Lay summary

Can polar molecular crystals really exist? We intend to elaborate a fundamental answer to this question. Growth of molecular crystals nucleating into a polar point group can result in a pronounced anisotropy of the growth speed. Experimental observations have found explanation by the effect of solvents, additives, impurities or genuine building blocks providing kinetic hindrance for attachment sites.

Here a new basic thesis is presented to investigate this phenomenon by (i) vapour growth and (ii) computational methods.

Theoretical work (2000-2001) by the applicant predicts a fundamental growth instability for molecular crystals built from dipolar components near thermodynamic equilibrium: Simulations show a bi-polar state in the sense that along one direction of the polar axis, growth leads to a full reversal of dipoles. It is proposed that the demand for dipole reversal is effecting kinetic hindrance for one direction of the polar axis.

The analysis will involve two steps: (i) High vacuum growth of selected materials suitable for both experimental (determination of growth speed and morphology) and (ii) force field calculations for estimating energy differences responsible for probabilities driving  orientational defect formation. At least three selected molecular examples will be analysed in detail.

In case of agreement of experimental and computational work, this would allow us to answer the initial question: Monopolar pyroelectric (no ferroelectric) molecular crystals can only exist because of a fundamental growth instability leading to kinetic hindrance of one of the two polar growth directions. This would lead to the final conclusion: In principle, a monopolar pyroelectric crystal of this type can not exist.