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

Novel heterometallic formate [(CH3)(2)NH2][(FeMgII)-Mg-III(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)(2)NH2][(FeFeII)-Fe-III(HCOO)(6)] (DMFeFe). DMFeMg crystallizes in the niccolite structure (P (3) over bar 1c space group). In contrast to the known DMFeFe, [(CH3)(2)NH2][(FeMnII)-Mn-III(HCOO)(6)] (DMFeMn) and [(CH3)(2)NH2][(FeCoII)-Co-III(HCOO)(6)] (DMFeCo) formates, the metal ions in DMFeMg are distributed statistically over the two available octahedral sites. Temperaturedependent 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 T-C = 13.5(5) K and the shape of M(T) measured in the field-cooling regime suggests ferromagnetic character of the ordering.