Li2PbGeS4 and Li2EuGeS4:: Polar chalcopyrites with a severe tetragonal compression

Li2PbGeS4 and Li2EuGeS4 form by dissolving Pb/Eu and Ge in molten Li2S9 at 600-650 degreesC. They crystallize in the tetragonal, noncentrosymmetric space group I-42m with alpha = 6.5224(5) Angstrom, c = 7.7603(8) Angstrom, Z = 2, R1 = 0.0253, and wR2 = 0.0646 for Li2PbGeS4 and alpha = 6.5447(4) Angstrom, c = 7.6960(6) Angstrom, Z 2, R1 = 0.0224, and wR2 = 0.0551 for Li2EuGeS4. 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/EuS8 dodecahedra share edges and corners with the GeS4 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 S2- 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 Li2PbGeS4 and report infrared spectroscopic characterization.