Polymorphism of anhydrous iron(II) oxalate

Abstract 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 α -FeC 2 O 4 ·2H 2 O and β -FeC 2 O 4 ·2H 2 O polymorphs at different heating rates and calcination temperatures by X-ray powder diffraction, Mössbauer spectroscopy and scanning electron microscopy. After dehydration, iron(II) oxalates formed two polymorphs with different XRD patterns: α -FeC 2 O 4 with sharp and narrow diffraction lines and β -FeC 2 O 4 with very broadened lines, which were attributed to the monoclinic structure with space group P2 1 /n.