Ferroelastic Phase Transition and Switchable Dielectric Constant in Heterometallic Niccolite Formate Frameworks.

Four heterometallic formate frameworks templated by various alkylamine cations with the general formula [cat][GaIIIMnII(HCOO)6] {cat is MA (CH3NH3+) for 1, DMA [(CH3)2NH2+] for 2, EtA (CH3CH2NH3+) for 3, and DEtA [(CH3CH2)2NH2+] for 4} have been prepared and characterized by X-ray diffraction, differential scanning calorimetry, and dielectric studies. All of the complexes have niccolite-like structures, which possess the same [GaMn(HCOO)6]- anionic framework with binodal (412·63)(49·66) topology; only the counterions in the cavity are different. Complex 4 undergoes a reversible ferroelastic phase transition around 220 K accompanied by a thermally switchable dielectric constant transition triggered by the freezing of the order-disorder DEtA cations.