Static disorder in a perovskite mixed-valence metal–organic framework

Abstract

Variable-temperature and variable-pressure single-crystal diffraction studies are carried out on a mixed-valence perovskite dimethylammonium (DMA) iron formate compound, with the formula [(DMA₃)(H₂O)][Fe^II₃Fe^III(HCOO)₁₂], in order to investigate potential electric ordering of the DMA cation from its fourfold type of dynamic disorder at ambient conditions. Mössbauer spectroscopy is additionally carried out at ambient conditions to confirm the presence and ratio of Fe²⁺ and Fe³⁺ cations. Below 200 K, a dynamic to static disorder of the DMA cation is observed, while the crystal symmetry and iron formate framework structure remain the same. Upon application of pressure, however, a phase transition occurs that lowers the symmetry above 3.3 GPa. This work highlights the further chemical modifications that are possible within the dimethylammonium metal formates family, i.e. doping upon the A-site with neutral molecules, leading to additional opportunities to tune their physical properties.