SYNTHESIS, SINGLE-CRYSTAL GROWTH AND STRUCTURAL DETERMINATION OF NEW COPPER OXYCARBONATES A(4)CUM(CO3)(2)O-4,A=SR,BA,M=LI,NA,CA

New layered perovskite compounds A(4)CuM(CO3)(2)O-4, where A = Sr, M = Li, or A = Ba, M = Li, Na or Ca, have been synthesized. Good-quality single crystals of the Li- and Na-based compounds have been grown. The crystal structure of these isomorphic compounds has been determined b y powder X-ray and single-crystal X-ray diffraction, HRTEM and electro n diffraction. The unit cells are tetragonal with symmetry I4/mmm. The lattice constants are a = 5.530(5)Angstrom, c = 14.637(3)Angstrom (A = Sr, M = Li); a = 5.754(1)Angstrom, c = 15.264(4)Angstrom (A = Ba,M = Li); a = 5.829(1)Angstrom, c = 15.364(2)Angstrom (A = Ba,M = Na); a = 5.844(4)Angstrom c = 15.459(7)Angstrom (A = Ba,M = Ca). All structure s consist of an alternate stacking of Cu,M-O-2 planes in which the Cu and M ions are ordered, and planes occupied by carbonate groups that p oint toward the nearest M ions. The carbon and oxygen estimations indi cate that if M is a monovalent metal, the formal valence of Cu is +3.