Polarization Reversal in AlPO4-5 Crystals Containing Polar or Nonpolar Organic Molecules:  A Scanning Pyroelectric Microscopy Study

Large AlPO4-5 crystals containing different polar or nonpolar guest molecules have been studied with scanning pyroelectric microscopy. Calcined crystals are pyroelectric with a polar structure that reverses in polarity at the center of the crystal. Consequently all these crystals are twinned. The temperature dependence of the pyroelectric coefficient reveals a genuine pyroelectric crystal structure, which spontaneous polarization decreases from 30 μC/m2 to almost zero at a Curie temperature, T c, of 50 °C. The presence of guest molecules, such as template molecules, adsorbates, or water, modifies the position and sharpness of the phase transition. Short-range interactions between polar guest molecules with the distinct capping faces of the asymmetric lattice were found to be the reason for the bidirectional adsorption (and formation of dipole chains) of polar molecules such as p-nitroaniline (pNA). The specific temperature dependence of the pyroelectric activity of AlPO4-5/pNA and the observation of a relaxation process were assigned to cooperative motions of the one-dimensional pNA chains. This is in line with theoretical predictions of the coexistence of a solid-like and a liquid-like pNA phase.