Infrared and Raman studies of phase transitions in metal-organic frameworks of [(CH3)2NH2][M(HCOO)3] with M=Zn, Fe

We report on temperature-dependent infrared (IR) and Raman studies of [(CH3)2NH2][M(HCOO)3] metal-organic frameworks (MOFs) with M=Zn, Fe. Based on Raman and IR data, an assignment of the observed modes to respective vibrations of atoms is proposed. Temperature-dependent studies revealed abrupt changes below 160 K that are attributed to the onset of first-order structural phase transition. The most pronounced changes are observed for the modes corresponding to the dimethylammonium cation, especially those involving motion of hydrogen atoms. This behavior proves that the phase transition has an order-disorder character and is associated with the ordering of protons. The abrupt splitting of some modes related to the formate ion indicates that this transition is also associated with significant distortion of the metal-formate framework.