indicate that the CdI2 underwent a first-order transition from a hexagonal to monoclinic phase at ∼32 GPa due to collapses in the c-lattice parameter and volume. The second structural phase transition (SPT) from monoclinic-to-tetragonal occurred at 48 GPa. The pressure-induced insulator-to-semiconductor-to-metal of CdI2 is attributed to the structural transition from hexagonal-to-monoclinic-to-tetragonal. Our first-principle calculations further confirm the sequence of the SPTs and the semiconducting and metallic band structure of the phases, respectively. Namely, the metallization is observed due to the filled 5p-iodide to shift and overlap with the filled 5s-cadmium band in metal phase. These findings pave the way for investigating crystal structure evolution, and the optical and electrical properties in CdI2-type compounds under extreme conditions.