Orientational disorder at growing surfaces of molecular crystals: general comments on polarity formation and on secondary defects

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Abstract
Spontaneous symmetry reduction and twinning in centric molecular crystals, organic solid solutions, inclusion compounds and loaded zeolites containing dipolar molecules, is interpreted in terms of a unifying principle of polarity formation: A pyroelectric real-structure of growth sectors corresponds to the minimum energy state of as-grown crystals containing dipolar compounds. As-grown refers here to the kinetic stability of intrinsic and extrinsic defect configurations formed at the surface and which do not relax when transformed into the bulk. In order to illustrate examples from the molecular world, a mechanical analogue portraying the principle of polarity formation is presented. Furthermore, two mechanisms are discussed which either heal or sustain primary intrinsic or extrinsic defects. The latter mechanism is preferred by dipolar compounds which form strong, polarisation-induced intermolecular bonds and pack in a chain-like fashion.