Structural inhibition of ferroelectric switching in triglycine sulphate-I. additives

The recently reported permanent poling effects in triglycine sulphate (TGS) containing some α-alanine additive is further investigated by pyroeiectric and dielectric measurements. The probable role of an L-alanine molecule in the lattice of TGS is examined in terms of the atomic motions normally required to reverse the polarity of the ferroelectric crystal. The observations can be explained by a model which relates the lack in α-alanine of the prototypic symmetry possessed by glycine I to the inhibition of the ferroelectric switching mechanism. Substituted α-alanine molecules are shown to lead to local irreversible dipoles which can "pole" a TGS crystal in a single sense provided that only one optically active form (L) is used. In this case crystals grown either below or above Tc behave similarly. Thus crystals can be grown in the non-polar phase, which become spontaneously, fully poled when cooled below Tc. Using dl-alanine as the additive for crystals grown below Tc, large permanently poled regions of each polarity are obtained. The model for the inhibition of ferroelectric switching by alanine is extended to cover other additives.