Semiconductor-metal transition in lead iodide under pressure

The two-dimensional (2D) semiconductor lead iodide (PbI2) has recently attracted considerable attention owing to its favorable properties in both applications as photodetectors and as a precursor for lead halide perovskite solar cells. Although earlier experiments have investigated the structural stability of PbI2 below 20 GPa, the electric structures at high pressure remain ambiguous, as does the crystal structure at higher pressures. Herein, a structural phase transition and a semiconductor-metal transition are revealed in PbI2 through high-pressure femtosecond optical pump-probe spectroscopy combined with Raman spectra, synchrotron x-ray diffraction (XRD), and resistance measurements up to 70 GPa. Two discontinuities appear in the pressure-dependent amplitude of the ultrafast spectroscopy at approximately 24.8 and 37.6 GPa. Raman spectra and in situ XRD patterns confirm a structural phase transition from orthorhombic Pnma to tetragonal I4/MMM symmetry at the first discontinuity. The second discontinuity is ascribed to the closure of the bandgap and the enhanced electron-phonon interaction across the semiconductor-metal transition, which is also revealed by the temperature dependencies of resistance for PbI2 under pressure. Our results not only help to design optical devices based on lead iodide but also highlight that ultrafast spectroscopy is an efficient noncontact tool to investigate the crystalline and electric phase transition under pressures simultaneously.