New alkaline earth-zirconium oxalates M2Zr(C2O4)4 • nH2O (M = Ba, Sr, Ca) synthesis, crystal structure and thermal behavior

Three new alkaline earth-zirconium oxalates M2Zr(C2O4)(4).nH(2)O have been synthesized by precipitation methods for M = Ba, Sr, Ca. For each compound the crystal structure was determined from single crystals obtained by controlled diffusion of M2+ and Zr4+ ions through, silica gel containing oxalic acid. Ba2Zr(C2O4)(4).7H(2)O, monoclinic, space group C2/c, a = 9.830(2), b = 29.019(6), c = 9.178(2) Angstrom, beta = 122.248(4)degrees, V = 2214.2(8) Angstrom(3), Z = 4, R = 0.0427; Sr2Zr(C2O4)(4).11H(2)O, tetragonal, space group I4(1)/acd, a = 16.139(4), c = 18.247(6) Angstrom, V = 4753(2) Angstrom(3), Z = 8, R = 9.0403; Ca2Zr(C2O4)(4).5H(2)O, orthorhombic, space group Pna2(1), a = 8.4181(5), b = 15.8885(8), c = 15.8885(8) Angstrom, V = 2125(2) Angstrom(3), Z = 4, R = 0.0622. The structures of the three compounds consist of chains of edge-shared MO6(H2O)(x) (x = 2 or 3) polyhedra connected to ZrO8 polyhedra through oxalate groups. Depending on the arrangement of chains.. the ZrO8 polyhedron geometry (dodecahedron or square antiprism) and the connectivity, two types of three-dimensional frameworks are obtained. For the smallest M2+ cations (Sr 21, Ca 21), large tunnels are obtained, running down the c direction of the unit cell, which can accommodate zeolitic water molecules. For the largest Ba 21 cation, the second framework is formed and is closely related to that of Pb2Zr(C2O4)(4).nH(2)O. The decomposition at 800degreesC into strontium carbonate, barium carbonate or calcium oxide and MZrO3 (M= Sr, Ba, Ca) perovskite is reported from thermal analyses studies and high temperature X-ray powder diffraction. (C) 2004 Elsevier Inc. All rights reserved.