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 P2(1)/c (a = 13.1877(9)angstrom, b = 10.6915(7)angstrom, c = 14.2148(9)angstrom, beta = 105.649(1)degrees, and R-1 = 0.019 for the Ca11Sn3C8 analog). The structure features isolated E4- anions as well as acetylide (C-2(2-)) and allenylide (C-3(4-)) 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 (E-g < 0.1 eV) and the lead analog has a pseudogap at the Fermi level. Reactions of these compounds with water produce acetylene and allene.