Conflict between the Electronic Factors and Structure-Directing Rules in the Intergrowth Structure of Ca4Ag2+xGe4–x with x = 1/2

Combined experimental and theoretical efforts to conceptually understand the structure directing forces in intergrowth structures have led to the discovery of the new ternary phase Ca4Ag2+xGe4–x (x = 0.5), obtained from high-temperature reaction of the elements. It crystallizes in a new structure type according to single-crystal diffraction methods: monoclinic space group C2/m–i10 with a = 10.7516(2) Å, b = 4.5475(1) Å, c = 18.7773(4) Å, β = 93.69(2)°, V = 916.17(3) Å3, Z = 4. The compound corresponds to the n = 2 member of the homologous series Ca2+nAg2+xGe2+n–x, that are built up by linear intergrowths of slabs cut from the CaGe (CrB-type) and the CaAg1+xGe1–x (KHg2 or TiNiSi-type) structures, and may be partitioned in Ag-rich and Ag-free domains. Instead of the predicted Zr2CoSi2-type (C2/m–i5), a simultaneous doubling of the size of the two building blocks is observed with the dimerization of the (Ge2) pairs into Ag-substituted tetramers (AgxGe4–x) due to valence electron shortage. However, the Ag/Ge mixing at one atomic site with roughly one-to-one atomic ratio is therefore unexplained. The electronic band structure calculations and analysis of the chemical bonding provided evidence that the Ag/Ge mixing is rather the result of a direct conflict between the Zintl-Klemm concept and empirically established “structure-directing rules”. The implications of these findings for the poorly understood ordered staging structural interfaces, typically observed in secondary Li-ion batteries during charge/discharge process, are briefly discussed.