Short-Range Order of the Rotational Degree of Freedom of the Glycine Molecule in Triglycine Sulfate Glass State

The unit cell of triglycine sulfate (TGS) contains the following glycine ions: dimers (GIISHBGIII)⁺ with a short hydrogen bond (SHB) and a monomer (SO₄LHBGI)⁺ with a long hydrogen bond (LHB) to the SO₄ ²⁻ anion. The spontaneous polarization results from a statistical charge distribution and the Coulomb interaction between SO₄ ²⁻ and fast rotating –NH₃ ⁺ groups. In the lamellar model, chains of –SO₄–(GIISHBGIII)–SO₄–(GIISHBGIII)–SO₄ lying along the polar b-axis are linked perpendicularly to this axis by SO₄LHBGI⁺ units. Tilting the LHBGI⁺ ion around the axis in the mirror plane decides on the direction of the charge displacement from this symmetry plane. The reversal of the spontaneous polarization, i.e., −P _S↔ +P _S, is related to the rotation of the NH₃ group. If this rotation becomes slowed down at low temperature and/or under high pressure, the coercive field increases dramatically. Application of an external electric field E _⊥ perpendicular to the b-axis leads to a hysteresis loop of the polarity P(E), and finally to its disappearance. This phenomenon comes from an ordering of protons in hydrogen bonds perpendicular to the polar axis. This is the first study of the E _⊥ effect by nuclear magnetic resonance. Authors' address: Jan Stankowski, Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, Poznań 60-179, Poland