New Physical Methods for a Space Resolved Mapping of the Macroscopic Polarisation in Molecular Crystals and a Stochastic Theory for Understanding

Authors

  • Jürg Hulliger Department of Chemistry and Biochemistry, University of Bern, Freiestr. 3, CH-3012 Bern

DOI:

https://doi.org/10.2533/chimia.2001.554

Keywords:

Orientational disorder, Phase sensitive second harmonic microscopy, Polar domains, Scanning pyroelectric microscopy, Stochastic theory

Abstract

The spatial distribution of polarity in molecular crystals built up by dipolar entities was imaged by two new techniques: Scanning pyroelectric microscopy (SPEM) and phase-sensitive second harmonic microscopy (PS-SHM). SPEM maps the variation in electrical polarisation due to a change in temperature (pyroelectric effect). The PS-SHM contrast is caused by a change of the sign of the molecular hyperpolarisability (nonlinear optical effect). A spatial inhomogeneity of polar properties in molecular crystals is predicted by a 2D Ising-type model describing 180° orientational disorder at the crystal-nutrient interface. As a result of (slow) growth, centric and acentric seed crystals can continuously transform into bipolar (twinned) crystals showing an inhomogeneous spatial distribution of polarity in the as-grown state.

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Published

2001-06-27