Thermally induced phase transitions of barium oxalates

The thermal decomposition of BaC2O4 center dot 3.5H(2)O and BaC2O4 center dot 0.5H(2)O was investigated using in situ synchrotron X-ray and neutron powder diffraction. The decomposition routes for the barium oxalate hydrates were observed to depend on the applied heating rate. Thermal decomposition of BaC2O4 center dot 0.5H(2)O showed transformation to alpha-BaC2O4 and to beta-BaC2O4 prior to the formation of BaCO3. The decomposition of BaC2O4 center dot 3.5H(2)O showed formation of BaC2O4 center dot 0.5H(2)O at 58 degrees C and the hemi hydrate transforms to alpha-BaC2O4 at 187 degrees C using a relatively fast heating rate of 6.25 degrees C/min. The phase transitions were more complicated using lower heating rate, which also reveal formation of beta-BaC2O4 coexisting with alpha-BaC2O4 along with an unidentified compound. Heating alpha- and beta-BaC2O4 to higher temperatures (T > 400 degrees C) produced BaCO3. A sample of alpha-BaC2O4 was prepared in situ by thermal decomposition of BaC2O4 center dot 3.5H(2)O on a powder neutron diffractometer. The neutron diffraction data has broad diffraction peaks due to small crystallite sizes and overlapping Bragg reflections. [A structural model for alpha-BaC2O4 was derived from the neutron pattern, triclinic, space group P-1, a = 5.127(7), b = 8.905(12), c = 9.068(12) angstrom, alpha = 82.74(1), beta = 99.46(2), gamma = 100.10(1)degrees measured at T= 300 degrees C. The average Ba-O distances are 2.84(3) angstrom and 2.66(3) angstrom for Ba 1 and Ba2 respectively, C-O atom distances in the oxalate ions were found in the range 1.25(3)-1.26(4) angstrom, and C-C distances were 1.60(1)-1.61(1) angstrom]. (C) 2011 Elsevier Masson SAS. All rights reserved