Ca6Cu2Sn7: novel 3D open framework with unusual Sn4 tetramers

The new ternary polar intermetallic phase, Ca6Cu2Sn7, has been synthesized by the solid-state reaction of the stoichiometric mixture of the pure elements in welded Ta tubes at high temperature. Its structure was established by single-crystal X-ray diffraction studies. Ca6Cu2Sn7 crystallizes in the monoclinic space group C2/m (No. 12) with cell parameters of a=14.257(7), b=4.564(2), and c=12.376(7) A, beta=93.979(6) degrees, V=803.3(7) A3, and Z=2. The structure of Ca6Cu2Sn7 belongs to a new structure type and features a 3D anionic open-framework composed of Cu2Sn3] layers interconnected by unusual Sn4 tetramers, forming large tunnels along the b axis which are composed of Cu4Sn12 16-membered rings. The calcium atoms are located in these large tunnels. Ca6Cu2Sn7 is metallic and exhibits temperature-independent paramagnetism.