Pyroelectricity in nonpolar directions in crystals: Enantiomeric disorder and surface wetting in racemic α-amino-acids

Display of pyroelectricity along nonpolar directions of crystals or from surfaces implies structural disorder or presence of polar surface layers. Nonpolar {210} faces of polar dl-alanine crystals display far larger pyroelectric effect than that at the polar {001} faces. Similarly, pyroelectricity is reported from {110} faces of centrosymmetric crystals of dl-aspartic acid. The origin of the disorder is due to an interchange of enantiomers at specific chiral crystal sites as supported by atom-atom potential energy computations and by pyroelectric effect observed on the nonpolar crystals of l-alanine intentionally doped with opposite enantiomers. These results should explain the riddle of the needle-like morphology of dl-alanine. The {100} faces of dl-serine and the {021} faces of dl-glutamic acid monohydrate crystals exhibit pyroelectricity due to surface wetting, whereas pyroelectricity originating from the {210} faces of enantiomerically doped l-alanine crystals could be deciphered as arising both from surface wetting and enantiomeric bulk disorder.