Pb4S3I2-A high-pressure phase in the PbS-PbI2 system

The reaction of a stoichiometric ratio of Pb, S and PbI2 at 4 GPa and 600 degrees C yields Pb4S3I2, a compound that has not been reported in the ambient pressure PbS-PbI2 system. Its crystal structure, determined from single crystal X-ray diffraction data, is orthorhombic (space group Pnma) with a = 8.1293(6) angstrom, b = 15.5613(11) angstrom, c = 8.1820(6) angstrom, and Z = 4 at ambient temperature and pressure. The structure consists of sheets of distorted edge-sharing lead-centered polyhedra and saw-tooth-shaped lead-lead bonded chains. Density Functional Theory based band structure calculations suggest Pb4S3I2 to be an indirect band gap semiconductor, and, consistently, a band gap value of approximately 1.6 eV is determined by diffuse reflectance measurements. The material is diamagnetic above 1.8 K.