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A Markov-mean-field model is developed to describe growth-induced polarity in singlecomponent organic crystals formed by dipolar molecules. Results of an analytical theory agree well with corresponding Monte Carlo simulations. Polarity formation is analyzed in terms of three basic energy differences, two of them resulting from the interaction of functional groups (synthons) and a third one accounting for the lateral interaction. Basic packing types are discussed with respect to polarity formation. The Markov model provides a general description of the phenomenological behavior when moving energetically from centric to polar structures. Keeping synthon interactions within reasonable limits, the range for designing lateral coupling is limited to a few kilojoules per mole for a square lattice: Between a gap of about -2 kJ/mol e ∆E ⊥ e 3-4 kJ/mol (300 K), the stochastic process of orientational disorder is either significantly increasing (centric structures) or decreasing (polar structures) polarity. Outside of these borders, orientational disorder represents only a small perturbation to centric or polar structures. With respect to the existence of structure types, the model predicts that a molecular packing where the lateral energy difference between a parallel and an antiparallel alignment of molecules is larger than about 3 kJ/mol (square lattice, 300 K) most likely results in a centric structure featuring a low level of orientational disorder. A fundamentally new behavior for the growth of polar crystals is predicted: As a consequence of the Markov model, one of the two inequivalent growth directions of a polar axis is metastable against a 180°flipping of most of the dipoles. This type of a continuous process of twinning is different from geometrical twinning. A first set of data on real crystals is given, demonstrating polarity formation in crystal structures for which the X-ray analysis has reported only centric space groups.

Hydrogen storage in new nano-structured carbonic materials is a topic for lively discussion. The measured storage capacities of these materials, which have been announced in the literature during the last ten years are spread over an enormous range from about 0.1 wt% up to 67 wt%. This paper will give a report on the state of the art of hydrogen storage in carbon nanostructures. We shall critically review the recent 'key publications' on this topic, which claim storage capacities clearly above the technological bench mark set by the US Department of Energy, and we shall report new results which have been obtained in a joint project sponsored by the Federal Ministry for Education and Research in Germany (BMBF).

The polarization distribution in a polar perhydrotriphenylene (PHTP) inclusion compound has been studied by scanning pyroelectric microscopy. Measurements have been performed on solution-grown crystals of PHTP-AD (AD: acceptor (A) and donor (D) disubstituted linear chromophores). The AD guest molecules form aligned dipole chains within the parallel channels created by the honeycomb-type host lattice of PHTP. Along the channel direction we found a 180°twinned macrodomain state with wedge-shaped domain boundaries adjacent to the middle of the needle-shaped crystals. In each domain the polarization is nearly constant, which implies that the majority of molecular dipoles are aligned parallel. This agrees well with predictions arising from the application of Markov's theory of stochastic processes, which explains the occurrence of polar domains due to a mechanism of dipole incorporation driven by the difference in the-A‚‚‚Aand-D‚‚‚Dintermolecular interaction energies.