We report that the hybrid organic–inorganic compound [(CH₃)₂NH₂][Mg(HCOO)₃] shows a marked dielectric transition around T_t ∼ 270 K, associated to a structural phase transition from SG Rc (centrosymmetric) to Cc (non-centrosymmetric). This is the highest T_t reported so far for a perovskite-like formate that is thus a promising candidate to display electric order very close to room temperature.

In this work we explore the overall structural behaviour of the [(CH₃)₂NH₂][Mn(HCOO)₃] multiferroic compound across the temperature range where its ferroelectric transition takes place by means of calorimetry, thermal expansion measurements and variable temperature powder and single crystal X-ray diffraction. The results clearly prove the presence of a structural phase transition at T_t ~ 187 K (the temperature at which the dielectric transition occurs) that involves a symmetry change from Rc to Cc, twinning of the crystals, a discontinuous variation of the unit cell parameters and unit cell volume, and a sharp first-order-like anomaly in the thermal expansion. In addition, the calorimetric results show a 3-fold order–disorder transition. The calculated pressure dependence of the transition temperature is rather large (dT_t/dP = 4.6 ± 0.1 K kbar⁻¹) in that it should be feasible to shift it to room temperature under adequate thermodynamic conditions, for instance by application of an external pressure.

In this work we further the structural characterization of the recently discovered (C₃N₂H₅)[Mn(HCOO)₃] metal–organic framework with perovskite-like structure, and we present its magnetic and dielectric properties up to 350 K. At low temperature, the C₃N₂H₅ ⁺ imidazolium cations, that sit oblique within the cavities of the [Mn(HCOO)₃]⁻ framework structure, show a cooperative order resulting in an antiparallel arrangement of their electrical dipole moments. Very interestingly, it is only above 220 K that thermal energy seems to be able to break this antiferroelectric order, resulting in a linear increase of its dielectric constant with temperature. In addition, this Mn(II) compound is antiferromagnetic below T_N = 9 K, with a slightly non-collinear arrangement of its magnetic moments, yielding to a weak ferromagnetism. Therefore, this is a new multiferroic material which exhibits coexistence of magnetic and electric ordering.