Synthesis and characterization of the barium oxalates BaC2O4•0.5H2O, α-BaC2O4 and β-BaC2O4

The synthesis of BaC2O4.0.5H(2)O and its thermal decomposition to alpha-BaC2O4 and beta-BaC2O4 was investigated. BaC2O4.0.5H(2)O is precipitated at room temperature from aqueous solutions of barium chloride and ammonium oxalate. The deuterated compound BaC2O4.0.5D(2)O was made in analogy with D2O as the solvent. The compounds were characterized by X-ray and neutron diffraction analysis. Single-crystal X-ray diffraction of BaC2O4.0.5H(2)O measured at 120 K gave the triclinic cell a = 8.692 (1), b = 9.216 (1), c = 6.146 (1) Angstrom, alpha = 95.094 (3), beta = 95.492 (3), gamma = 64.500 (3)degrees, space group P (1) over bar, Z = 4. Two independent Ba atoms are each coordinated to nine O atoms at distances from 2.73 (1) to 2.99 (1) Angstrom. One of the two oxalate ions deviates significantly from planarity. The water molecule does form weak hydrogen bonds. In situ X-ray powder diffraction was used to study the thermal decomposition of BaC2O4.0.5H(2)O and the formation of alpha-BaC2O4. The X-ray powder pattern of alpha-BaC2O4 measured at 473 K was indexed on a triclinic cell with a = 5.137 (3), b = 8.764 (6), c = 9.006 (4) Angstrom, alpha = 83.57 (4), beta = 98.68 (5), gamma = 99.53 (5)degrees, and the space group P (1) over bar with Z = 4.