Polymorphism of anhydrous iron(II) oxalate

Recently, iron(II) oxalate has experienced a renewed interest due to their newly found application in lithium-ion batteries. Lithium is expected to be embedded between the oxalate sheets, dramatically increasing the need to understand the oxalate structure. Despite being known for decades, the discrepancies still exist regarding the anhydrous iron(II) oxalate. In this work, we explore the dehydration process of both alpha-FeC2O4 center dot 2H2O and beta-FeC2O4 center dot 2H2O polymorphs at different heating rates and calcination temperatures by X-ray powder diffraction, Mossbauer spectroscopy and scanning electron microscopy. After dehydration, iron(II) oxalates formed two polymorphs with different XRD patterns: alpha-FeC2O4 with sharp and narrow diffraction lines and beta-FeC2O4 with very broadened lines, which were attributed to the monoclinic structure with space group P21/n.