Temperature-dependent studies of [(CH3)2NH2][Fe(III)M(II)(HCOO)6] frameworks (M(II) = Fe and Mg): structural, magnetic, dielectric and phonon properties.

Novel heterometallic formate [(CH3)2NH2][Fe(III)Mg(II)(HCOO)6] (DMFeMg) was prepared and characterized by single crystal X-ray diffraction, DSC, dielectric, magnetic susceptibility, Raman and IR methods. We also report thermal, Raman and IR studies of the known compound [(CH3)2NH2][Fe(III)Fe(II)(HCOO)6] (DMFeFe). DMFeMg crystallizes in the niccolite structure (P3[combining macron]1c space group). In contrast to the known DMFeFe, [(CH3)2NH2][Fe(III)Mn(II)(HCOO)6] (DMFeMn) and [(CH3)2NH2][Fe(III)Co(II)(HCOO)6] (DMFeCo) formates, the metal ions in DMFeMg are distributed statistically over the two available octahedral sites. Temperature-dependent studies show that whereas DMFeFe exhibits an order-disorder phase transition at 151.8 K upon cooling, freezing-in of re-orientational motions of DMA(+) cations does not lead to any structural phase transition in DMFeMg. We discuss the origin of this difference. The low-temperature studies also show that DMFeMg orders magnetically at TC = 13.5(5) K and the shape of M(T) measured in the field-cooling regime suggests ferromagnetic character of the ordering.