First-order structural transition in the multiferroic perovskite-like formate [(CH3)2NH2][Mn(HCOO)3]

Abstract

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.