Preferences of polarity and chirality in triglycine sulfate crystals by alanine ghost

Triglycine sulfate (NH₂CH₂COOH)₃⋅H₂SO₄, (TGS) has attracted much attention because it exhibits excellent ferroelectricity below the Curie temperature. Although TGS is composed of glycine and sulfuric acid, which are achiral molecules, it forms chiral crystals below the Curie temperature. Ferroelectric domain observations and polarization-electric field hysteresis loop measurements showed that TGS polarity become preferred after a small amount of doping with enantiomeric alanine. It has been confirmed that the preferred polarity appears when the internal bias electric field, caused by alanine doping, is larger than the coercive field. Furthermore, X-ray crystal structure analysis has revealed that L-alanine-doped TGS (LATGS) and D-alanine-doped TGS (DATGS) with the nominal amount of alanine above 50 mol% exhibited great preferences for either chirality, although the alanine content in the crystal was too low to be detected; alanine was a “ghost”. Correlation between the polarity and chirality, thus the “absolute polarity”, of LATGS and DATGS were identified, and the preferences induced by the alanine ghost were noted. We have found that the chirality of TGS grown from achiral molecules can be preferred using an extremely simple method; doping with a tiny amount of alanine.