The crystal structure of DMABA has been determined at 293 and 180 K. It
crystallizes in the monoclinic P21/c space group. The structure was refined
to R1 = 0.0466 for 1269 independent reflections at 293 K and R1 = 0.038 for
1728 reflections at 180 K. DMABA is isomorphous with the chlorine analogue,
DMACA. Its anionic sublattice forms corrugated two-dimensional layers in the
bc plane. There are two crystallographically independent, disordered
dimethylammonium cations in the crystal structure. One of them occupies
cavities inside the polyanionic layers; the other is located between the
layers. The temperature dependence of lattice parameters has been determined
between 100 and 290 K. The presence of the ferro-paraelectric phase
transition at 164 K has been confirmed. The complex dielectric permittivity of
DMABA single crystals in the frequency range 100 Hz-10 GHz between 10 K and
300 K has been measured. The dynamic dielectric behaviour of DMABA is found to
be determined by the properties of two independent relaxators: a low frequency
one (~1 MHz at Tc) with a critical slowing down of its dynamics and
a high frequency one (~5 GHz), which is thermally activated and
contributes to the electric permittivity at least from about 80 K.
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