More complex than originally thought: revisiting the origins of the relaxation processes in dimethylammonium zinc formate

Abstract

Metal formates are a subclass of coordination polymers that is renowned for the rich phase transition behavior arising from the complex interplay of molecular dynamics of organic guests and the surrounding coordination net. This contribution challenges current consensus on the origins of the relaxation processes present in the low temperature phase of dimethylammonium (DMA⁺) zinc formate [(CH₃)₂NH₂][Zn(HCOO)₃] (DMAZn). Thus far, it was believed that below 156 K the order–disorder structural phase transition leads to the reduction of the crystal structure symmetry from hexagonal to monoclinic, as well as causes nearly complete freezing of the DMA⁺ cation. Herein, we assign the crystal symmetry of low-temperature phase DMAZn to triclinic (P1), based on the observed splitting of Bragg peaks into six different components originating from six ferroelastic domains. Noncentrosymmetry of the triclinic phase of DMAZn is confirmed with second harmonic generation measurements. Full reversibility of phase transition between triclinic (noncentrosymmetric) and trigonal (centrosymmetric) crystal phases allowed for the demonstration of nonlinear optical switching of the SHG-on–SHG-off type. In turn, a new set of experimental and theoretical data on cation dynamics shows that in the low temperature phase the flipping of organic cations does not completely freeze as previously thought. Dielectric measurements on the DMAZn sample synthesized in an electric field display enhanced intensity of two relaxation phenomena in dielectric spectra. The origin of these processes, previously misinterpreted as methyl group rotations, has been explored with the use of DFT calculations. It was found that the high- and low-frequency processes can be attributed to the hopping between stable and metastable positions of the DMA⁺ cation, respectively.