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.