Raman spectroscopic studies across the ferroelectric transition in cobalt substituted dimethyl amine manganese formate

Abstract

AbstractTemperature‐dependent Raman studies, across the ferroelectric transition, Tc, in the metal organic framework material, [(CH3)2NH2][Mn(HCOO)3]—DMMn, and its cobalt (Co) doped analogues have been carried out to look for the changes in the lattice and intramolecular modes associated with the orientational ordering of the central dimethyl ammonium cation (DMA+) driven by the hydrogen‐bonded interaction between the DMA cation and the formate anion. A systematic decrease of the transition temperature with increased Co doping has been observed that correlates with results of specific heat measurements. The low‐wavenumber librational mode of the DMA cation shows a dramatic softening and narrowing of line width, across the transition, providing direct evidence of the slowing down of the rotation of the DMA cation associated with the orientational ordering. The evolution with temperature of the N‐H stretch modes of the DMA cation suggests that invoking the hydrogen‐bonding interactions alone is not enough to understand the observed decrease in the ferroelectric transition with Co doping in DMMn. A subtle interplay between the framework flexibility, size/charge of the metal cation, and the hydrogen‐bonding tendency finally decides the transition temperature in these materials.