We report the synthesis, crystal structure, thermal, dielectric, Raman, infrared, and magnetic properties of [(CH3)2NH2][Na(0.5)Fe(0.5)(HCOO)3] (DMNaFe), the first metal formate framework templated by organic cations, presenting an ABO3 perovskite architecture with NaO6 octahedra as building blocks of the framework. On the basis of Raman and IR data, assignment of the observed modes to respective vibrations of atoms is proposed. We have found that DMNaFe undergoes a structural phase transition at 167 K on cooling. According to the X-ray diffraction, the compound shows R3[combining macron] symmetry at 293 K and triclinic P1[combining macron] symmetry at 110 K. The DMA(+) cations are dynamically disordered in the high-temperature phase and the phase transition is associated with ordering of the DMA(+) cations and distortion of the metal formate framework. The dielectric studies reveal pronounced dielectric dispersion that can be attributed to slow dynamics of the DMA(+) cations. Based on the low-temperature magnetic studies, this compound is a weak ferromagnet with a critical temperature 8.5 K.

A novel formate [(CH3)2NH2][Na0.5Cr0.5(HCOO)3] (DMNaCr) was prepared by a solvothermal method. This compound crystallizes in the perovskite-type metal formate framework (space group R3[combining macron]) with disordered dimethylammonium (DMA(+)) cations. X-ray diffraction, DSC, Raman and IR studies show that in contrast to the isostructural iron analogue [(CH3)2NH2][Na0.5Fe0.5(HCOO)3] (DMNaFe), DMNaCr does not exhibit any structural phase transition at low temperatures. This behavior has been attributed to the smaller flexibility of the perovskite-like framework in DMNaCr when compared with that of DMNaFe. Dielectric permittivity data reveal pronounced dielectric relaxation that is attributed to the dynamical rotation of DMA(+) ions. Electron absorption and photoluminescence studies show that this material exhibits efficient emission at low temperatures. A detailed analysis of the optical properties shows that chromium ions are located at the site of intermediate crystal field strength with Dq/B = 2.29.

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.

In this paper, we propose a transparent subordination approach to anomalous diffusion processes underlying the nonexponential relaxation. We investigate properties of a coupled continuous-time random walk that follows from modeling the occurrence of jumps with compound counting processes. As a result, two different diffusion processes corresponding to over- and undershooting operational times, respectively, have been found. We show that within the proposed framework, all empirical two-power-law relaxation patterns may be derived. This work is motivated by the so-called "less typical" relaxation behavior observed, e.g., for gallium-doped Cd0.99Mn0.01Te mixed crystals.