Order-disorder phase transition and multiferroic behaviour in a metal organic framework compound (CH3)2NH2Co(HCOO)3

We have investigated the multiferroic and glassy behaviour of metal-organic framework (MOF) material (CH3)2NH2Co(CHOO)3. The compound has perovskite–like architecture in which the metal–formate forms a framework. The organic cation (CH3)2NH2+ occupies the cavities in the formate framework in the framework via N–H···O hydrogen bonds. At room temperature, the organic cation is disordered and occupies three crystallographically equivalent positions. Upon cooling, the organic cation is ordered which leads to a structural phase transition at 155 K. The structural phase transition is associated with a para-ferroelectric phase transition and is revealed by dielectric and pyroelectric measurements. Further, a PE hysteresis loop below 155 K confirms the ferroelectric behaviour of the material. Analysis of dielectric data reveal large frequency dispersion in the values of dielectric constant and tanδ which signifies the presence of glassy dielectric behaviour. The material displays a antiferromagnetic ordering below 15 K which is attributed to the super-exchange interaction between Co2+ ions mediated via formate linkers. Interestingly, another magnetic transition is also found around 11 K. The peak of the transition shifts to lower temperature with increasing frequency, suggesting glassy magnetism in the sample.