Remarkable resilience of the formate cage in a multiferroic metal organic framework material: dimethyl ammonium manganese formate (DMAMnF).

Dimethyl ammonium (DMA) metal formate, an important member of the dense metal organic framework (MOF) family, is known to exhibit a low temperature ferroelectric transition, caused by the ordering of the hydrogen bonds. In this study, we probed the effect of pressure on the disordered hydrogen bond and the HCOO- linkers of DMA manganese formate, with the help of XRD, IR and Raman spectroscopic studies up to ∼20 GPa. We observed that though a phase transition was initiated at ∼3.4 GPa, it was complete only by 6 GPa, indicating its first order nature. Beyond 7 GPa, this compound becomes highly disordered and shows an almost amorphous character, indicating a total collapse of the formate network. The reversibility of the initial structure of DMAMnF on the release of pressure from 20 GPa (i.e. from a highly disordered phase) shows the remarkable resilience of the formate cage. At the first crystal to crystal transition at 3.4 GPa, the distortion of the formate cage causes the ordering of the dynamically disordered hydrogen bond, resulting in a rearrangement of the DMA+ cation. Lifting of the mutual exclusivity of the Raman and IR modes (C-H out of plane and O-C-O bending modes) of HCOO- linkers, at this transition, indicates that the high pressure phase may be non-centro-symmetric.