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

Abstract

Novel heterometallic formate [(CH₃)₂NH₂][Fe^IIIMg^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^IIIFe^II(HCOO)₆] (DMFeFe). DMFeMg crystallizes in the niccolite structure (P1c space group). In contrast to the known DMFeFe, [(CH₃)₂NH₂][Fe^IIIMn^II(HCOO)₆] (DMFeMn) and [(CH₃)₂NH₂][Fe^IIICo^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 T_C = 13.5(5) K and the shape of M(T) measured in the field-cooling regime suggests ferromagnetic character of the ordering.