Li₂PbGeS₄ and Li₂EuGeS₄: Polar Chalcopyrites with a Severe Tetragonal Compression

Li 2 PbGeS 4 and Li 2 EuGeS 4 form by dissolving Pb/Eu and Ge in molten Li 2 S 9 at 600−650 °C. They crystallize in the tetragonal, noncentrosymmetric space group I-42m with a = 6.5224(5) Å, c = 7.7603(8) Å, Z = 2, R1 = 0.0253, and wR2 = 0.0646 for Li 2 PbGeS 4 and a = 6.5447(4) Å, c = 7.6960(6) Å, Z = 2, R1 = 0.0224, and wR2 = 0.0551 for Li 2 EuGeS 4 . These polar compounds possess a compressed chalcopyrite-like structure. The compression occurs along the c axis and results in an expanded coordination sphere for Pb and Eu that can be described as a distorted dodecahedron. The Pb/EuS 8 dodecahedra share edges and corners with the GeS 4 tetrahedra in such a way as to create small tunnels down the c axis in which the lithium cations reside. The Li + cations are surrounded by four S 2 - anions that create a highly distorted tetrahedral pocket. Both compounds are semiconductors with indirect band gaps of 2.41 and 2.54 eV, respectively. We also discuss the results of an LAPW band structure calculation for Li 2 PbGeS 4 and report infrared spectroscopic characterization.