Thermally induced phase transitions of barium oxalates

The thermal decomposition of BaC2O4·3.5H2O and BaC2O4·0.5H2O 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 BaC2O4·0.5H2O showed transformation to α-BaC2O4 and to β-BaC2O4 prior to the formation of BaCO3. The decomposition of BaC2O4·3.5H2O showed formation of BaC2O4·0.5H2O at 58 °C and the hemi hydrate transforms to α-BaC2O4 at 187 °C using a relatively fast heating rate of 6.25 °C/min. The phase transitions were more complicated using lower heating rate, which also reveal formation of β-BaC2O4 coexisting with α-BaC2O4 along with an unidentified compound. Heating α- and β-BaC2O4 to higher temperatures (T > 400 °C) produced BaCO3.A sample of α-BaC2O4 was prepared in situ by thermal decomposition of BaC2O4·3.5H2O 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 α-BaC2O4 was derived from the neutron pattern, triclinic, space group P-1, a = 5.127(7), b = 8.905(12), c = 9.068(12) Å, α = 82.74(1), β = 99.46(2), γ = 100.10(1)° measured at T = 300 °C. The average Ba–O distances are 2.84(3) Å and 2.66(3) Å for Ba1 and Ba2 respectively, C–O atom distances in the oxalate ions were found in the range 1.25(3)–1.26(4) Å, and C–C distances were 1.60(1)–1.61(1) Å].

Barium oxalate hydrates can be converted to two polymorphs of barium oxalate α- and β-BaC2O4 observed by in situ synchrotron radiation powder X-ray diffraction.Figure optionsDownload as PowerPoint slide