Plastic Crystals with Polar Halochromate Anion: Thermosensitive Dielectrics Based upon Plastic Transition and Dipole Rotation

Plastic
crystals functioning with rotatable components offer new
opportunities in areas such as modern optoelectronic materials. Here,
by taking advantage of controllable rotation of the polar component
within the ion-pair plastic-crystal system, we present two such crystals,
namely, (Et4N)­(CrO3X) (X = Cl or Br),
which are unusual examples exhibiting two-staged thermosensitive dielectric
responses above room temperature. The frequency-dependent response
in the first stage is due to the structural phase transitions, whereas
that in the second stage is induced by dynamic rotation of the polar
halochromate anions in their NaCl-type plastic-crystal phases. The
intrinsic mechanisms were also explicated by molecular dynamics simulations,
providing a direct insight into the dynamic characteristics of these
two compounds. These studies show that ionic plastic crystals functioning
with polar groups are an attractive candidate as sensitive thermoresponsive
dielectric materials.