Thermal decomposition of Prussian blue under inert atmosphere

The thermal decomposition of Prussian blue (iron(III) hexacyanoferrate) under inert atmosphere of argon was monitored by thermal analysis from room temperature up to 1000 degrees C. X-ray powder diffraction and Fe-57 Mossbauer spectroscopy were the techniques used for phase identification before and after sample heating. The decomposition reaction is based on a successive release of cyanide groups from the Prussian blue structure. Three principal stages were observed including dehydration, change of crystal structure of Prussian blue, and its decomposition. At 400 degrees C, a monoclinic Prussian blue analogue was identified, while at higher temperatures the formation of various polymorphs of iron carbides was observed, including an orthorhombic Fe2C. Increase in the temperature above 700 degrees C induced decomposition of primarily formed Fe7C3 and Fe2C iron carbides into cementite, metallic iron, and graphite. The overall decomposition reaction can be expressed as follows: Fe-4[Fe(CN)(6)](3)center dot 4H(2)O -> 4Fe + Fe3C + 7C + 5(CN)(2) + 4N(2) + 4H(2)O.