Static disorder in a perovskite mixed-valence metal–organic framework
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)][FeII3FeIII(HCOO)12], in order to investigate potential electric ordering of the DMA cation from its four-fold 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 remains 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 in tuning their physical properties.