3D imaging and simulation of the polarisation distribution in molecular crystals

The spatial polarisation distribution in inhomogeneously polar molecular cr ystals has been imaged by scanning pyroelectric tomography and simulated by a Markov process. The experimental technique combines scanning pyroelectri c microscopy (SPEM) with layerwise thinning of crystals. The SPEM probes th e local spontaneous polarisation by its temperature dependence (pyroelectri c effect). A focused and intensity modulated laser beam scans the surface o f a polar sample and induces temperature changes in a volume depending on t he laser spot size and the thermal diffusion length lambda(th) Since high l ateral resolution of the SPEM is only available for small values of lambda( th) depth information to a resolution of similar to 10 mu m is achieved by repeated scanning and stepwise thinning of crystals. A layer by layer techn ique can provide 3D imaging of the polar ordering with a lateral resolution of similar to 20 mu m at a probed layer thickness of similar to 10 mu m. A pplied to perhydrotriphenylene (PI-ITP) co-crystallised with 1-(4-nitrophen yl)piperazine (NPP) two conical macro-domains of opposite and nearly consta nt polarisation were found. The SPEM results are in good agreement with a h omogeneous Markov chain model driving dipolar molecules into a parallel sta te within channels of PHTP. The new tomographic view reveals that the oppos ing cones are partially overlapping at the tips. Lateral growth processes a re assumed to be responsible for this. A theoretical section covers aspects of the convergency into the polar state and the length of polar chains dis cussed in terms of intermolecular interaction energies.