title: Synthesis and characterization of the barium oxalates BaC2O4.0.5H2O, alpha-BaC2O4 and beta-BaC2O4.
abstract: The synthesis of BaC(2)O(4).0.5H(2)O and its thermal decomposition to alpha-BaC(2)O(4) and beta-BaC(2)O(4) was investigated. BaC(2)O(4).0.5H(2)O is precipitated at room temperature from aqueous solutions of barium chloride and ammonium oxalate. The deuterated compound BaC(2)O(4).0.5D(2)O was made in analogy with D(2)O as the solvent. The compounds were characterized by X-ray and neutron diffraction analysis. Single-crystal X-ray diffraction of BaC(2)O(4).0.5H(2)O measured at 120 K gave the triclinic cell a = 8.692 (1), b = 9.216 (1), c = 6.146 (1) A, alpha = 95.094 (3), beta = 95.492 (3), gamma = 64.500 (3) degrees, space group P1, Z = 4. Two independent Ba atoms are each coordinated to nine O atoms at distances from 2.73 (1) to 2.99 (1) A. 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 BaC(2)O(4).0.5H(2)O and the formation of alpha-BaC(2)O(4). The X-ray powder pattern of alpha-BaC(2)O(4) measured at 473 K was indexed on a triclinic cell with a = 5.137 (3), b = 8.764 (6), c = 9.006 (4) A, alpha = 83.57 (4), beta = 98.68 (5), gamma = 99.53 (5) degrees, and the space group P1 with Z = 4.