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ABSTRACT: Scanning pyroelectric microscopy on large crystals reveals the three-dimensional polarization distribution in AlPO4-5 crystals. AlPO4-5 probably grows in such a way that one particular absolute configuration is formed. In general twinned crystals are formed. The twin domains exhibit an opposite polarization. A single domain is found when crystals are grown from a substrate. In some cases the crystals show a more complex domain structure, which can be attributed to specific growth conditions. From the polarization distribution the history of the crystal growth can be retrieved. A mechanism for the adsorption of p-nitroaniline molecules is proposed on the basis of the polarization distribution in AlPO4-5 crystals and earlier observed preferential orientation of this guest molecule in the host structure.
Chemistry of Materials 11/1999; 11(12):3497-3503. · 7.29 Impact Factor
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ABSTRACT: The dynamics of molecular chains of hydrogen-bonded p-nitroaniline (pNA), confined in the one-dimensional pore system of AlPO4-5, was studied by broadband dielectric spectroscopy (10−2–106 Hz). Two relaxation processes were observed. A fast process (the α process) shows above 72 °C a Vogel–Fulcher–Tammann behavior typical of glass-forming materials, indicating cooperative dynamics of the “condensed” pNA chains. Below the glass temperature the α-relaxation peak broadens significantly and obeys an Arrhenius law. Both observations imply a change in length scale of cooperativity ξ from a temperature-controlled to a chain-length-dependent one. The second relaxation process is tentatively assigned to structural relaxations of the AlPO4-5 host lattice itself, since it is also present in calcined (empty) crystals. © 1999 American Institute of Physics.
The Journal of Chemical Physics 09/1999; 111(13):5637-5640. · 3.33 Impact Factor
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ABSTRACT: In order to study the polarisation direction and possible domain
structures of the macroscopic polarisation in polar materials, we have
developed a new scanning probe technique based on local pyroelectric
measurements (scanning pyroelectric microscopy, SPEM). The principles of
SPEM and some selected results on polar inclusion crystals and
pNA-loaded zeolites are given
Electrets, 1999. ISE 10. Proceedings. 10th International Symposium on; 02/1999
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ABSTRACT: In the present paper the authors describe results from (dynamic)
pyroelectric experiments on the p-nitroaniline molecule confined in AlPO
<sub>4</sub>-5 nanopores, which allow a study of individual crystals. A
comparison between dielectric and pyroelectric relaxation times is made
Electrets, 1999. ISE 10. Proceedings. 10th International Symposium on; 02/1999
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ABSTRACT: The specific adsorption of polar molecules in AlPO4-5 is caused by the nonsymmetric lattice structure of the crystals. This has been proven using scanning pyroelectric microscopy (SPEM). This novel technique provides information about both the zeolite structure and the orientation of adsorbed polar molecules.
12th International Zeolite Conference, Baltimore; 01/1999
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ABSTRACT: The dynamics of molecular chains of hydrogen-bonded p-nitroaniline (pNA), confined in the one-dimensional pore system of AlPO4 -5, was studied by broadband dielectric spectroscopy (10 -2 –106 Hz). Two relaxation processes were observed. A fast process (the a process) shows above 72 °C a Vogel–Fulcher–Tammann behavior typical of glass-forming materials, indicating cooperative dynamics of the ‘‘condensed’’ pNA chains. Below the glass temperature the a-relaxation peak broadens significantly and obeys an Arrhenius law. Both observations imply a change in length scale of cooperativity x from a temperature-controlled to a chain-length-dependent one. The second relaxation process is tentatively assigned to structural relaxations of the AlPO4 -5 host lattice itself, since it is also present in calcined (empty) crystals.
The Journal of Chemical Physics 01/1999; 111:5637-5640. · 3.33 Impact Factor
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ABSTRACT: 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, Tc, 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.
The Journal of Physical Chemistry B 10/1998; 102:9518-9524. · 3.70 Impact Factor
