Experimental and theoretical investigations of the novel ternary compound Ca4InGe4.

A new polar intermetallic compound with a novel structure type has been synthesized and characterized by both powder and single-crystal X-ray diffraction. Ca(4)InGe(4) crystallizes in the monoclinic crystal system (space group C2/c, Z = 4, Pearson symbol mS36) with five crystallographically unique atomic positions in the asymmetric unit. The corresponding lattice parameters at 200(2) K are a = 18.452(8) Å, b = 5.819(2) Å, c = 8.339(3) Å, and β = 99.330(6)°. The overall crystal structure can be described as a linear intergrowth of two imaginary fragments--Ca(2)InGe(2) with the Gd(2)AlGe(2) type- and CaGe with the FeB type-structures. Another way to rationalize the bonding is to focus on the polyanionic framework, which in this case is made up of unique nets of "seesaw"-shaped [InGe(4)] units. They are interconnected via Ge-Ge dimers into an open three-dimensional framework with Ca(2+) cations occupying the voids within it. Tight-binding linear muffin-tin orbital (LMTO) calculations provide a rationale for the unique local coordination geometry around In and the two distinct types of Ge-Ge bond distances.