Layered Compounds BaM2Ge4Ch6 (M = Rh, Ir and Ch = S, Se) with   Pyrite-Type Building Blocks and Ge-Ch Heteromolecule-Like Anions
  Structure and chemical features of layered compounds BaM2Ge4Ch6 (M = Rh, Ir; Ch = S, Se) synthesized under high-pressure and high-temperature method are systematically studied. These compounds crystallize in an orthorhombic phase with space group Pbca (No. 61). The remarkable structural feature is to have the M-Ge-Ch pyrite-type building units, stacking with Ba-Ch layers alternatively along c axis. It is very rare and novel that pyrite-type subunits as the building blocks in the layered compounds. Theoretical calculations and experimental results indicate that there are strongly polarized covalent bonds between Ge and Ch atoms, forming heteromolecule-like anions in these compounds. Moreover, Ge atoms in this structure exhibit unusual valence state (~ +1) due to the tetrahedral coordination environment of Ge atoms along with M and Ch atoms simultaneously. 
Hechang Lei, Jun-ichi Yamaura, Jiangang Guo, Yanpeng Qi, Yoshitake   Toda, and Hideo Hosono
cond-mat.mtrl-sci