  We study the dynamics of inorganic antiferroelectric nanoclusters formed
during an order-disorder transition and demonstrate the coexistence of the two
phases in a region of 2-3 K around the transition temperature TN~215 K. Single
crystals of NH4H2AsO4, a model hydrogen-bonded compound, show an
antiferroelectric-paraelectric transition studied by means of highly sensitive
magic angle spinning 15N NMR at 21.1 T. The phase co-existence is demonstrated
by a double-peak structure of the chemical shift. Two-dimensional chemical
exchange spectroscopy and spin-lattice relaxation time (T1) measurements show
that the clusters are dynamic with sizes ~50 nm and lifetimes approaching
seconds as T->TN. Their occupancy increases rapidly to fill the crystal volume
below $T_N$. This study provides evidence for the commonality of the phase
transitions in systems with electric properties and provides an improved
spectroscopic method for such studies.
