  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 proof the presence of structural phase
transition at Tt ~187 K (temperature at which the dielectric transition occurs)
that involves a symmetry change from R-3c 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 (dTt/dP = 4.6
$\pm$ 0.1 K/kbar), so that it should be feasible to shift it to room
temperature using adequate thermodynamic conditions, for instance by
application of external pressure.
