  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.
