Local structure study of the Fe ions in mixed-valence iron(II)-iron(III) metal formate frameworks

Abstract

X-ray Absorption Spectroscopy (XAS) methods have been used to study the mixed-valence iron(II)-iron(III) metal–organic frameworks (MOFs) that undergo order–disorder phase transitions. In particular, this paper reports X-ray Absorption Fine Structure (XAFS) study of the electronic and local structure at the Fe K-edge (7 keV) for three mixed-valence iron(II)-iron(III) formate frameworks, namely dietylammonium ((C2H5)2NH2[FeIIIFeII(HCOO)6], DEtAFeFe), dimethylammonium ((CH3)2NH2[FeIIIFeII(HCOO)6], DMAFeFe) and N-etylmethylammonium (CH3NH2C2H5[FeIIIFeII(HCOO)6], EtMeAFeFe) analogues. DMAFeFe (DEtAFeFe) was studied at 140 K and 155 K (240 K and 250 K) in detail to obtain information on the structural changes during the phase transitions. The near-edge x-ray absorption fine structure (XANES) analysis confirms the octahedrally coordinated Fe in the studied samples and the electron transfer from divalent to trivalent iron ions. A slight variation was found in the Fe-O bond lengths and quantified the order in the bond length distribution measured by the Debye-Waller factor (σ2) around the phase transition temperature. The Fe2+/Fe3+ ions are homogeneously distributed in the order–disorder phase transition samples. The slight variation in the local structure defines the nature of the phase transition.