Ca11E3C8 (E = Sn, Pb): New Complex Carbide Zintl Phases Grown from Ca/Li Flux

New carbide Zintl phases Ca11E3C8 (E = Sn, Pb) were grown from reactions of carbon and heavy tetrelides in Ca/Li flux. They form with a new structure type in space group P21/c (a = 13.1877(9)­Å, b = 10.6915(7)­Å, c = 14.2148(9)­Å, β = 105.649(1)°, and R1 = 0.019 for the Ca11Sn3C8 analog). The structure features isolated E4– anions as well as acetylide (C22–) and allenylide (C34–) anions; the vibrational modes of the carbide anions are observed in the Raman spectrum. The charge-balanced nature of these phases is confirmed by DOS calculations which indicate that the tin analog has a small band gap (Eg < 0.1 eV) and the lead analog has a pseudogap at the Fermi level. Reactions of these compounds with water produce acetylene and allene.