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.nH2O 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)4.7H2O, monoclinic, space group C2/c, a=9.830(2), b=29.019(6), c=9.178(2)A, β=122.248(4)o, V=2214.2(8)A3, Z=4, R=0.0427; Sr2Zr(C2O4)4.11H2O, tetragonal, space group I41/acd, a=16.139(4), c=18.247(6)A, V=4753(2)A3, Z=8, R=0.0403; Ca2Zr(C2O4)4.5H2O, orthorhombic, space group Pna21, a=8.4181(5), b=15.8885(8), c=15.8885(8)A, V=2125(2)A3, 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 (Sr2+, Ca2+), large tunnels are obtained, running down the c direction of the unit cell, which can accommodate zeolitic water molecules. For the largest Ba2+ cation, the second framework is formed and is closely related to that of Pb2Zr(C2O4)4.nH2O. The decomposition at 800oC 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