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Scanning pyroelectric microscopy on large crystals reveals the three-dimensional polarization distribution in AlPO 4 -5 crystals. AlPO 4 -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 AlPO 4 -5 crystals and earlier observed preferential orientation of this guest molecule in the host structure.

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.

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