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

Abstract

Large AlPO 4 -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/m 2 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 AlPO 4 -5/pNA and the observation of a relaxation process were assigned to cooperative motions of the onedimensional pNA chains. This is in line with theoretical predictions of the coexistence of a solid-like and a liquid-like pNA phase.

Insights into Structure Direction of Microporous Aluminophosphates: Competition between Organic Molecules and Water

“…Klap et al have shown in a series of papers [24][25][26] that the Al À O À P vector (of the T À O À T bonds parallel to c) always points in the same direction (in fact, AFI crystal growth only takes place in that direction), thus giving a permanent polarisation to the crystal along the c direction. This macroscopic polarisation may cause the alignment of the water molecules in the 6-MR channels.…”

Glass transition of one-dimensional molecular chains of p-nitroaniline confined in AlPO4-5 nanopores revealed by dielectric spectroscopy

“…12 In the present paper we want to address the relaxational behavior of AlPO 4 -5/pNA by dielectric spectroscopy. …”

“…This host-guest system is of particular interest since the highly polar ͑and hyperpolarizable͒ pNA molecules will be adsorbed in a preferred direction, leading to a macroscopic polarization which gives rise to a secondharmonic generation activity 10 and to pyroelectricity. 11,12 Owing to the small channel diameter of 0.73 nm, the pNA molecules can neither pass each other nor flip around their short molecular axis. Infrared measurements revealed a high molecular orientation with a mean deviation angle of the long molecular axis from the channel direction of less than 15°at 273 K. 13 Another important feature of the aligned pNA molecules is their ability to form single or even double hydrogen bonds, the probability of which is temperature dependent.…”

Resonant second-harmonic generation in monosized and aligned single-walled carbon nanotubes

“…AFI crystals contain aluminum-oxygen-phosphorus pairs pointing in the same direction along the crystal axis, forming a macroscopically preferred direction. 10 Hence, an excess of one type of enantiomers may be achieved during the growth process of chiral CNTs because of this asymmetry. Indeed, molecular mechanics computations 11 demonstrate that for some organic molecules, one helical arrangement is more stable than the other in the channel of AFI crystal, and this can lead to a longrange chirality in the whole solid.…”