Synthetic Cosalite Pb2Bi2S5: Vibrational and Nonlinear Optical Properties

ABSTRACT Cosalite (), a layered sulfosalt mineral with an orthorhombic structure, is investigated through a combined experimental and first‐principles theoretical approach to elucidate its vibrational, nonlinear optical, and thermal transport properties. The unambiguous determination of the non‐centrosymmetric space group for the most stable phase of cosalite is performed using DFT calculations, supported by experimental EBSD data. The experimental Raman spectra recorded at liquid nitrogen temperature reveal eight intense modes in the range 50–300 cm, assigned to Bi‐S and Pb‐S stretching vibrations. The direct optical transition is established by calculation of the band structure. The optical absorption spectrum calculated by TDDFT including SOC effects yields a bandgap value of = 1.1 eV, which is substantially similar to the fundamental Shockley‐Queisser limit value, thus opening the possibility of using cosalite as a photovoltaic element. The strong third‐order nonlinear optical response is obtained in cosalite ( m/V) and is attributed to the dynamics of the electronic lone pair localized on Pb and Bi atoms. The lattice thermal conductivity is established by analysis of the heat flux current using classical molecular dynamics, employing a machine‐learning technique for constructing the interatomic potential. The estimated ultralow value gives rise to potential applications of the crystal as a thermoelectric generator.